52 2 Vibration DynamicsEmploying the momentum pi = mi vi of the mass mi , the Newton equation providesus with the equation of motion of the system: d d Fi = pi = (mi vi ) (2.1) dt dtWhen the motion of a massive body with mass moment Ii is rotational, then its equa-tion of motion will be found by Euler equation, in which we employ the moment ofmomentum Li = Ii ω of the mass mi : d d Mi = Li = (Ii ω) (2.2) dt dt For example, Fig. 2.1 illustrates a one degree-of-freedom (DOF) vibrating sys-tem. Figure 2.1(b) depicts the system when m is out of the equilibrium position at x ˙and moving with velocity x, both in positive direction. The FBD of the system is asshown in Fig. 2.1(c). The Newton equation generates the equations of motion: ma = −cv − kx + f (x, v, t) (2.3) The equilibrium position of a vibrating system is where the potential energy ofthe system, V , is extremum: ∂V =0 (2.4) ∂xWe usually set V = 0 at the equilibrium position. Linear systems with constantstiffness have only one equilibrium or inﬁnity equilibria, while nonlinear systemsmay have multiple equilibria. An equilibrium is stable if ∂ 2V >0 (2.5) ∂x 2and is unstable if ∂ 2V <0 (2.6) ∂x 2 The geometric arrangement and the number of employed mechanical elementscan be used to classify discrete vibrating systems. The number of masses times theDOF of each mass makes the total DOF of the vibrating system n. Each indepen-dent DOF of a mass is indicated by an independent variable, called the generalizedcoordinate. The ﬁnal set of equations would be n second-order differential equa-tions to be solved for n generalized coordinates. When each moving mass has oneDOF, then the system’s DOF is equal to the number of masses. The DOF mayalso be deﬁned as the minimum number of independent coordinates that deﬁnes theconﬁguration of a system. The equation of motion of an n DOF linear mechanical vibrating system of canalways be arranged as a set of second-order differential equations [m]¨ + [c]˙ + [k]x = F x x (2.7)
2.1 Newton–Euler Method 53Fig. 2.2 Two, three, and oneDOF models for verticalvibrations of vehiclesFig. 2.3 A 1/8 car modeland its free body diagramin which, x is a column array of describing coordinates of the system, and f is acolumn array of the associated applied forces. The square matrices [m], [c], [k] arethe mass, damping, and stiffness matrices.Example 30 (The one, two, and three DOF model of vehicles) The one, two, andthree DOF model for analysis of vertical vibrations of a vehicle are shown inFig. 2.2(a)–(c). The system in Fig. 2.2(a) is called the quarter car model, in whichms represents a quarter mass of the body, and mu represents a wheel. The param-eters ku and cu are models for tire stiffness and damping. Similarly, ks and cu aremodels for the main suspension of the vehicle. Figure 2.2(c) is called the 1/8 carmodel, which does not show the wheel of the car, and Fig. 2.2(b) is a quarter carwith a driver md . The driver’s seat is modeled by kd and cd .Example 31 (1/8 car model) Figure 2.3(a) shows the simplest model for verticalvibrations of a vehicle. This model is sometimes called 1/8 car model. The mass msrepresents one quarter of the car’s body, which is mounted on a suspension made ofa spring ks and a damper cs . When ms is at a position such as shown in Fig. 2.3(b),its free body diagram is as in Fig. 2.3(c).
54 2 Vibration DynamicsFig. 2.4 Equivalentmass–spring vibrator for apendulum Applying Newton’s method, the equation of motion would be ms x = −ks (xs − y) − cs (xs − y) ¨ ˙ ˙ (2.8)which can be simpliﬁed to ms x + cs xs + ks xs = ks y + cs y ¨ ˙ ˙ (2.9)The coordinate y indicates the input from the road and x indicates the absolutedisplacement of the body. Absolute displacement refers to displacement with respectto the motionless background.Example 32 (Equivalent mass and spring) Figure 2.4(a) illustrates a pendulum madeby a point mass m attached to a massless bar with length l. The coordinate θ showsthe angular position of the bar. The equation of motion for the pendulum can befound by using the Euler equation and employing the FBD shown in Fig. 2.4(b): ¨ IA θ = MA (2.10) ¨ ml 2 θ = −mgl sin θ (2.11)Simplifying the equation of motion and assuming a very small swing angle yields ¨ l θ + gθ = 0 (2.12)This equation is equivalent to an equation of motion of a mass–spring system madeby a mass me ≡ l, and a spring with stiffness ke ≡ g. The displacement of the masswould be xe ≡ θ . Figure 2.4(c) depicts such an equivalent mass–spring system.Example 33 (Gravitational force in rectilinear vibrations) When the direction ofthe gravitational force on a mass m is not varied with respect to the direction of
2.1 Newton–Euler Method 55Fig. 2.5 A mass–spring–damper system indicating that the gravitational force in rectilinear vibra-tions provides us with a static deﬂectionmotion of m, the effect of the weight force can be ignored in deriving the equationof motion. In such a case the equilibrium position of the system is at a point wherethe gravity is in balance with a deﬂection in the elastic member. This force–balanceequation will not be altered during vibration. Consequently we may ignore bothforces; the gravitational force and the static elastic force. It may also be interpretedas an energy balance situation where the work of gravitational force is always equalto the extra stored energy in the elastic member. Consider a spring k and damper c as is shown in Fig. 2.5(a). A mass m is put onthe force free spring and damper. The weight of m compresses the spring a staticlength xs to bring the system at equilibrium in Fig. 2.5(b). When m is at equilibrium,it is under the balance of two forces, mg and −kxs : mg − kxs = 0 (2.13)While the mass is in motion, its FBD is as shown in Fig. 2.5(c) and its equation ofmotion is mx = −kx − cx + mg − kxs ¨ ˙ = −kx − cx ˙ (2.14)It shows that if we examine the motion of the system from equilibrium, we canignore both the gravitational force and the initial compression of the elastic memberof the system.Example 34 (Force proportionality) The equation of motion for a vibrating systemis a balance between four different forces: a force proportional to displacement,−kx, a force proportional to velocity, −cv, a force proportional to acceleration, ma,and an applied external force f (x, v, t), which can be a function of displacement,velocity, and time. Based on Newton method, the force proportional to acceleration,ma, is always equal to the sum of all the other forces: ma = −cv − kx + f (x, v, t) (2.15)
56 2 Vibration DynamicsFig. 2.6 A 1/4 car modeland its free body diagramExample 35 (A two DOF base excited system) Figure 2.6(a)–(c) illustrate the equi-librium, motion, and FBD of a two DOF system. The FBD is plotted based on theassumption xs > x u > y (2.16)Applying Newton’s method provides us with two equations of motion: ms xs = −ks (xs − xu ) − cs (xs − xu ) ¨ ˙ ˙ (2.17) mu xu = ks (xs − xu ) + cs (xs − xu ) ¨ ˙ ˙ − ku (xu − y) − cu (xu − y) ˙ ˙ (2.18) The assumption (2.16) is not necessary. We can ﬁnd the same Eqs. (2.17)and (2.18) using any other assumption, such as xs < xu > y, xs > xu < y, orxs < xu < y. However, having an assumption helps to make a consistent free bodydiagram. We usually arrange the equations of motion for a linear system in a matrix formto take advantage of matrix calculus: [M]˙ + [C]˙ + [K]x = F x x (2.19)Rearrangement of Eqs. (2.17) and (2.18) yields ms 0 ¨ xs cs −cs ˙ xs + 0 mu ¨ xu −cs cs + cu ˙ xu ks −ks xs 0 + = (2.20) −ks ks + ku xu ku y + c u y ˙Example 36 (Inverted pendulum and negative stiffness) Figure 2.7(a) illustratesan inverted pendulum with a tip mass m and a length l. The pendulum is supported
2.1 Newton–Euler Method 57Fig. 2.7 An invertedpendulum with a tip mass mand two supportive springsby two identical springs attached to point B at a distance a < l from the pivot A.A free body diagram of the pendulum is shown in Fig. 2.7(b). The equation ofmotion may be found by taking a moment about A: ¨ MA = I A θ (2.21) ¨ mg(l sin θ ) − 2kaθ (a cos θ ) = ml 2 θ (2.22)To derive Eq. (2.22) we assumed that the springs are long enough to remain almoststraight when the pendulum oscillates. Rearrangement and assuming a very small θshow that the nonlinear equation of motion (2.22) can be approximated by ¨ ml 2 θ + mgl − 2ka 2 θ = 0 (2.23)which is equivalent to a linear oscillator: ¨ me θ + ke θ = 0 (2.24)with an equivalent mass me and equivalent stiffness ke : me = ml 2 ke = mgl − 2ka 2 (2.25) The potential energy of the inverted pendulum is V = −mgl(1 − cos θ ) + ka 2 θ 2 (2.26)which has a zero value at θ = 0. The potential energy V is approximately equal tothe following equation if θ is very small: 1 V ≈ − mglθ 2 + ka 2 θ 2 (2.27) 2
58 2 Vibration Dynamicsbecause 1 cos θ ≈ 1 − θ 2 + O θ 4 (2.28) 2To ﬁnd the equilibrium positions of the system, we may solve the equation ∂V /∂θ =0 for any possible θ : ∂V = −2mglθ + 2ka 2 θ = 0 (2.29) ∂θThe solution of the equation is θ =0 (2.30)which shows that the upright vertical position is the only equilibrium of the invertedpendulum as long as θ is very small. However, if mgl = ka 2 (2.31)then any θ around θ = 0 would be an equilibrium position and, hence, the invertedpendulum would have an inﬁnity of equilibria. The second derivative of the potential energy ∂ 2V = −2mgl + 2ka 2 (2.32) ∂x 2indicates that the equilibrium position θ = 0 is stable if ka 2 > mgl (2.33)A stable equilibrium pulls the system back if it deviates from the equilibrium, whilean unstable equilibrium repels the system. Vibration happens when the equilibriumis stable. This example also indicates the fact that having a negative stiffness is possibleby geometric arrangement of mechanical components of a vibrating system.Example 37 (Force function in equation of motion) Qualitatively, force is what-ever changes the motion, and quantitatively, force is whatever is equal to mass timesacceleration. Mathematically, the equation of motion provides us with a vectorialsecond-order differential equation m¨ = F(˙ , r, t) r r (2.34)We assume that the force function may generally be a function of time t, position r, ˙and velocity r. In other words, the Newton equation of motion is correct as long aswe can show that the force is only a function of r, r, t.˙ If there is a force that depends on the acceleration, jerk, or other variables that ˙cannot be reduced to r, r, t, the system is not Newtonian and we do not know theequation of motion, because ... ˙ ¨ F(r, r, r, r , . . . , t) = m¨ r (2.35)
2.2 Energy 59 ˙In Newtonian mechanics, we assume that force can only be a function of r, r, t andnothing else. In real world, however, force may be a function of everything; however, ˙we always ignore any other variables than r, r, t. Because Eq. (2.34) is a linear equation for force F, it accepts the superpositionprinciple. When a mass m is affected by several forces F1 , F2 , F3 , . . . , we maycalculate their summation vectorially F = F1 + F2 + F3 + · · · (2.36)and apply the resultant force on m. So, if a force F1 provides us with acceleration¨ ¨r1 , and F2 provides us with r2 , m¨ 1 = F1 r m¨ 2 = F2 r (2.37) ¨then the resultant force F3 = F1 + F2 provides us with the acceleration r3 such that ¨ ¨ ¨ r3 = r1 + r 2 (2.38) To see that the Newton equation of motion is not correct when the force is not ˙only a function of r, r, t, let us assume that a particle with mass m is under two ¨ ¨acceleration dependent forces F1 (x) and F2 (x) on x-axis: ¨ mx1 = F1 (x1 ) ¨ mx2 = F2 (x2 ) ¨ ¨ (2.39) ¨The acceleration of m under the action of both forces would be x3 mx3 = F1 (x3 ) + F2 (x3 ) ¨ ¨ ¨ (2.40)however, though we must have ¨ x3 = x1 + x2 ¨ ¨ (2.41)we do have ¨ m(x1 + x2 ) = F1 (x1 + x2 ) + F2 (x1 + x2 ) ¨ ¨ ¨ ¨ ¨ = F1 (x1 ) + F2 (x2 ) ¨ ¨ (2.42)2.2 EnergyIn Newtonian mechanics, the acting forces on a system of bodies can be dividedinto internal and external forces. Internal forces are acting between bodies of thesystem, and external forces are acting from outside of the system. External forcesand moments are called the load. The acting forces and moments on a body arecalled a force system. The resultant or total force F is the sum of all the externalforces acting on the body, and the resultant or total moment M is the sum of all
60 2 Vibration Dynamicsthe moments of the external forces about a point, such as the origin of a coordinateframe: F= Fi M= Mi (2.43) i i The moment M of a force F, acting at a point P with position vector rP , about apoint Q at rQ is MQ = (rP − rQ ) × F (2.44)and, therefore, the moment of F about the origin is M = rP × F (2.45)The moment of the force about a directional line l passing through the origin is Ml = u · (rP × F) ˆ (2.46) ˆwhere u is a unit vector on l. The moment of a force may also be called torque ormoment. The effect of a force system is equivalent to the effect of the resultant force andresultant moment of the force system. Any two force systems are equivalent if theirresultant forces and resultant moments are equal. If the resultant force of a forcesystem is zero, the resultant moment of the force system is independent of the originof the coordinate frame. Such a resultant moment is called a couple. When a force system is reduced to a resultant FP and MP with respect to areference point P , we may change the reference point to another point Q and ﬁndthe new resultants as FQ = FP (2.47) MQ = MP + (rP − rQ ) × FP = MP + Q rP × FP (2.48) The momentum of a moving rigid body is a vector quantity equal to the total massof the body times the translational velocity of the mass center of the body: p = mv (2.49)The momentum p is also called the translational momentum or linear momentum. Consider a rigid body with momentum p. The moment of momentum, L, about adirectional line l passing through the origin is Ll = u · (rC × p) ˆ (2.50) ˆwhere u is a unit vector indicating the direction of the line, and rC is the positionvector of the mass center C. The moment of momentum about the origin is L = rC × p (2.51)The moment of momentum L is also called angular momentum.
2.2 Energy 61 Kinetic energy K of a moving body point P with mass m at a position G rP , andhaving a velocity G vP , in the global coordinate frame G is 1 1 K = mG vP · G vP = mG v2 P (2.52) 2 2where G indicates the global coordinate frame in which the velocity vector vP isexpressed. The work done by the applied force G F on m in moving from point 1 topoint 2 on a path, indicated by a vector G r, is 2 1 W2 = G F · dG r (2.53) 1However, 2 2 Gd 1 2 d 2 G F · dG r = m G v · G v dt = m v dt 1 1 dt 2 1 dt 1 = m v2 − v1 = K2 − K1 2 2 (2.54) 2which shows that 1 W2 is equal to the difference of the kinetic energy of terminaland initial points: 1 W2 = K2 − K1 (2.55)Equation (2.55) is called the principle of work and energy. If there is a scalar po-tential ﬁeld function V = V (x, y, z) such that dV ∂V ∂V ∂V ˆ F = −∇V = − =− ı+ ˆ j+ ˆ k (2.56) dr ∂x ∂y ∂zthen the principle of work and energy (2.55) simpliﬁes to the principle of conserva-tion of energy, K1 + V1 = K2 + V2 (2.57)The value of the potential ﬁeld function V = V (x, y, z) is the potential energy ofthe system.Proof Consider the spatial integral of Newton equation of motion 2 2 F · dr = m a · dr (2.58) 1 1We can simplify the right-hand side of the integral (2.58) by the change of variable r2 r2 t2 dv F · dr = m a · dr = m · v dt r1 r1 t1 dt v2 1 =m v · dv = m v2 − v2 2 1 (2.59) v1 2
62 2 Vibration DynamicsThe kinetic energy of a point mass m that is at a position deﬁned by G r and havinga velocity G v is deﬁned by (2.52). Whenever the global coordinate frame G is theonly involved frame, we may drop the superscript G for simplicity. The work doneby the applied force G F on m in going from point r1 to r2 is deﬁned by (2.53).Hence the spatial integral of equation of motion (2.58) reduces to the principle ofwork and energy (2.55): 1 W2 = K2 − K1 (2.60)which says that the work 1 W2 done by the applied force G F on m during the dis-placement r2 − r1 is equal to the difference of the kinetic energy of m. If the force F is the gradient of a potential function V , F = −∇V (2.61)then F · dr in Eq. (2.58) is an exact differential and, hence, 2 2 F · dr = dV = −(V2 − V1 ) (2.62) 1 1 E = K1 + V1 = K2 + V2 (2.63)In this case the work done by the force is independent of the path of motion betweenr1 and r2 and depends only upon the value of the potential V at start and end pointsof the path. The function V is called the potential energy; Eq. (2.63) is called theprinciple of conservation of energy, and the force F = −∇V is called a potential, or aconservative force. The kinetic plus potential energy of the dynamic system is calledthe mechanical energy of the system and is denoted by E = K + V . The mechanicalenergy E is a constant of motion if all the applied forces are conservative. A force F is conservative only if it is the gradient of a stationary scalar function.The components of a conservative force will only be functions of space coordinates: ı ˆ ˆ F = Fx (x, y, z)ˆ + Fy (x, y, z)j + Fz (x, y, z)k (2.64)Example 38 (Energy and equation of motion) Whenever there is no loss of energyin a mechanical vibrating system, the sum of kinetic and potential energies is aconstant of motion: E = K + V = const (2.65)A system with constant energy is called a conservative system. The time derivativeof a constant of motion must be zero at all time. The mass–spring system of Fig. 2.10 is a conservative system with the total me-chanical energy of 1 1 ˙ E = mx 2 + kx 2 (2.66) 2 2
2.2 Energy 63Fig. 2.8 A multi DOFconservative vibrating systemHaving a zero rate of energy, ˙ E = mx x + kx x = x(mx + kx) = 0 ˙¨ ˙ ˙ ¨ (2.67) ˙and knowing that x cannot be zero at all times provides us with the equation ofmotion: mx + kx = 0 ¨ (2.68)Example 39 (Energy and multi DOF systems) We may use energy method anddetermine the equations of motion of multi DOF conservative systems. Considerthe system in Fig. 2.8 whose mechanical energy is 1 1 1 E = K + V = m1 x1 + m2 x2 + m3 x3 ˙2 ˙2 ˙2 2 2 2 1 1 1 1 + k1 x1 + k2 (x1 − x2 )2 + k3 (x2 − x3 )2 + k4 x3 (2.69) 2 2 2 2 2 2To ﬁnd the ﬁrst equation of motion associated to x1 , we assume x2 and x3 are con-stant and take the time directive: ˙ E = m1 x1 x1 + k1 x1 x1 + k2 (x1 − x2 )x1 = 0 ˙ ¨ ˙ ˙ (2.70) ˙Because x1 cannot be zero at all times, the ﬁrst equation of motion is m1 x1 + k1 x1 + k2 (x1 − x2 ) = 0 ¨ (2.71)To ﬁnd the second equation of motion associated to x2 , we assume that x1 and x3are constant and we take a time directive of E ˙ E = m2 x2 x2 − k2 (x1 − x2 )x2 + k3 (x2 − x3 )x2 = 0 ˙ ¨ ˙ ˙ (2.72)which provides us with m2 x2 − k2 (x1 − x2 ) + k3 (x2 − x3 ) = 0 ¨ (2.73)To ﬁnd the second equation of motion associated to x2 , we assume that x1 and x3are constant and we take the time directive of E ˙ E = m3 x3 x3 − k3 (x2 − x3 )x3 + k4 x3 x3 = 0 ˙ ¨ ˙ ˙ (2.74)
64 2 Vibration DynamicsFig. 2.9 A two DOFconservative nonlinearvibrating systemwhich provides us with m3 x3 − k3 (x2 − x3 ) + k4 x3 = 0 ¨ (2.75)We may set up the equations in a matrix form: ⎡ ⎤⎡ ⎤ m1 0 0 ¨ x1 ⎣ 0 m2 0 ⎦ ⎣x2 ⎦ ¨ 0 0 m3 ¨ x3 ⎡ ⎤⎡ ⎤ k1 + k2 −k2 0 x1 + ⎣ −k2 k2 + k3 −k3 ⎦ ⎣x2 ⎦ = 0 (2.76) 0 −k3 k3 + k4 x3Example 40 (Energy and nonlinear multi DOF systems) The energy method canbe applied on every conservative system regardless of linearity of the system. Fig-ure 2.9 illustrates a two DOF nonlinear system whose kinetic and potential energiesare 1 1 ˙ ˙ ˙˙ K = m1 x 2 + m2 x 2 + k 2 θ 2 + 2l x θ sin θ ˙ 2 (2.77) 2 2 1 V = kx 2 − m2 g(x − l cos θ ) (2.78) 2We assumed that the motionless hanging down position is the equilibrium of inter-est, and that the gravitational energy is zero at the level of m1 at the equilibrium.Example 41 (Maximum energy and frequency of vibrations) The mechanical vibra-tions is a continuous exchange of energy between kinetic and potential. If there isno waste of energy, their maximum values must be equal. Consider the simple mass–spring system of Fig. 2.10. The harmonic motion,kinetic energy, and potential energy of the system are x = X sin ωt (2.79) 1 1 ˙ K = mx 2 = mX 2 ω2 cos2 ωt (2.80) 2 2
2.2 Energy 65Fig. 2.10 A mass–springsystemFig. 2.11 A wheel turning,without slip, over acylindrical hill 1 1 V = kx 2 = kX 2 sin2 ωt (2.81) 2 2Equating the maximum K and V 1 1 mX 2 ω2 = kX 2 (2.82) 2 2provides us with the frequency of vibrations: k ω2 = (2.83) mExample 42 (Falling wheel) Figure 2.11 illustrates a wheel turning, without slip,over a cylindrical hill. We may use the conservation of mechanical energy to ﬁndthe angle at which the wheel leaves the hill. Initially, the wheel is at point A. We assume the initial kinetic and potential, andhence, the mechanical energies E = K + V are zero. When the wheel is turningover the hill, its angular velocity, ω, is v ω= (2.84) rwhere v is the speed at the center of the wheel. At any other point B, the wheelachieves some kinetic energy and loses some potential energy. At a certain angle,where the normal component of the weight cannot provide more centripetal force, mv 2 mg cos θ = (2.85) R+r
66 2 Vibration DynamicsFig. 2.12 A turning wheelmoving up a stepthe wheel separates from the surface. Employing the conservation of energy, wehave KA + VA = KB + VB (2.86)The kinetic and potential energy at the separation point B are 1 1 KB = mv 2 + IC ω2 (2.87) 2 2 VB = −mg(R + r)(1 − cos θ ) (2.88)where IC is the mass moment of inertia for the wheel about its center. Therefore, 1 2 1 mv + IC ω2 = mg(R + r)(1 − cos θ ) (2.89) 2 2and substituting (2.84) and (2.85) yields IC 1+ (R + r)g cos θ = 2g(R + r)(1 − cos θ ) (2.90) mr 2and, therefore, the separation angle is 2mr 2 θ = cos−1 (2.91) IC + 3mr 2 Let us examine the equation for a disc wheel with 1 IC = mr 2 (2.92) 2and ﬁnd the separation angle: 4 θ = cos−1 ≈ 0.96 rad ≈ 55.15 deg (2.93) 7Example 43 (Turning wheel over a step) Figure 2.12 illustrates a wheel of radiusR turning with speed v to go over a step with height H < R. We may use the
2.2 Energy 67principle of energy conservation and ﬁnd the speed of the wheel after getting acrossthe step. Employing the conservation of energy, we have KA + VA = KB + VB (2.94) 1 2 1 1 2 1 mv + IC ω1 + 0 = 2 mv + IC ω2 + mgH 2 (2.95) 2 1 2 2 2 2 IC 2 IC 2 m + 2 v1 = m + 2 v2 + 2mgH (2.96) R Rand, therefore, 2gH v2 = v1 − 2 IC (2.97) 1+ mR 2The condition for having a real v2 is 2gH v1 > IC (2.98) 1+ mR 2The second speed (2.97) and the condition (2.98) for a solid disc with IC = mR 2 /2are 4 v2 = v1 − Hg 2 (2.99) 3 4 v1 > Hg (2.100) 3Example 44 (Newton equation) The application of a force system is emphasized byNewton’s second law of motion, which states that the global rate of change of linearmomentum is proportional to the global applied force: Gd Gd G F= G p= mG v (2.101) dt dtThe second law of motion can be expanded to include rotational motions. Hence, thesecond law of motion also states that the global rate of change of angular momentumis proportional to the global applied moment: Gd G M= G L (2.102) dt
68 2 Vibration DynamicsProof Differentiating the angular momentum (2.51) shows that Gd Gd G dr G dp C G L= (rC × p) = × p + rC × dt dt dt dt G dp = G rC × = G rC × G F = G M (2.103) dtExample 45 (Integral and constant of motion) Any equation of the form ˙ f (q, q, t) = c (2.104) ˙ c = f (q0 , q0 , t0 ) (2.105) q = q1 q2 · · · qn (2.106)with total differential n df ∂f ∂f ∂f = qi + ˙ qi + ¨ =0 (2.107) dt ∂qi ˙ ∂ qi ∂t i=1 ˙that the generalized positions q and velocities q of a dynamic system must satisfyat all times t is called an integral of motion. The parameter c, of which the valuedepends on the initial conditions, is called a constant of motion. The maximumnumber of independent integrals of motion for a dynamic system with n degreesof freedom is 2n. A constant of motion is a quantity of which the value remainsconstant during the motion. Any integral of motion is a result of a conservation principle or a combinationof them. There are only three conservation principles for a dynamic system: en-ergy, momentum, and moment of momentum. Every conservation principle is theresult of a symmetry in position and time. The conservation of energy indicates thehomogeneity of time, the conservation of momentum indicates the homogeneity inposition space, and the conservation of moment of momentum indicates the isotropyin position space.Proof Consider a mechanical system with fC degrees of freedom. Mathematically,the dynamics of the system is expressed by a set of n = fC second-order differentialequations of n unknown generalized coordinates qi (t), i = 1, 2, . . . , n: ¨ qi = Fi (qi , qi , t) i = 1, 2, . . . , n ˙ (2.108)The general solution of the equations contains 2n constants of integrals. ˙ qi = qi (c1 , c2 , . . . , cn , t) ˙ i = 1, 2, . . . , n (2.109) qi = qi (c1 , c2 , . . . , c2n , t) i = 1, 2, . . . , n (2.110)
2.2 Energy 69To determine these constants and uniquely identify the motion of the system, it is ˙necessary to know the initial conditions qi (t0 ), qi (t0 ), which specify the state of thesystem at some given instant t0 : ˙ cj = cj q(t0 ), q(t0 ), t0 j = 1, 2, . . . , 2n (2.111) ˙ ˙ fj q(t), q(t), t = cj q(t0 ), q(t0 ), t0 (2.112)Each of these functions fj is an integral of the motion and each ci is a constant ofthe motion. An integral of motion may also be called a ﬁrst integral, and a constantof motion may also be called a constant of integral. When an integral of motion is given, ˙ f1 (q, q, t) = c1 (2.113)we can substitute one of the equations of (2.108) with the ﬁrst-order equation of q1 = f (c1 , qi , qi+1 , t) i = 1, 2, . . . , n ˙ ˙ (2.114)and solve a set of n − 1 second-order and one ﬁrst-order differential equations: qi+1 = Fi+1 (qi , qi , t) ¨ ˙ i = 1, 2, . . . , n (2.115) q1 = f (c1 , qi , qi+1 , t) ˙ ˙If there exist 2n independent ﬁrst integrals fj , j = 1, 2, . . . , 2n, then instead of solv-ing n second-order equations of motion (2.108), we can solve a set of 2n algebraicequations ˙ ˙ fj (q, q) = cj q(t0 ), q(t0 ), t0 j = 1, 2, . . . , 2n (2.116)and determine the n generalized coordinates qi , i = 1, 2, . . . , n: qi = qi (c1 , c2 , . . . , c2n , t) i = 1, 2, . . . , n (2.117) Generally speaking, an integral of motion f is a function of generalized coordi- ˙nates q and velocities q such that its value remains constant. The value of an integralof motion is the constant of motion c, which can be calculated by substituting the ˙given value of the variables q(t0 ), q(t0 ) at the associated time t0 .Example 46 (A mass–spring–damper vibrator) Consider a mass m attached to aspring with stiffness k and a damper with damping c. The equation of motion of thesystem and its initial conditions are mx + cx + kx = 0 ¨ ˙ (2.118) x(0) = x0 x(0) = x0 ˙ ˙ (2.119)Its solution is x = c1 exp(s1 t) + c2 exp(s2 t) (2.120)
70 2 Vibration DynamicsFig. 2.13 A planar pendulum √ √ c− c2 − 4km c+ c2 − 4km s1 = s2 = (2.121) −2m −2m ˙Taking the time derivative, we ﬁnd x: x = c1 s1 exp(s1 t) + c2 s2 exp(s2 t) ˙ (2.122) ˙Using x and x, we determine the integrals of motion f1 and f2 : x − xs2 ˙ f1 = = c1 (2.123) (s1 − s2 ) exp(s1 t) x − xs1 ˙ f2 = = c2 (2.124) (s2 − s1 ) exp(s2 t)Because the constants of integral remain constant during the motion, we can calcu-late their value at any particular time such as t = 0: x0 − x0 s2 ˙ x0 − x0 s1 ˙ c1 = c2 = (2.125) (s1 − s2 ) (s2 − s1 )Substituting s1 and s2 provides us with the constants of motion c1 and c2 : √ √ c2 − 4km(cx0 + x0 c2 − 4km + 2mx0 ) ˙ c1 = 2 − 4km) (2.126) 2(c √ √ c2 − 4km(cx0 − x0 c2 − 4km + 2mx0 ) ˙ c2 = 2 − 4km) (2.127) 2(cExample 47 (Constraint and ﬁrst integral of a pendulum) Figure 2.13(a) illus-trates a planar pendulum. The free body diagram of Fig. 2.13(b) provides us withtwo equations of motion: x mx = −T ¨ (2.128) l y my = −mg + T ¨ (2.129) l
2.2 Energy 71Eliminating the tension force T , we have one second-order equation of two vari-ables: yx + xy + gx = 0 ¨ ¨ (2.130)Because of the constant length of the connecting bar we have a constraint equationbetween x and y: x2 + y2 − l2 = 0 (2.131)Having one constraint indicates that we can express the dynamic of the system byonly one generalized coordinate. Choosing θ as the generalized coordinate, we canexpress x and y by θ , writing the equation of motion (2.130) as g ¨ θ+ sin θ = 0 (2.132) l ˙Multiplying the equation by θ and integrating provides us with the integral of en-ergy: 1 g ˙ ˙ f (θ, θ ) = θ 2 − cos θ = E (2.133) 2 l 1 2 g ˙ E = θ0 − cos θ0 (2.134) 2 lThe integral of motion (2.133) is a ﬁrst-order differential equation: g ˙ θ= 2E + 2 cos θ (2.135) lThis equation expresses the dynamic of the pendulum upon solution. Let us assume that θ is too small to approximate the equation of motion as g ¨ θ + θ =0 (2.136) lThe ﬁrst integral of this equation is 1 g ˙ ˙ f (θ, θ ) = θ 2 − θ =E (2.137) 2 l 1 2 g ˙ E = θ0 − θ0 (2.138) 2 lthat provides us with a separated ﬁrst-order differential equation: g ˙ θ= 2E + 2 θ (2.139) lIts solution is dθ √ l g t= = 2 θ +E−p (2.140) 2E + 2gθ g l l
72 2 Vibration Dynamicswhere p is the second constant of motion: l p= ˙ θ0 (2.141) g Now, let us ignore the energy integral and solve the second-order equation ofmotion (2.136): g g θ = c1 cos t + c2 sin t (2.142) l lThe time derivative of the solution l g g ˙ θ = −c1 sin t + c2 cos t (2.143) g l lcan be used to determine the integrals and constants of motion: g l g f1 = θ cos t− ˙ θ sin t (2.144) l g l g l g f2 = θ sin t+ ˙ θ cos t (2.145) l g l ˙ ˙Using the initial conditions θ (0) = θ0 , θ (0) = θ0 , we have l c1 = θ 0 c2 = ˙ θ0 (2.146) gA second-order equation has only two constants of integrals. Therefore, we shouldbe able to express E and p in terms of c1 and c2 or vice versa: 1 2 g 1g 2 g ˙ E = θ0 − θ0 = c − c1 (2.147) 2 l 2l 2 l l l g p= ˙ θ0 = c2 (2.148) g g l l g l c2 = ˙ θ0 = p (2.149) g l g 1g 2 l c1 = θ 0 = p − E (2.150) 2l gE is the mechanical energy of the pendulum, and p is proportional to its moment ofmomentum.
2.3 Rigid Body Dynamics 73Fig. 2.14 A globally ﬁxedG-frame and a body B-framewith a ﬁxed common originat O2.3 Rigid Body DynamicsA rigid body may have three translational and three rotational DOF. The transla-tional and rotational equations of motion of the rigid body are determined by theNewton–Euler equations.2.3.1 Coordinate Frame TransformationConsider a rotation of a body coordinate frame B(Oxyz) with respect to a globalframe G(OXY Z) about their common origin O as illustrated in Fig. 2.14. The com-ponents of any vector r may be expressed in either frame. There is always a trans-formation matrix G RB to map the components of r from the frame B(Oxyz) to theother frame G(OXY Z): G r = G RB B r (2.151) −1In addition, the inverse map B r = G RB G r can be done by B RG , B r = B RG G r (2.152)where G B RB = RG = 1 (2.153)and −1 B RG = G RB = G RB T (2.154)When the coordinate frames B and G are orthogonal, the rotation matrix GR B iscalled an orthogonal matrix. The transpose R T and inverse R −1 of an orthogonalmatrix [R] are equal: R T = R −1 (2.155)
74 2 Vibration DynamicsBecause of the matrix orthogonality condition, only three of the nine elements ofG R are independent. BProof Employing the orthogonality condition ˆı ˆ ˆ ˆ ˆ r = (r · ı )ˆ + (r · j )j + (r · k)k (2.156)and decomposition of the unit vectors of G(OXY Z) along the axes of B(Oxyz), ˆ ˆ ˆı ˆ ˆ ˆ ˆ ˆ ˆ I = (I · ı )ˆ + (I · j )j + (I · k)k (2.157) ˆ ˆ ˆı ˆ ˆ ˆ ˆ ˆ ˆ J = (J · ı )ˆ + (J · j )j + (J · k)k (2.158) ˆ ˆ ˆı ˆ ˆ ˆ ˆ ˆ ˆ K = (K · ı )ˆ + (K · j )j + (K · k)k (2.159)introduces the transformation matrix G RB to map the local axes to the global axes: ⎡ ⎤ ⎡ ⎤⎡ ⎤ ⎡ ⎤ Iˆ ˆ ˆ I ·ı ˆ ˆ I ·j ˆ ˆ I ·k ˆ ı ˆ ı ⎣J ⎦ = ⎣J · ı ˆ ˆ ˆ ˆ ˆ J ·j J · k ⎦ ⎣j ⎦ = G RB ⎣j ⎦ ˆ ˆ ˆ ˆ (2.160) ˆ K ˆ ˆ K ·ı ˆ ˆ K ·j ˆ ˆ K ·k ˆ k ˆ kwhere ⎡ ⎤ ˆ ˆ ˆ ˆ I ·ı I ·j I ·k ˆ ˆ G RB = ⎣ J · ı J · j J · k ⎦ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ·ı K ·j K ·k K ˆ ˆ ˆ ˆ ˆ ⎡ ⎤ ˆ ˆ ˆ ˆ ˆ ˆ cos(I , ı ) cos(I , j ) cos(I , k) = ⎣ cos(J , ı ) cos(J , j ) cos(J , k) ⎦ ˆ ˆ ˆ ˆ ˆ ˆ (2.161) ˆ ˆ ˆ ˆ ˆ ˆ cos(K, ı ) cos(K, j ) cos(K, k)Each column of G RB is the decomposition of a unit vector of the local frameB(Oxyz) in the global frame G(OXY Z): G RB = ˆ Gı ˆ Gj ˆ Gk (2.162)Similarly, each row of G RB is decomposition of a unit vector of the global frameG(OXY Z) in the local frame B(Oxyz). ⎡B ⎤ ˆ IT G RB = ⎣ B J T ⎦ ˆ (2.163) ˆ B KTso the elements of G RB are directional cosines of the axes of G(OXY Z) inB(Oxyz) or B in G. This set of nine directional cosines completely speciﬁes theorientation of B(Oxyz) in G(OXY Z) and can be used to map the coordinates ofany point (x, y, z) to its corresponding coordinates (X, Y, Z).
2.3 Rigid Body Dynamics 75 Alternatively, using the method of unit-vector decomposition to develop the ma-trix B RG leads to −1 r = B RG G r = G RB G r B (2.164) ⎡ ⎤ ˆ ˆ ˆ ˆ ˆ ˆ ı ·I ı ·J ı ·K B RG = ⎣ j · I j · J j · K ⎦ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ k·I k·J k·K ⎡ ⎤ ı ˆ ı ˆ ı ˆ cos(ˆ, I ) cos(ˆ, J ) cos(ˆ, K) = ⎣cos(j , I ) cos(j , J ) cos(j , K)⎦ ˆ ˆ ˆ ˆ ˆ ˆ (2.165) ˆ I ) cos(k, J ) cos(k, K) cos(k, ˆ ˆ ˆ ˆ ˆIt shows that the inverse of a transformation matrix is equal to the transpose of thetransformation matrix, −1 G RB = G RB T (2.166)or G RB · G RB = I T (2.167) A matrix with condition (2.166) is called an orthogonal matrix. Orthogonality ofGR B comes from the fact that it maps an orthogonal coordinate frame to anotherorthogonal coordinate frame. An orthogonal transformation matrix G RB has only three independent elements.The constraint equations among the elements of G RB will be found by applying thematrix orthogonality condition (2.166): ⎡ ⎤⎡ ⎤ ⎡ ⎤ r11 r12 r13 r11 r21 r31 1 0 0 ⎣r21 r22 r23 ⎦ ⎣r12 r22 r32 ⎦ = ⎣0 1 0⎦ (2.168) r31 r32 r33 r13 r23 r33 0 0 1Therefore, the inner product of any two different rows of G RB is zero, and the innerproduct of any row of G RB by itself is unity: r11 + r12 + r13 = 1 2 2 2 r21 + r22 + r23 = 1 2 2 2 r31 + r32 + r33 = 1 2 2 2 (2.169) r11 r21 + r12 r22 + r13 r23 = 0 r11 r31 + r12 r32 + r13 r33 = 0 r21 r31 + r22 r32 + r23 r33 = 0
76 2 Vibration DynamicsThese relations are also true for columns of G RB and evidently for rows andcolumns of B RG . The orthogonality condition can be summarized by the equation 3 rij rik = δj k j, k = 1, 2, 3 (2.170) i=1where rij is the element of row i and column j of the transformation matrix G RBand δj k is the Kronecker delta δij , 1 i=j δij = δj i = (2.171) 0 i=j Equation (2.170) provides us with six independent relations that must be satisﬁedby the nine directional cosines. Therefore, there are only three independent direc-tional cosines. The independent elements of the matrix G RB cannot be in the samerow or column or any diagonal. The determinant of a transformation matrix is equal to unity, G RB = 1 (2.172)because of Eq. (2.167) and noting that G G G G G G 2 RB · G RB = T RB · RB = T RB · RB = RB =1 (2.173) ˆ ˆ ˆUsing linear algebra and column vectors G ı , G j , and G k of G RB , we know that G RB = G ı · ˆ G ˆ j × Gk ˆ (2.174)and because the coordinate system is right handed, we have ˆ Gj ˆ × G k = G ı and, ˆtherefore, G RB = G ı T · G ı = +1 ˆ ˆ (2.175)Example 48 (Global position using B r and B RG ) The position vector r of a pointP may be described in either the G(OXY Z) or the B(Oxyz) frame. If B r = 10ˆ −ı ˆ5j + 15k and the transformation matrix to map G r to B r is ˆ ⎡ ⎤ 0.866 0 0.5 B r = B RG G r = ⎣−0.353 0.707 0.612 ⎦ G r (2.176) 0.353 0.707 −0.612then the components of G r in G(OXY Z) would be ⎡ ⎤ 15.72 G r = G RB B r = B RG B r = ⎣ 7.07 ⎦ T (2.177) −7.24
2.3 Rigid Body Dynamics 77Example 49 (Two-point transformation matrix) The global position vectors of twopoints P1 and P2 , of a rigid body B are ⎡ ⎤ ⎡ ⎤ 1.077 −0.473 G rP1 = ⎣1.365⎦ G rP2 = ⎣ 2.239 ⎦ (2.178) 2.666 −0.959The origin of the body B(Oxyz) is ﬁxed on the origin of G(OXY Z), and the pointsP1 and P2 are lying on the local x- and y-axis, respectively. ˆ ˆ To ﬁnd G RB , we use the local unit vectors G ı and G j , ⎡ ⎤ ⎡ ⎤ Gr 0.338 Gr −0.191 = ⎣0.429⎦ = ⎣ 0.902 ⎦ P1 P2 G ı= G ˆ G j= G ˆ (2.179) | rP1 | 0.838 | rP2 | −0.387 ˆto obtain G k: ⎡ ⎤ −0.922 Gˆ k = ı × j = ⎣−0.029⎦ ˆ ˆ (2.180) 0.387Hence, the transformation matrix G RB would be ⎡ ⎤ 0.338 −0.191 −0.922 G RB = G ı G j G k = ⎣0.429 0.902 −0.029⎦ ˆ ˆ ˆ (2.181) 0.838 −0.387 0.387Example 50 (Length invariant of a position vector) Expressing a vector in differentframes utilizing rotation matrices does not affect the length and direction propertiesof the vector. Therefore, the length of a vector is an invariant property: G B |r| = r = r (2.182) The length invariant property can be shown as G TG |r|2 = G rTG r = RB B r RB B r = B rTG RB G RB B r T = B rTB r (2.183)Example 51 (Multiple rotation about global axes) Consider a globally ﬁxed pointP at ⎡ ⎤ 1 G r = ⎣2⎦ (2.184) 3The body B will turn 45 deg about the X-axis and then 45 deg about the Y -axis.An observer in B will see P at
78 2 Vibration Dynamics B r = Ry,−45 Rx,−45 G r ⎡ ⎤ ⎡1 ⎤⎡ ⎤ cos −π 0 − sin −π 0 0 1 ⎦ ⎢0 cos 4 sin −π ⎥ ⎣2⎦ 4 4 −π =⎣ 0 1 0 ⎣ 4 ⎦ sin −π 0 cos −π 4 4 0 − sin −π cos −π 3 4 4 ⎡ ⎤⎡ ⎤ ⎡ ⎤ 0.707 0.5 0.5 1 3.207 =⎣ 0 0.707 −0.707⎦ ⎣2⎦ = ⎣−0.707⎦ (2.185) −0.707 0.5 0.5 3 1.793To check this result, let us change the role of B and G. So, the body point at ⎡ ⎤ 1 B r = ⎣2⎦ (2.186) 3undergoes an active rotation of 45 deg about the x-axis followed by 45 deg aboutthe y-axis. The global coordinates of the point would be B r = Ry,45 Rx,45 G r (2.187)so G r = [Ry,45 Rx,45 ]TB r = Rx,45 Ry,45 B r T T (2.188)Example 52 (Multiple rotations about body axes) Consider a globally ﬁxed point Pat ⎡ ⎤ 1 G r = ⎣2⎦ (2.189) 3The body B will turn 45 deg about the x-axis and then 45 deg about the y-axis.An observer in B will see P at B r = RY,−45 RX,−45 G r ⎡ ⎤ ⎡1 ⎤⎡ ⎤ cos −π 0 sin −π 0 0 1 4 4 ⎢ −π −π ⎥ =⎣ 0 1 0 ⎦ ⎣0 cos 4 − sin 4 ⎦ ⎣2⎦ − sin −π 0 cos −π 4 4 0 sin −π cos −π 3 4 4 ⎡ ⎤⎡ ⎤ ⎡ ⎤ 0.707 0.5 −0.5 1 0.20711 =⎣ 0 0.707 0.707⎦ ⎣2⎦ = ⎣ 3.5356 ⎦ (2.190) 0.707 −0.5 0.5 3 1.2071Example 53 (Successive rotations about global axes) After a series of sequentialrotations R1 , R2 , R3 , . . . , Rn about the global axes, the ﬁnal global position of abody point P can be found by G r = G RB B r (2.191)
2.3 Rigid Body Dynamics 79where G RB = Rn · · · R3 R2 R1 (2.192)The vectors G r and B r indicate the position vectors of the point P in the global andlocal coordinate frames, respectively. The matrix G RB , which transforms the localcoordinates to their corresponding global coordinates, is called the global rotationmatrix. Because matrix multiplications do not commute, the sequence of performing ro-tations is important and indicates the order of rotations.Proof Consider a body frame B that undergoes two sequential rotations R1 andR2 about the global axes. Assume that the body coordinate frame B is initiallycoincident with the global coordinate frame G. The rigid body rotates about a globalaxis, and the global rotation matrix R1 gives us the new global coordinate G r1 ofthe body point: G r 1 = R1 B r (2.193)Before the second rotation, the situation is similar to the one before the ﬁrst rota-tion. We put the B-frame aside and assume that a new body coordinate frame B1is coincident with the global frame. Therefore, the new body coordinate would beB1 r ≡ G r . The second global rotation matrix R provides us with the new global 1 2position G r2 of the body points B1 r: B1 r = R2 B1 r (2.194)Substituting (2.193) into (2.194) shows that G r = R 2 R1 B r (2.195)Following the same procedure we can determine the ﬁnal global position of a bodypoint after a series of sequential rotations R1 , R2 , R3 , . . . , Rn as (2.192).Example 54 (Successive rotations about local axes) Consider a rigid body B witha local coordinate frame B(Oxyz) that does a series of sequential rotations R1 , R2 ,R3 , . . . , Rn about the local axes. Having the ﬁnal global position vector G r of a bodypoint P , we can determine its local position vector B r by B r = B RG G r (2.196)where B R G = R n · · · R3 R 2 R 1 (2.197)The matrix B RG is called the local rotation matrix and it maps the global coordi-nates of body points to their local coordinates.
80 2 Vibration DynamicsProof Assume that the body coordinate frame B was initially coincident with theglobal coordinate frame G. The rigid body rotates about a local axis, and a localrotation matrix R1 relates the global coordinates of a body point to the associatedlocal coordinates: B r = R1 G r (2.198)If we introduce an intermediate space-ﬁxed frame G1 coincident with the new posi-tion of the body coordinate frame, then G1 r ≡ Br (2.199)and we may give the rigid body a second rotation about a local coordinate axis. Nowanother proper local rotation matrix R2 relates the coordinates in the intermediateﬁxed frame to the corresponding local coordinates: B r = R2 G1 r (2.200)Hence, to relate the ﬁnal coordinates of the point, we must ﬁrst transform its globalcoordinates to the intermediate ﬁxed frame and then transform to the original bodyframe. Substituting (2.198) in (2.200) shows that B r = R 2 R1 G r (2.201)Following the same procedure we can determine the ﬁnal global position of a bodypoint after a series of sequential rotations R1 , R2 , R3 , . . . , Rn as (2.197). Rotation about the local coordinate axes is conceptually interesting. This is be-cause in a sequence of rotations each rotation is about one of the axes of the localcoordinate frame, which has been moved to its new global position during the lastrotation.2.3.2 Velocity KinematicsConsider a rotating rigid body B(Oxyz) with a ﬁxed point O in a reference frameG(OXY Z), as shown in Fig. 2.15. We express the motion of the body by a time-varying rotation transformation matrix between B and G to transform the instanta-neous coordinates of body points to their coordinates in the global frame: G r(t) = G RB (t)B r (2.202)The velocity of a body point in the global frame is G ˙ ˙ v(t) = G r(t) = G RB (t)B r = G ωB G r(t) = G ωB × G r(t) ˜ (2.203)where G ωB is the angular velocity vector of B with respect to G. It is equal to a ˙ ˆrotation with angular speed φ about an instantaneous axis of rotation u:
2.3 Rigid Body Dynamics 81Fig. 2.15 A rotating rigidbody B(Oxyz) with a ﬁxedpoint O in a global frameG(OXY Z) ⎤⎡ ω1 ω = ⎣ω2 ⎦ = φ u ˙ˆ (2.204) ω3 ˜The angular velocity vector is associated with a skew-symmetric matrix G ωB calledthe angular velocity matrix, ⎡ ⎤ 0 −ω3 ω2 ω = ⎣ ω3 ˜ 0 −ω1 ⎦ (2.205) −ω2 ω1 0where ˜ G ωB ˙ ˙˜ = G R B G RB = φ u T (2.206)The B-expression of the angular velocity is similarly deﬁned: T ˙ B ˜ G ωB = G RB G R B (2.207)Employing the global and body expressions of the angular velocity of the body ˜ G˜relative to the global coordinate frame, G ωB and B ωB , we determine the global andbody expressions of the velocity of a body point as G G vP = G ω B × G rP G (2.208) B G vP = B ω B × B rP G (2.209) ˜ G˜ The G-expression G ωB and B-expression B ωB of the angular velocity matrixcan be transformed to each other using the rotation matrix G RB : ˜ G ωB = G RB B ω B G RB G˜ T (2.210) B ˜ G ωB = G RB G ω B G RB T G˜ (2.211)They are also related to each other directly by ˜ G ωB G RB = G RB B ω B G˜ T (2.212)
82 2 Vibration DynamicsFig. 2.16 A body ﬁxed pointP at B r in the rotating bodyframe B G RB G ω B = B ω B G RB T ˜ G˜ T (2.213) The relative angular velocity vectors of relatively moving rigid bodies can bedone only if all the angular velocities are expressed in one coordinate frame: n 0 ωn = 0 ω1 + 1 ω2 + 2 ω3 + · · · + n−1 ωn = 0 0 0 0 i−1 ωi (2.214) i=1 ˜ G˜ The inverses of the angular velocity matrices G ωB and B ωB are ˜ −1 G ωB ˙ −1 = G RB G R B (2.215) B −1 ˜ G ωB ˙ −1 = G R B G RB (2.216)Proof Consider a rigid body with a ﬁxed point O and an attached frame B(Oxyz)as shown in Fig. 2.16. The body frame B is initially coincident with the global frameG. Therefore, the position vector of a body point P at the initial time t = t0 is G r(t0 ) = B r (2.217)and at any other time is found by the associated transformation matrix G RB (t): G r(t) = G RB (t)B r = G RB (t)G r(t0 ) (2.218)The global time derivative of G r is Gd Gd Gd G ˙ v = Gr = G r(t) = G RB (t)B r = G RB (t)G r(t0 ) dt dt dt G ˙ G ˙ = RB (t) r(t0 ) = RB (t)B r G (2.219)Eliminating B r between (2.218) and (2.219) determines the velocity of the globalpoint in the global frame: G ˙ v = G RB (t)G RB (t)G r(t) T (2.220)
2.3 Rigid Body Dynamics 83 ˜We denote the coefﬁcient of G r(t) by G ωB ˜ G ωB ˙ = G R B G RB T (2.221)and rewrite Eq. (2.220) as G v = G ωB G r(t) ˜ (2.222)or equivalently as G v = G ωB × G r(t) (2.223)where G ωB is the instantaneous angular velocity of the body B relative to the globalframe G as seen from the G-frame. Transforming G v to the body frame provides us with the body expression of thevelocity vector: T ˙ B G vP = G RB G v = G RB G ω B G r = G RB G R B G RB G r T T ˜ T T ˙ = G RB G R B B r (2.224) G˜We denote the coefﬁcient of B r by B ωB T ˙ B ˜ G ωB = G RB G R B (2.225)and rewrite Eq. (2.224) as B G vP = B ω B B rP G˜ (2.226)or equivalently as B G vP = B ω B × B rP G (2.227)where Bω is the instantaneous angular velocity of B relative to the global frame G BG as seen from the B-frame. The time derivative of the orthogonality condition, G RB G RB = I, introduces an Timportant identity, G ˙ T ˙T R B G RB + G RB G R B = 0 (2.228) ˜ ˙which can be used to show that the angular velocity matrix G ωB = [G RB G RB ] is Tskew-symmetric: G ˙T RB G R B = G ˙ T R B G RB T (2.229) Generally speaking, an angular velocity vector is the instantaneous rotation ofa coordinate frame A with respect to another frame B that can be expressed in orseen from a third coordinate frame C. We indicate the ﬁrst coordinate frame A bya right subscript, the second frame B by a left subscript, and the third frame C bya left superscript, C ωA . If the left super and subscripts are the same, we only show Bthe subscript.
84 2 Vibration Dynamics We can transform the G-expression of the global velocity of a body point P ,Gv and the B-expression of the global velocity of the point P , B vP , to each other P, Gusing a rotation matrix: B G vP = B RG G vP = B RGG ωB G rP = B RGG ωB G RB B rP ˜ ˜ ˙ ˙ = B RG G R B G RB G RB B r P = B RG G R B B r P T T ˙ = G RB G R B B r P = B ω B B r P = B ω B × B r P G˜ G (2.230) G vP = G RB B v P G = G G˜ RB B ω B B r P = G G˜ T RB B ω B G RB G r P T ˙ ˙ = G RB G RB G R B G RB G r P = G R B G RB G r P T T B = G ω B G r P = G ω B × G r P = G RB ˜ ω G B × B rP (2.231) ˜ G˜From the deﬁnitions of G ωB and B ωB in (2.221) and (2.225) and comparing with(2.230) and (2.231), we are able to transform the two angular velocity matrices by ˜ G ωB = G RB B ω B G RB G˜ T (2.232) B ˜ G ωB = G RB G ω B G RB T ˜ (2.233)and derive the following useful equations: G ˙ R B = G ω B G RB ˜ (2.234) G ˙ RB = G G˜ RB B ω B (2.235) ˜ G ωB G RB = G RB B ω B G˜ (2.236) The angular velocity of B in G is negative of the angular velocity of G in B ifboth are expressed in the same coordinate frame: G ˜ G ωB = −G ω G B ˜ G G ωB = −G ω G B (2.237) B ˜ G ωB = −B ω G B˜ B G ωB = −B ω G B (2.238)The vector G ωB can always be expressed in the natural form G ωB = ωu ˆ (2.239) ˆwith the magnitude ω and a unit vector u parallel to G ωB that indicates the instan-taneous axis of rotation. To show the addition of relative angular velocities in Eq. (2.214), we start froma combination of rotations, 0 R2 = 0 R1 1 R2 (2.240)and take the time derivative: 0 ˙ ˙ ˙ R 2 = 0 R 1 1 R2 + 0 R1 1 R 2 (2.241)
2.3 Rigid Body Dynamics 85Substituting the derivative of the rotation matrices with 0 ˙ R 2 = 0 ω 2 0 R2 ˜ (2.242) 0 ˙ R 1 = 0 ω 1 R1 ˜ 0 (2.243) 1 ˙ R 2 = 1 ω 2 1 R2 ˜ (2.244)results in ˜ 0 ω2 0 R2 = 0 ω1 0 R1 1 R2 + 0 R11 ω2 1 R2 ˜ ˜ = 0 ω1 0 R2 + 0 R11 ω2 0 R1 0 R1 1 R2 ˜ ˜ T = 0 ω 1 0 R2 + 0 ω 2 0 R2 ˜ 1˜ (2.245)where 0 R11 ω2 0 R1 = 0 ω2 ˜ T 1˜ (2.246)Therefore, we ﬁnd ˜ 0 ω2 = 0 ω1 + 0 ω2 ˜ 1˜ (2.247)which indicates that two angular velocities may be added when they are expressedin the same frame: 0 ω2 = 0 ω1 + 0 ω2 1 (2.248)The expansion of this equation for any number of angular velocities would beEq. (2.214). Employing the relative angular velocity formula (2.248), we can ﬁnd the relativevelocity formula of a point P in B2 at 0 rP : 0 v2 = 0 ω 2 0 rP = 0 ω1 + 0 ω 2 0 rP = 0 ω 1 0 rP + 0 ω 2 0 rP 1 1 = 0 v1 + 0 v2 1 (2.249) ˜ G˜ The angular velocity matrices G ωB and B ωB are skew-symmetric and not invert-ible. However, we can deﬁne their inverse by the rules ˜ −1 G ωB ˙ −1 = G RB G R B (2.250) B −1 ˜ G ωB ˙ −1 = G R B G RB (2.251)to get ˜ −1 ˜ G ωB G ωB ˜ ˜ −1 = G ωB G ωB = [I] (2.252) B −1 B ˜ ˜ G ωB G ωB = B ˜ B ˜ −1 G ωB G ωB = [I] (2.253)
86 2 Vibration DynamicsExample 55 (Rotation of a body point about a global axis) Consider a rigid bodyis turning about the Z-axis with a constant angular speed α = 10 deg/s. The global ˙velocity of a body point at P (5, 30, 10) when the body is at α = 30 deg is G ˙ vP = G RB (t)B rP ⎛⎡ ⎤⎞ ⎡ ⎤ Gd cos α − sin α 0 5 = ⎝⎣ sin α cos α 0⎦⎠ ⎣30⎦ dt 0 0 1 10 ⎡ ⎤⎡ ⎤ − sin α − cos α 0 5 = α ⎣ cos α − sin α 0⎦ ⎣30⎦ ˙ 0 0 0 10 ⎡ ⎤⎡ ⎤ ⎡ ⎤ − sin π − cos π 0 5 −4.97 10π ⎣ 6 6 = cos π 6 − sin π 0⎦ ⎣30⎦ = ⎣−1.86⎦ 6 (2.254) 180 0 0 0 10 0The point P is now at G r P = G RB B r P ⎡ ⎤⎡ ⎤ ⎡ ⎤ cos π − sin π 6 6 0 5 −10.67 = ⎣ sin π cos π 0⎦ ⎣30⎦ = ⎣ 28.48 ⎦ (2.255) 6 6 0 0 1 10 10Example 56 (Rotation of a global point about a global axis) A body point P atBr P = [5 30 10]T is turned α = 30 deg about the Z-axis. The global position of Pis at G r P = G RB B r P ⎡ ⎤⎡ ⎤ ⎡ ⎤ cos π − sin π 6 6 0 5 −10.67 = ⎣ sin π cos π 0⎦ ⎣30⎦ = ⎣ 28.48 ⎦ (2.256) 6 6 0 0 1 10 10If the body is turning with a constant angular speed α = 10 deg/s, the global velocity ˙of the point P would be G ˙ v P = G R B G RB G r P T ⎡ π ⎤⎡ π ⎤T ⎡ ⎤ −s −c π 0 c6 −s π 0 −10.67 10π ⎣ π6 6 6 = c6 −s π 6 0⎦ ⎣ s π 6 cπ6 0⎦ ⎣ 28.48 ⎦ 180 0 0 0 0 0 1 10 ⎡ ⎤ −4.97 = ⎣−1.86⎦ (2.257) 0
2.3 Rigid Body Dynamics 87Example 57 (Simple derivative transformation formula) Consider a point P thatcan move in the body coordinate frame B(Oxyz). The body position vector B rP isnot constant, and, therefore, the B-expression of the G-velocity of such a point is Gd Bd G˙ rP = B rP = rP + B ω B × B rP B B G (2.258) dt dt The result of Eq. (2.258) is used to deﬁne the transformation of the differentialoperator on a B-vector B from the body to the global coordinate frame: Gd Bd B =B ˙ = G B + B ωB × B G (2.259) dt dtHowever, special attention must be paid to the coordinate frame in which the vectorB and the ﬁnal result are expressed. The ﬁnal result is B ˙ , showing the global G(G) time derivative expressed in the body frame (B) or simply the B-expressionof the G-derivative of B . The vector B may be any vector quantity such as po-sition, velocity, angular velocity, momentum, angular momentum, a time-varyingforce vector. Equation (2.259) is called a simple derivative transformation formula and relatesthe derivative of a B-vector as it would be seen from the G-frame to its derivativeas seen from the B-frame. The derivative transformation formula (2.259) is moregeneral and can be applied to every vector for a derivative transformation betweenevery two relatively moving coordinate frames.2.3.3 Acceleration KinematicsConsider a rotating rigid body B(Oxyz) with a ﬁxed point O in a reference frameG(OXY Z) such as shown in Fig. 2.17. When the body rotates in G, the globalacceleration of a body point P is given by G ˙ ¨ a = G v = G r = G SB G r (2.260) = G α B × G r + G ωB × G ωB × Gr (2.261) = ˜ α G B + G ωB ˜2 G r (2.262) ¨ = G R B G RB G r T (2.263)where G α B is the angular acceleration vector of B relative to G, Gd GαB = G ωB (2.264) dt ˜and G αB is the angular acceleration matrix ˜ G αB ˙ ¨ ˙ ˙T = G ω B = G R B G RB + G R B G R B ˜ T (2.265)
88 2 Vibration DynamicsFig. 2.17 A rotating rigidbody B(Oxyz) with a ﬁxedpoint O in a reference frameG(OXY Z)and G SB is the rotational acceleration transformation: G SB ¨ = G RB G RB = G αB + G ωB = G αB − G ωB G ωB T ˜ ˜2 ˜ ˜ ˜T (2.266) ˜The angular velocity vector G ωB and matrix G ωB are ˜ G ωB ˙ = G R B G RB T (2.267) G ωB ˙ˆ ˙ˆ = φ u = φ uω (2.268) The relative angular acceleration of two bodies B1 , B2 in the global frame G canbe combined as Gd Gα2 = G ω2 = G α 1 + G α2 1 (2.269) dt G S2 = G S 1 + G S 2 + 2G ω 1 G ω 2 1 ˜ 1 ˜ (2.270)The B-expressions of G a and G SB are B Ga = B α B × B r + B ωB × G G B G ωB × Br (2.271) B G SB = B ¨ RG RB = B αB G G˜ + B ωB G˜ 2 (2.272)The global and body expressions of the rotational acceleration transformations G SBand B SB can be transformed to each other by the following rules: G G SB = G RB B S B G RB G T (2.273) B G SB = G RB G S B G RB T (2.274)Proof The global position and velocity vectors of the body point P are G r = G RB B r (2.275) G ˙G ˙ v = r = RB r = G ω B r = G ω B × r G ˜ B G G (2.276) ˜where G ωB is also the rotational velocity transformation because it transforms theglobal position vector of a point, G r, to its velocity vector G v.
2.3 Rigid Body Dynamics 89 Gd Differentiating Eq. (2.276) and using the notation G αB = dt G ωB yieldEq. (2.261): G ¨ ˙ a = G r = G ωB × G r + G ω B × G r ˙ = G α B × G r + G ωB × G ωB × Gr (2.277) We may substitute the matrix expressions of angular velocity and acceleration in(2.277) to derive Eq. (2.262): G ¨ r = G αB × G r + G ωB × G ωB × Gr = G αB G r + G ωB G ωB G r ˜ ˜ ˜ = ˜ α G B + G ωB ˜2 G r (2.278)Recalling that ˜ G ωB ˙ = G R B G RB T (2.279) G ˙ r(t) = G ωB G r(t) ˜ (2.280)we ﬁnd Eqs. (2.263) and (2.265): Gd G ¨ r= G ˙ T R B G RB G r dt ¨ ˙ ˙T = G R B G RB G r + G R B G R B G r + T G ˙ T R B G RB G ˙ T R B G RB G r = G ¨ ˙ ˙T R B G RB + G R B G R B + T G ˙ T R B G RB 2 G r = G ¨ R B G RB − T G ˙ T R B G RB 2 + G ˙ T R B G RB 2 G r ¨ = G R B G RB G r T (2.281) ˜ G αB ˙ ¨ ˙ ˙T = G ω B = G R B G RB + G R B G R B ˜ T ¨ ˙ ˙T = G R B G RB + G R B G RB G RB G R B T T ¨ = G R B G RB + T G ˙ T R B G RB G ˙ T R B G RB T ¨ T ˜T ¨ = G R B G RB + G ω B G ω B = G R B G RB − G ω B ˜ T ˜2 (2.282)which indicates that G ¨ RB G RB = G αB + G ωB = G SB T ˜ ˜2 (2.283)
90 2 Vibration Dynamics ˜ The expanded forms of the angular accelerations G α B , G αB and rotational ac-celeration transformation G SB are ⎡ ⎤ 0 −ω 3 ω 2 ˙ ˙ ˜ ˙ ˜ ¨˜ ˙˙ G αB = G ωB = φ u + φ u = ⎣ ω3 ˜ ˙ 0 −ω 1 ⎦ ˙ −ω 2 ω 1 ˙ ˙ 0 ⎡ ⎤ 0 ˙ ˙ ¨ ˙ ˙ −u3 φ − u3 φ u2 φ + u2 φ ¨ = ⎣ u3 φ + u3 φ ˙ ˙ ¨ 0 −u1 φ − u1 φ ⎦ ˙ ˙ ¨ (2.284) −u2 φ ˙ ˙ − u2 φ u1 φ + u1 φ ¨ ˙ ˙ ¨ 0 ⎡ ⎤ ⎡ ⎤ ˙ ω1 ˙ ˙ ¨ u1 φ + u1 φ G α B = ⎣ω2 ⎦ = ⎣u2 φ + u2 φ ⎦ ˙ ˙ ˙ ¨ (2.285) ˙ ω3 ˙ ˙ + u3 φ u3 φ ¨ G SB ˙ = G ωB + G ωB = G αB + G ωB ˜ ˜2 ˜ ˜2 ⎡ ⎤ −ω2 − ω3 ω1 ω2 − ω3 ω2 + ω1 ω3 2 2 ˙ ˙ = ⎣ω3 + ω1 ω2 −ω1 − ω3 ω2 ω3 − ω1 ⎦ ˙ 2 2 ˙ (2.286) ω1 ω3 − ω2 ω1 + ω2 ω3 −ω1 − ω2 ˙ ˙ 2 2 G SB ¨˜ ˙˙ ˙ ˜ = φ u + φ u + φ 2 u2 ˜ ⎡ ⎤ −(1 − u2 )φ 2˙ ˙ ˙ ˙ ¨ u1 u2 φ 2 − u3 φ − u3 φ ˙ ˙ ˙ ¨ u1 u3 φ 2 + u2 φ + u2 φ 1 = ⎣u1 u2 φ 2 + u3 φ + u3 φ ˙ ˙ ˙ ¨ −(1 − u2 ˙ 2 )φ 2 u2 u3 φ 2 − u1 φ − u1 φ ⎦ ˙ ˙ ˙ ¨ ˙ 2 − u2 φ − u2 φ u1 u3 φ ˙ ˙ ¨ ˙ ˙ ˙ ¨ u2 u3 φ 2 + u1 φ + u1 φ −(1 − u3 ˙ 2 )φ 2 (2.287) The angular velocity of several bodies rotating relative to each other can be re-lated according to (2.214): 0 ωn = 0 ω1 + 0 ω2 + 0 ω3 + · · · + n−1 ωn 1 2 0 (2.288)The angular accelerations of several relatively rotating rigid bodies follow the samerule: 0αn = 0 α1 + 0 α 2 + 0 α 3 + · · · + n−1 α n 1 2 0 (2.289)To show this fact and develop the relative acceleration formula, we consider a pairof relatively rotating rigid links in a base coordinate frame B0 with a ﬁxed point atO. The angular velocities of the links are related as 0 ω2 = 0 ω1 + 0 ω2 1 (2.290)So, their angular accelerations are 0d 0α1 = 0 ω1 (2.291) dt
2.3 Rigid Body Dynamics 91 0d 0α2 = 0 ω2 = 0 α 1 + 0 α2 1 (2.292) dtand, therefore, 2 0 S2 = 0 α2 + 0 ω2 = 0 α1 + 0 α2 + ˜ ˜2 ˜ 1˜ ˜ 0 ω1 + 0 ω2 1˜ = 0 α1 + 0 α2 + 0 ω1 + 0 ω2 + 20 ω1 0 ω2 ˜ 1˜ ˜2 1 ˜2 ˜ 1˜ = 0 S1 + 0 S2 + 20 ω 1 0 ω 2 1 ˜ 1˜ (2.293)Equation (2.293) is the required relative acceleration transformation formula. Itindicates the method of calculation of relative accelerations for a multibody. As amore general case, consider a six-link multibody. The angular acceleration of link(6) in the base frame would be 0 S6 = 0 S1 + 0 S2 + 0 S3 + 0 S4 + 0 S5 + 0 S6 1 2 3 4 5 + 20 ω 1 0 ω 2 + 0 ω 3 + 0 ω 4 + 0 ω 5 + 0 ω 6 ˜ 1˜ 2˜ 3˜ 4˜ 5˜ + 20 ω 2 0 ω 3 + 0 ω 4 + 0 ω 5 + 0 ω 6 1˜ 2˜ 3˜ 4˜ 5˜ . . . + 20 ω 5 0 ω 6 4˜ 5˜ (2.294) We can transform the G and B-expressions of the global acceleration of a bodypoint P to each other using a rotation matrix: B G aP = B RG G aP = B RGG SB G rP = B RG G SB G RB B rP ¨ ¨ = B RG G R B G RB G RB B r P = B RG G R B B r P T T ¨ B = G RB G R B B r P = B S B B r P = G ˜ α G B + B ωB G˜ 2 B rP = B α B × B r + B ωB × G G B G ωB × Br (2.295) G a P = G RB B a P = G RB B S B B r P = G RB B S B G RB G r P G G G T T ¨ ¨ = G RB G RB G R B G RB G r P = G R B G RB G r P T T = G S B G rP = ˜ α G B + G ωB ˜2 G r = G α B × G r + G ωB × G ωB × Gr (2.296)From the deﬁnitions of G SB and B SB in (2.266) and (2.272) and comparing with G(2.295) and (2.296), we are able to transform the two rotational acceleration trans-formations by G SB = G RB B S B G RB G T (2.297)
92 2 Vibration Dynamics B G SB = G RB G S B G RB T (2.298)and derive the useful equations G ¨ R B = G S B G RB (2.299) G ¨ R B = G RB B S B G (2.300) G SB G RB = G RB B S B G (2.301) The angular acceleration of B in G is negative of the angular acceleration of Gin B if both are expressed in the same coordinate frame: ˜ G αB = −G αG B˜ GαB = −G α G B (2.302) B ˜ G αB = −B αG ˜ B GαB = −B α G (2.303) The term G α B × G r in (2.277) is called the tangential acceleration, which is afunction of the angular acceleration of B in G. The term G ωB × (G ωB × G r) inG a is called centripetal acceleration and is a function of the angular velocity of Bin G.Example 58 (Rotation of a body point about a global axis) Consider a rigidbody is turning about the Z-axis with a constant angular acceleration α = 2 rad/s2 . ¨The global acceleration of a body point at P (5, 30, 10) cm when the body is atα = 10 rad/s and α = 30 deg is˙ G ¨ aP = G RB (t)B rP ⎡ ⎤⎡ ⎤ ⎡ ⎤ −87.6 48.27 0 5 1010 = ⎣−48.27 −87.6 0⎦ ⎣30⎦ = ⎣−2869.4⎦ cm/s (2.304) 0 0 0 10 0where G d2 Gd Gd G d2 G ¨ RB = G RB = α ˙ RB = α G RB + α 2 2 G RB G ¨ ˙ dt 2 dα dα dα ⎡ ⎤ ⎡ ⎤ − sin α − cos α 0 − cos α sin α 0 = α ⎣ cos α ¨ − sin α 0⎦ + α 2 ⎣ − sin α − cos α ˙ 0⎦ (2.305) 0 0 0 0 0 0At this moment, the point P is at G r P = G RB B r P ⎡ ⎤⎡ ⎤ ⎡ ⎤ cos π − sin π 6 6 0 5 −10.67 = ⎣ sin π 6 cos π 6 0⎦ ⎣30⎦ = ⎣ 28.48 ⎦ cm (2.306) 0 0 1 10 10
2.3 Rigid Body Dynamics 93Example 59 (Rotation of a global point about a global axis) A body point P atB r = [5 30 10]T cm is turning with a constant angular acceleration α = 2 rad/s2 ¨ Pabout the Z-axis. When the body frame is at α = 30 deg, its angular speedα = 10 deg/s.˙ The transformation matrix G RB between the B- and G-frames is ⎡ ⎤ ⎡ ⎤ cos π − sin π 0 6 6 0.866 −0.5 0 G RB = ⎣ sin π 6 cos π6 0⎦ ≈ ⎣ 0.5 0.866 0⎦ (2.307) 0 0 1 0 0 1and, therefore, the acceleration of point P is ⎡ ⎤ 1010 G ¨ aP = G RB G RB G rP = ⎣−2869.4⎦ cm/s2 T (2.308) 0where G d2 Gd G d2 G RB = α ¨ G RB − α 2 ˙ G RB (2.309) dt 2 dα dα 2is the same as (2.305).Example 60 (B-expression of angular acceleration) The angular acceleration ex-pressed in the body frame is the body derivative of the angular velocity vector. Toshow this, we use the derivative transport formula (2.259): Gd G˙ = B ωB = B B GαB G ωB dt Bd Bd = B G ωB + B ωB × B ωB = G G B G ωB (2.310) dt dtInterestingly, the global and body derivatives of B ωB are equal: G Gd Bd B G ωB = B G ωB = B αB G (2.311) dt dt ˆThis is because G ωB is about an axis u that is instantaneously ﬁxed in both B and G. A vector α can generally indicate the angular acceleration of a coordinate frameA with respect to another frame B. It can be expressed in or seen from a thirdcoordinate frame C. We indicate the ﬁrst coordinate frame A by a right subscript,the second frame B by a left subscript, and the third frame C by a left superscript,C α . If the left super and subscripts are the same, we only show the subscript. So,B Athe angular acceleration of A with respect to B as seen from C is the C-expressionof B αA : C B αA = C RB B αA (2.312)
94 2 Vibration DynamicsFig. 2.18 A simplependulumExample 61 (B-expression of acceleration) Transforming G a to the body frameprovides us with the body expression of the acceleration vector: T ¨ B G aP = G RB G a = T G RB G S B G r = G RB G R B G RB G r T T T ¨ = G RB G R B B r (2.313)We denote the coefﬁcient of B r by B SB G T ¨ B G SB = G RB G R B (2.314)and rewrite Eq. (2.313) as B G aP = B S B B rP G (2.315)where B SB is the rotational acceleration transformation of the B-frame relative to GG-frame as seen from the B-frame.Example 62 (Velocity and acceleration of a simple pendulum) A point mass at-tached to a massless rod hanging from a revolute joint is what we call a simplependulum. Figure 2.18 illustrates a simple pendulum. A local coordinate frame B isattached to the pendulum, which rotates in a global frame G about the Z-axis. Thekinematic information of the mass is given by B r = lˆ ı (2.316) ⎡ ⎤ l sin φ G r = RBG B r= ⎣−l cos φ ⎦ (2.317) 0 B G ωB ˙ˆ = φk (2.318) G ωB = G T RB B ω B G ˙ ˆ =φK (2.319)
2.3 Rigid Body Dynamics 95Fig. 2.19 A sphericalpendulum ⎡ ⎤ cos( 3 π + φ) − sin( 3 π + φ) 0 2 2 ⎢ ⎥ G RB = ⎣ sin( 3 π + φ) cos( 3 π + φ) 0⎦ 2 2 0 0 1 ⎡ ⎤ sin φ cos φ 0 = ⎣− cos φ sin φ 0⎦ (2.320) 0 0 1Therefore, B Gv ˙ G G ˙ˆ ı ˙ˆ = B r + B ωB × B r = 0 + φ k × lˆ = l φ j (2.321) ⎡ ⎤ ˙ l φ cos φ G v = RB v = G B ⎣ l φ sin φ ⎦ ˙ (2.322) 0and B Ga G˙ ¨ ˆ ˙ˆ ˙ˆ ¨ˆ ˙ ˆ = B v + B ωB × B v = l φ j + φ k × l φ j = l φ j − l φ 2 ı G G (2.323) ⎡ ⎤ ¨ ˙ l φ cos φ − l φ 2 sin φ G a = G RB B a = ⎣l φ sin φ + l φ 2 cos φ ⎦ ¨ ˙ (2.324) 0Example 63 (Spherical pendulum) A pendulum free to oscillate in any plane iscalled a spherical pendulum. This name comes from the codominants that we useto locate the tip mass. Consider a pendulum with a point mass m at the tip point ofa long, massless, and straight string with length l. The pendulum is hanging from apoint A(0, 0, 0) in a local coordinate frame B1 (x1 , y1 , z1 ). To indicate the mass m, we attach a coordinate frame B2 (x2 , y2 , z2 ) to the pen-dulum at point A as is shown in Fig. 2.19. The pendulum makes an angle β with the
96 2 Vibration Dynamicsvertical z1 -axis. The pendulum swings in the plane (x2 , z2 ) and makes an angle γwith the plane (x1 , z1 ). Therefore, the transformation matrix between B2 and B1 is 2 R1 = Ry2 ,−β Rz2 ,γ ⎡ ⎤ cos γ cos β cos β sin γ sin β = ⎣ − sin γ cos γ 0 ⎦ (2.325) − cos γ sin β − sin γ sin β cos βThe position vectors of m are ⎡ ⎤ ⎡ ⎤ 0 l cos γ sin β 2 r=⎣ 0 ⎦ 1 r = 1 R2 2 r = ⎣ l sin β sin γ ⎦ (2.326) −l −l cos βThe equation of motion of m is 1 M = I 1α2 (2.327) r × m g = ml 1 α 2 1 1 2 (2.328) ⎡ ⎤ ⎡ ⎤ l cos γ sin β 0 ⎣ l sin β sin γ ⎦ × m ⎣ 0 ⎦ = ml 2 1 α 2 (2.329) −l cos β −g0Therefore, ⎡ ⎤ − sin β sin γ g0 ⎣ 1α2 = cos γ sin β ⎦ (2.330) l 0To ﬁnd the angular acceleration of B2 in B1 , we use 2 R1 : ˙ d d 2 1 ˙ R 2 = β 2 R1 + γ ˙ R1 dβ dγ ⎡ ⎤ ˙ −βcγ sβ − γ cβsγ −γ cγ ˙ ˙ ˙ γ sβsγ − βcβcγ ˙ = ⎣ γ cβcγ − βsβsγ ˙ ˙ −γ sγ ˙ −βcβsγ − γ cγ sβ ⎦ ˙ ˙ (2.331) ˙ βcβ 0 ˙ −βsβ ⎡ ⎤ 0 −γ ˙ ˙ −β cos γ 1 ˙ 1 T 1 ω 2 = R 2 R2 = ⎣ γ ˜ ˙ 0 −β sin γ ⎦ ˙ (2.332) ˙ β cos γ ˙ β sin γ 0 ¨ d d2 d2 2 1 ¨ ˙ ˙˙ R 2 = β 2 R1 + β 2 2 2 R1 + β γ R1 dβ dβ dγ dβ d 2 d2 2 d2 +γ ¨ ˙˙ R1 + γ β R1 + γ 2 2 2 R1 ˙ (2.333) dγ dβ dγ dγ
2.3 Rigid Body Dynamics 97 ˜ 1 α2 ¨ = 1 R 2 1 R2 − 1 ω 2 T ˜2 ⎡ ⎤ 0 −γ ¨ ¨ ˙˙ −βcγ + β γ sγ =⎣ γ¨ 0 −βsγ − β γ cγ ⎦ ¨ ˙˙ (2.334) ¨ ˙˙ βcγ − β γ sγ ¨ ˙˙ βsγ + β γ cγ 0Therefore, the equation of motion of the pendulum would be ⎡ ⎤ ⎡ ⎤ − sin β sin γ ¨ ˙˙ β sin γ + β γ cos γ g0 ⎣ cos γ sin β ⎦ = ⎣−β cos γ + β γ sin γ ⎦ ¨ ˙˙ (2.335) l 0 ¨ γThe third equation indicates that γ = γ0 ˙ ˙ γ = γ0 t + γ 0 ˙ (2.336)The second and third equations can be combined to form g02 ¨ β =− ˙ ˙2 sin2 β + β 2 γ0 (2.337) l 2 ˙which reduces to the equation of a simple pendulum if γ0 = 0.Example 64 (Equation of motion of a spherical pendulum) Consider a particleP of mass m that is suspended by a string of length l from a point A, as shown inFig. 2.19. If we show the tension of the string by T, then the equation of motion ofP is 1 ¨ T + m1 g = m1 r (2.338)or ¨ −T 1 r + m1 g = m1 r. To eliminate 1 T, we multiply the equation by 1 r, r × 1g = 1r × 1r 1 ¨ ⎡ ⎤ ⎡ ⎤ ⎡ ⎤ ⎡ ⎤ l cos γ sin β 0 l cos γ sin β ¨ x (2.339) ⎣ l sin β sin γ ⎦ × ⎣ 0 ⎦ = ⎣ l sin β sin γ ⎦ × ⎣y ⎦ ¨ −l cos β −g0 −l cos β ¨ zand ﬁnd ⎡ ⎤ ⎡ ⎤ −lg0 sin β sin γ l y cos β + l z sin β sin γ ¨ ¨ ⎣ lg0 cos γ sin β ⎦ = ⎣−l x cos β − l z cos γ sin β ⎦ ¨ ¨ (2.340) 0 l y cos γ − l x sin γ ¨ ¨These are the equations of motion of m. However, we may express the equationsonly in terms of γ and β. To do so, we may either take time derivatives of 1 r or use 1¨1 α 2 from Example 64 and ﬁnd r: 1 ¨ r = 1α2 × 1r (2.341)In either case, Eq. (2.335) would be the equation of motion in terms of γ and β.
98 2 Vibration DynamicsFig. 2.20 Foucault pendulumis a simple pendulum hangingfrom a point A above a pointP on Earth surfaceExample 65 (Foucault pendulum) Consider a pendulum with a point mass mat the tip of a long, massless, and straight string with length l. The pendulum ishanging from a point A(0, 0, l) in a local coordinate frame B1 (x1 , y1 , z1 ) at a pointP on the Earth surface. Point P at longitude ϕ and latitude λ is indicated by E din the Earth frame E(Oxyz). The E-frame is turning in a global frame G(OXY Z)about the Z-axis. To indicate the mass m, we attach a coordinate frame B1 (x1 , y1 , z1 ) to the pen-dulum at point A as shown in Fig. 2.20. The pendulum makes an angle β with thevertical z1 -axis. The pendulum swings in the plane (x2 , z2 ) and makes an angle γwith the plane (x1 , z1 ). Therefore, the transformation matrix between B2 and B1 is 1 T 2 = 1 D 2 1 R2 ⎡ ⎤ cos γ cos β − sin γ − cos γ sin β 0 ⎢ cos β sin γ cos γ − sin γ sin β 0⎥ =⎣⎢ ⎥ (2.342) sin β 0 cos β l⎦ 0 0 0 1The position vector of m is ⎡ ⎤ 0 2 r=⎣0⎦ (2.343) −l ⎡ ⎤ ⎡ ⎤ x1 l cos γ sin β 1 r = 1 T2 2 r = ⎣y1 ⎦ = ⎣ l sin β sin γ ⎦ (2.344) z1 l − l cos βEmploying the acceleration equation, Ga = a + Gα1 × 1 r + 21 ω 1 × 1 v + 1 ω 1 × × 1r 1 1 1 1 G G G ω1 (2.345)
2.3 Rigid Body Dynamics 99we can write the equation of motion of m as G F − mG g = mG a 1 1 1 (2.346)where 1 F is the applied nongravitational force on m. Recalling that 1 Gα1 =0 (2.347)we ﬁnd the general equation of motion of a particle in frame B1 as G F + mG g = m a + 2G ω1 × 1 v + 1 ω1 × × 1r 1 1 1 1 1 G G ω1 (2.348)The individual vectors in this equation are ⎡ ⎤ ⎡ ⎤ ⎡ ⎤ 0 Fx ωE cos λ 1 g=⎣ 0 ⎦ 1 ⎣ ⎦ G F = Fy G ω1 = 1 ⎣ 0 ⎦ (2.349) −g0 Fz ωE sin λ ⎡ ⎤ ⎡ ⎤ ˙ x1 ˙ l β cos β cos γ − l γ sin β sin γ ˙ 1 v = ⎣y1 ⎦ = ⎣l β cos β sin γ + l γ cos γ sin β ⎦ ˙ ˙ ˙ (2.350) ˙ z1 ˙ l β sin β ⎡ ⎤ ¨ ˙ ˙˙ l(β cos γ − β 2 sin γ − β γ sin γ ) cos β ⎡ ⎤ ¨ x ⎢ − l(γ sin γ + γ 2 cos γ + β γ cos γ ) sin β ⎥ ¨ ˙ ˙˙ ⎢ ⎥ 1 a= ⎣y ⎦ = ⎢l(β sin γ + β 2 cos γ + β γ cos γ ) cos β ¨ ¨ ˙ ˙˙ ⎥ (2.351) ⎢ ⎥ ¨ z ⎣ + l(γ cos γ − γ 2 sin γ − β γ sin γ ) sin β ⎦ ¨ ˙ ˙˙ ¨ l β sin βIn a spherical pendulum, the external force 1 F is the tension of the string: F1 GF = − 1 r (2.352) lSubstituting the above vectors in (2.348) provides us with three coupled ordinarydifferential equations for two angular variables γ and β. One of the equations is notindependent and the others may theoretically be integrated to determine γ = γ (t)and β = β(t). For example, let us use ωE ≈ 7.292 1 × 10−5 rad/s g0 ≈ 9.81 m/s2 l = 100 m λ = 28◦ 58 30 N ≈ 28.975 deg N ϕ = 50◦ 50 17 E ≈ 50.838 deg E x0 = l cos 10 = 17.365 m (2.353)
100 2 Vibration DynamicsFig. 2.21 The projection ofthe path of a pendulum withlength l = 100 m at latitudeλ ≈ 28.975 deg N on Earthfor a few oscillations (not toscale)and ﬁnd x = 8.6839 cos(0.313 16t) + 8.6811 cos(−0.313 26t) (2.354) y = 8.6839 sin(0.313 16t) + 8.6811 sin(−0.313 26t) (2.355)At the given latitude, which corresponds to Bushehr, Iran, on the Persian Gulfshore, the plane of oscillation turns about the local g-axis with an angular speedω = −3.532 5 × 10−5 rad/s ≈ −87.437 deg/d. These results are independent oflongitude. Therefore, the same phenomena will be seen at Orlando, Florida, or NewDelhi, India, which are almost at the same latitude. Figure 2.21 depicts the pro-jection of m on the (x, y)-plane for a few oscillations. It takes T ≈ 49.4 h for thependulum to turn 2π : 2π T= = 1.7787 × 105 s = 49.408 h (2.356) 3.5325 × 10−5However, the pendulum gets back to the (y, x)-plane after t = T /2 = 24.704 h. Bythat time, the pendulum must have oscillated about n ≈ 4433 times: ωn T 0.313 21 1.7787 × 105 n= = = 4433.3 (2.357) 2π 2 2π 2By shortening the length of the pendulum, say l = 1 m, the rotation speed remainsthe same while the number of oscillations increases to n ≈ 443220.127.116.11 Translational DynamicsFigure 2.22 depicts a moving body B in a global coordinate frame G. Assume thatthe body frame is attached at the mass center of the body. Point P indicates aninﬁnitesimal sphere of the body, which has a very small mass dm. The point massdm is acted on by an inﬁnitesimal force df and has a global velocity G vP .
2.3 Rigid Body Dynamics 101Fig. 2.22 A body point massmoving with velocity G vPand acted on by force df According to Newton’s law of motion df = G aP dm (2.358)However, the equation of motion for the whole body in a global coordinate frame is G F = mG aB (2.359)which can be expressed in the body coordinate frame as F = mB aB + mB ωB × B vB B G G (2.360) ⎡ ⎤ ⎡ ⎤ Fx max + m(ωy vz − ωz vy ) ⎣Fy ⎦ = ⎣ may − m(ωx vz − ωz vx ) ⎦ (2.361) Fz maz + m(ωx vy − ωy vx )In these equations, G aB is the G-expression of the acceleration vector of the bodymass center C, m is the total mass of the body, and F is the G-expression of theresultant of the external forces acted on the body at C.Proof A body coordinate frame at the mass center is called a central frame. If frameB is a central frame, then the center of mass, C, is deﬁned such that B rdm dm = 0 (2.362) BThe global position vector of dm is related to its local position vector by G rdm = G dB + G RB B rdm (2.363)
102 2 Vibration Dynamicswhere G dB is the global position vector of the central body frame, and, therefore, G rdm dm = G dB dm + G RB B rdm dm B B m = G dB dm = G dB dm = mG dB (2.364) B BThe time derivative of both sides shows that ˙ mG dB = mG vB = G ˙ rdm dm = G vdm dm (2.365) B Band the other derivative is ˙ mG vB = mG aB = G ˙ vdm dm (2.366) B ˙However, we have df = G vP dm and, therefore, mG aB = df (2.367) BThe integral on the right-hand side collects all the forces acting on the body. Theinternal forces cancel one another out, so the net result is the vector sum of all theexternally applied forces, F, and, therefore, G ˙ F = mG aB = m G vB (2.368)In the body coordinate frame we have B F = B RG G F = mB RG G aB = mB aB G = mB aB + mB ωB × B vB G (2.369)The expanded form of Newton’s equation in the body coordinate frame is then equalto F = mB aB + mB ωB × B vB B G ⎡ ⎤ ⎡ ⎤ ⎡ ⎤ ⎡ ⎤ Fx ax ωx vx ⎣Fy ⎦ = m ⎣ay ⎦ + m ⎣ωy ⎦ × ⎣vy ⎦ Fz az ωz vz ⎡ ⎤ max + m(ωy vz − ωz vy ) = ⎣ may − m(ωx vz − ωz vx ) ⎦ (2.370) maz + m(ωx vy − ωy vx )
2.3 Rigid Body Dynamics 1032.3.5 Rotational DynamicsThe rigid body rotational equation of motion is the Euler equation Gd B M= B ˙ L = B L + B ωB × B L G dt G˙ = B I B ωB + B ωB × B B G I G ωB (2.371)where L is the angular momentum B L = B I B ωB G (2.372)and I is the mass moment of the rigid body. ⎡ ⎤ Ixx Ixy Ixz I = ⎣Iyx Iyy Iyz ⎦ (2.373) Izx Izy IzzThe expanded form of the Euler equation (2.371) is Mx = Ixx ωx + Ixy ωy + Ixz ωz − (Iyy − Izz )ωy ωz ˙ ˙ ˙ −Iyz ωz − ωy − ωx (ωz Ixy − ωy Ixz ) 2 2 (2.374) My = Iyx ωx + Iyy ωy + Iyz ωz − (Izz − Ixx )ωz ωx ˙ ˙ ˙ −Ixz ωx − ωz − ωy (ωx Iyz − ωz Ixy ) 2 2 (2.375) Mz = Izx ωx + Izy ωy + Izz ωz − (Ixx − Iyy )ωx ωy ˙ ˙ ˙ −Ixy ωy − ωx − ωz (ωy Ixz − ωx Iyz ) 2 2 (2.376)which can be reduced to M1 = I1 ω1 − (I2 − I2 )ω2 ω3 ˙ M2 = I2 ω2 − (I3 − I1 )ω3 ω1 ˙ (2.377) M3 = I3 ω3 − (I1 − I2 )ω1 ω2 ˙when the body coordinate is the principal coordinate frame. The principal coordi-nate frame is denoted by numbers 123 to indicate the ﬁrst, second, and third princi-pal axes. The parameters Iij , i = j , are zero in the principal frame. The body andprincipal coordinate frame sit at the mass center C. The kinetic energy of a rotating rigid body is 1 K= Ixx ωx + Iyy ωy + Izz ωz 2 2 2 2 − Ixy ωx ωy − Iyz ωy ωz − Izx ωz ωx (2.378)
104 2 Vibration Dynamics 1 1 = ω · L = ωT I ω (2.379) 2 2which in the principal coordinate frame reduces to 1 K= I1 ω 1 + I2 ω 2 + I3 ω 3 2 2 2 (2.380) 2Proof Let mi be the mass of the ith particle of a rigid body B, which is made of nparticles and let T r i = B ri = xi yi zi (2.381)be the Cartesian position vector of mi in a central body ﬁxed coordinate frameB(Oxyz). Assume that T ω = B ω B = ωx G ωy ωz (2.382)is the angular velocity of the rigid body with respect to the global coordinate frameG(OXY Z), expressed in the body coordinate frame. The angular momentum of mi is ˙ Li = ri × mi ri = mi ri × (ω × ri ) = mi (ri · ri )ω − (ri · ω)ri = mi ri2 ω − mi (ri · ω)ri (2.383)Hence, the angular momentum of the rigid body would be n n L=ω mi ri2 − mi (ri · ω)ri (2.384) i=1 i=1Substitution for ri and ω gives us n ˆ ˆ L = (ωx ı + ωy j + ωz k) ˆ mi xi2 + yi2 + zi 2 i=1 n − ˆ ˆ ˆ mi (xi ωx + yi ωy + zi ωz ) · (xi ı + yi j + zi k) (2.385) i=1which can be rearranged as n n n L= mi yi2 + zi ωx ı + 2 ˆ mi zi + xi2 ωy j + 2 ˆ ˆ mi xi2 + yi2 ωz k i=1 i=1 i=1 n n − (mi xi yi )ωy + ˆ (mi xi zi )ωz ı i=1 i=1
2.3 Rigid Body Dynamics 105 n n − (mi yi zi )ωz + ˆ (mi yi xi )ωx j i=1 i=1 n n − (mi zi xi )ωx + ˆ (mi zi yi )ωy k (2.386) i=1 i=1 By introducing the mass moment matrix I with the following elements: n Ixx = mi yi2 + zi 2 (2.387) i=1 n Iyy = mi zi + xi2 2 (2.388) i=1 n Izz = mi xi2 + yi2 (2.389) i=1 n Ixy = Iyx = − (mi xi yi ) (2.390) i=1 n Iyz = Izy = − (mi yi zi ) (2.391) i=1 n Izx = Ixz = − (mi zi xi ) (2.392) i=1we may write the angular momentum L in concise form: Lx = Ixx ωx + Ixy ωy + Ixz ωz (2.393) Ly = Iyx ωx + Iyy ωy + Iyz ωz (2.394) Lz = Izx ωx + Izy ωy + Izz ωz (2.395)or in matrix form: L=I ·ω (2.396) ⎡ ⎤ ⎡ ⎤⎡ ⎤ Lx Ixx Ixy Ixz ωx ⎣Ly ⎦ = ⎣Iyx Iyy Iyz ⎦ ⎣ωy ⎦ (2.397) Lz Izx Izy Izz ωzFor a rigid body that is a continuous solid, the summations must be replaced byintegrations over the volume of the body as in Eq. (2.433).
106 2 Vibration Dynamics The Euler equation of motion for a rigid body is Gd B M= B L (2.398) dtwhere B M is the resultant of the external moments applied on the rigid body. Theangular momentum B L is a vector quantity deﬁned in the body coordinate frame.Hence, its time derivative in the global coordinate frame is G dB L ˙ = B L + B ωB × B L (2.399) G dtTherefore, dL ˙ B M= ˙ = L + ω × L = I ω + ω × (I ω) (2.400) dtor in expanded form B M = (Ixx ωx + Ixy ωy + Ixz ωz )ˆ + ωy (Ixz ωx + Iyz ωy + Izz ωz )ˆ ˙ ˙ ˙ ı ı − ωz (Ixy ωx + Iyy ωy + Iyz ωz )ˆ ı + (Iyx ωx + Iyy ωy + Iyz ωz )j + ωz (Ixx ωx + Ixy ωy + Ixz ωz )j ˙ ˙ ˙ ˆ ˆ − ωx (Ixz ωx + Iyz ωy + Izz ωz )j ˆ ˙ ˆ ˆ + (Izx ωx + Izy ωy + Izz ωz )k + ωx (Ixy ωx + Iyy ωy + Iyz ωz )k ˙ ˙ ˆ − ωy (Ixx ωx + Ixy ωy + Ixz ωz )k (2.401)and, therefore, the most general form of the Euler equations of motion for a rigidbody in a body frame attached to C are Mx = Ixx ωx + Ixy ωy + Ixz ωz − (Iyy − Izz )ωy ωz ˙ ˙ ˙ − Iyz ωz − ωy − ωx (ωz Ixy − ωy Ixz ) 2 2 (2.402) My = Iyx ωx + Iyy ωy + Iyz ωz − (Izz − Ixx )ωz ωx ˙ ˙ ˙ − Ixz ωx − ωz − ωy (ωx Iyz − ωz Ixy ) 2 2 (2.403) Mz = Izx ωx + Izy ωy + Izz ωz − (Ixx − Iyy )ωx ωy ˙ ˙ ˙ − Ixy ωy − ωx − ωz (ωy Ixz − ωx Iyz ) 2 2 (2.404)Assume that we are able to rotate the body frame about its origin to ﬁnd an orien-tation that makes Iij = 0, for i = j . In such a coordinate frame, which is called aprincipal frame, the Euler equations reduce to M1 = I1 ω1 − (I2 − I2 )ω2 ω3 ˙ (2.405) M2 = I2 ω2 − (I3 − I1 )ω3 ω1 ˙ (2.406)
2.3 Rigid Body Dynamics 107Fig. 2.23 A disc with massm and radius r, mounted on amassless turning shaft M3 = I3 ω3 − (I1 − I2 )ω1 ω2 ˙ (2.407) The kinetic energy of a rigid body may be found by the integral of the kineticenergy of the mass element dm, over the whole body: 1 1 K= ˙ v2 dm = (ω × r) · (ω × r) dm 2 B 2 B 2 2 ωy 2 ωx ωz = y 2 + z2 dm + z2 + x 2 dm + x 2 + y 2 dm 2 B 2 B 2 B − ωx ωy xy dm − ωy ωz yz dm − ωz ωx zx dm B B B 1 = Ixx ωx + Iyy ωy + Izz ωz 2 2 2 2 − Ixy ωx ωy − Iyz ωy ωz − Izx ωz ωx (2.408)The kinetic energy can be rearranged to a matrix multiplication form 1 1 K = ωT I ω = ω · L (2.409) 2 2When the body frame is principal, the kinetic energy will simplify to 1 K= I1 ω 1 + I2 ω 2 + I3 ω 3 2 2 2 (2.410) 2Example 66 (A tilted disc on a massless shaft) Figure 2.23 illustrates a disc withmass m and radius R, mounted on a massless shaft. The shaft is turning with aconstant angular speed ω. The disc is attached to the shaft at an angle α. Because ofα, the bearings at A and B must support a rotating force. We attach a principal body coordinate frame at the disc center as shown in theﬁgure. The G-expression of the angular velocity is a constant vector G ωB = ωIˆ (2.411)
108 2 Vibration Dynamicsand the expression of the angular velocity vector in the body frame is B G ωB = ω cos θ ı + ω sin θ j ˆ ˆ (2.412)The mass moment of inertia matrix is ⎡ ⎤ mR 2 /2 0 0 B I= ⎣ 0 mR 2 /4 0 ⎦ (2.413) 0 0 mR 2 /4Substituting (2.412) and (2.413) in (2.405)–(2.407), with 1 ≡ x, 2 ≡ y, 3 ≡ z, yields Mx = 0 (2.414) My = 0 (2.415) mr 2 Mz = ω cos θ sin θ (2.416) 4Therefore, the bearing reaction forces FA and FB are Mz mr 2 FA = −FB = − =− ω cos θ sin θ (2.417) l 4lExample 67 (Steady rotation of a freely rotating rigid body) Consider a situation inwhich the resultant applied force and moment on a rigid body are zero: G F = BF = 0 (2.418) G M= M=0 B (2.419)Based on Newton’s equation, G ˙ F = mG v (2.420)the velocity of the mass center will be constant in the global coordinate frame. How-ever, the Euler equation B M = I B ωB + B ω B × B L G˙ G (2.421)reduces to I2 − I3 ˙ ω1 = ω2 ω3 (2.422) I1 I3 − I1 ˙ ω2 = ω3 ω1 (2.423) I22 I1 − I2 ω3 = ˙ ω1 ω2 (2.424) I3
2.3 Rigid Body Dynamics 109which show that the angular velocity can be constant if I1 = I2 = I3 (2.425)or if two principal moments of inertia, say I1 and I2 , are zero and the third angularvelocity, in this case ω3 , is initially zero, or if the angular velocity vector is initiallyparallel to a principal axis.2.3.6 Mass Moment MatrixTwo types of integral arise in rigid body dynamics that depend solely on the geome-try and mass distribution of the body. The ﬁrst type deﬁnes the center of mass and isimportant when the translation motion of the body is considered. The second is themass moment, which appears when the rotational motion of the body is considered.The mass moment is also called moment of inertia, centrifugal moments, or devia-tion moment. Every rigid body has a 3 × 3 mass moment matrix I , which is denotedby ⎡ ⎤ Ixx Ixy Ixz I = ⎣Iyx Iyy Iyz ⎦ (2.426) Izx Izy IzzThe diagonal elements Iij , i = j , are called polar mass moments Ixx = Ix = y 2 + z2 dm (2.427) B Iyy = Iy = z2 + x 2 dm (2.428) B Izz = Iz = x 2 + y 2 dm (2.429) Band the off-diagonal elements Iij , i = j , are called products of inertia Ixy = Iyx = − xy dm (2.430) B Iyz = Izy = − yz dm (2.431) B Izx = Ixz = − zx dm (2.432) BThe elements of I are functions of the mass distribution of the rigid body and maybe deﬁned by Iij = ri2 δmn − xim xj n dm i, j = 1, 2, 3 (2.433) B
110 2 Vibration Dynamicswhere δij is Kronecker’s delta (2.171), 1 if m = n δmn = (2.434) 0 if m = n The elements of I are calculated in a body coordinate frame attached to the masscenter C of the body. Therefore, I is a frame-dependent quantity and must be writtenwith a frame indicator such as B I to show the frame in which it is computed: ⎡ 2 ⎤ y + z2 −xy −zx B I= ⎣ −xy z2 + x 2 −yz ⎦ dm (2.435) B −zx −yz x 2 + y2 = r 2 I − r rT dm = −˜ r dm r˜ (2.436) B B ˜where r is the associated skew-symmetric matrix of r: ⎡ ⎤ 0 −r3 r2 r = ⎣ r3 ˜ 0 −r1 ⎦ (2.437) −r2 r1 0 The moments of inertia can be transformed from a coordinate frame B1 to an-other coordinate frame B2 , both deﬁned at the mass center of the body, according tothe rule of the rotated-axes theorem: B2 I = B2 RB1 B1 I B2 RB1 T (2.438)Transformation of the moment of inertia from a central frame B1 located at B2 rC toanother frame B2 , which is parallel to B1 , is, according to the rule of the parallel-axes theorem: B2 I = B1 I + m rC rC ˜ ˜T (2.439) If the local coordinate frame Oxyz is located such that the products of inertiavanish, the local coordinate frame is the principal coordinate frame and the associ-ated mass moments are principal mass moments. Principal axes and principal massmoments can be found by solving the following characteristic equation for I : Ixx − I Ixy Ixz Iyx Iyy − I Iyz = 0 (2.440) Izx Izy Izz − I det [Iij ] − I [δij ] = 0 (2.441)Equation (2.441) is a cubic equation in I , so we obtain three eigenvalues, I1 = Ix I2 = Iy I3 = Iz (2.442)
2.3 Rigid Body Dynamics 111Fig. 2.24 Two coordinateframes with a common originat the mass center of a rigidbodywhich are the principal mass moments.Proof Two coordinate frames with a common origin at the mass center of a rigidbody are shown in Fig. 2.24. The angular velocity and angular momentum of a rigidbody transform from the frame B1 to the frame B2 by the vector transformation rule 2 ω = 2 R2 1 ω (2.443) 2 L = R1 L 2 1 (2.444)However, L and ω are related according to Eq. (2.372) 1 L = 1I 1ω (2.445)and, therefore, 2 L = 2 R1 1 I 2 R1 2 ω = 2 I 2 ω T (2.446)which shows how to transfer the mass moment from the coordinate frame B1 to arotated frame B2 2 I = 2 R1 1 I 2 R1 T (2.447) Now consider a central frame B1 , shown in Fig. 2.25, at 2 rC , which rotates aboutthe origin of a ﬁxed frame B2 such that their axes remain parallel. The angularvelocity and angular momentum of the rigid body transform from frame B1 to frameB2 by 2 ω = 1ω (2.448) 2 L = L + (rC × mvC ) 1 (2.449)Therefore, 2 L = 1 L + m2 rC × 2 ω × 2 rC = 1 L + m2 rC 2 rC 2 ω ˜ ˜T = 1 I + m2 rC 2 rC 2 ω ˜ ˜T (2.450)
112 2 Vibration DynamicsFig. 2.25 A centralcoordinate frame B1 and atranslated frame B2which shows how to transfer the mass moment from frame B1 to a parallel frame B2 2 I = 1 I + m rC rC ˜ ˜T (2.451)The parallel-axes theorem is also called the Huygens–Steiner theorem. Referring to Eq. (2.447) for transformation of the moment of inertia to a rotatedframe, we can always ﬁnd a frame in which 2 I is diagonal. In such a frame, we have 2 R1 1 I = 2 I 2 R1 (2.452)or ⎡ ⎤⎡ ⎤ r11 r12 r13 Ixx Ixy Ixz ⎣r21 r22 r23 ⎦ ⎣Iyx Iyy Iyz ⎦ r31 r32 r33 Izx Izy Izz ⎡ ⎤⎡ ⎤ I1 0 0 r11 r12 r13 =⎣0 I2 0 ⎦ ⎣r21 r22 r23 ⎦ (2.453) 0 0 I3 r31 r32 r33which shows that I1 , I2 , and I3 are eigenvalues of 1 I . These eigenvalues can befound by solving the following characteristic equation for λ: Ixx − λ Ixy Ixz Iyx Iyy − λ Iyz = 0 (2.454) Izx Izy Izz − λThe eigenvalues I1 , I2 , and I3 are principal mass moments, and their associatedeigenvectors are principal directions. The coordinate frame made by the eigenvec-tors is the principal body coordinate frame. In the principal coordinate frame, therigid body angular momentum simpliﬁes to ⎡ ⎤ ⎡ ⎤⎡ ⎤ L1 I1 0 0 ω1 ⎣ L2 ⎦ = ⎣ 0 I 2 0 ⎦ ⎣ ω 2 ⎦ (2.455) L3 0 0 I3 ω3
2.3 Rigid Body Dynamics 113Example 68 (Principal moments of inertia) Consider the inertia matrix I ⎡ ⎤ 10 −2 0 I = ⎣−2 20 −2⎦ (2.456) 0 −2 30We set up the determinant (2.441) 10 − λ −2 0 −2 20 − λ −2 = 0 (2.457) 0 −2 30 − λwhich leads to the following characteristic equation: −λ3 + 60λ2 − 1092λ + 5840 = 0 (2.458) Three roots of Eq. (2.458) are I1 = 9.6077 I2 = 20 I3 = 30.392 (2.459)and, therefore, the principal mass moment matrix is ⎡ ⎤ 9.6077 0 0 I =⎣ 0 20 0 ⎦ (2.460) 0 0 30.392Example 69 (Principal coordinate frame) Consider the inertia matrix I ⎡ ⎤ 10 −2 0 I = ⎣−2 20 −2⎦ (2.461) 0 −2 30the direction of a principal axis xi is established by solving ⎡ ⎤⎡ ⎤ ⎡ ⎤ Ixx − Ii Ixy Ixz cos αi 0 ⎣ Iyx Iyy − Ii Iyz ⎦ ⎣cos βi ⎦ = ⎣0⎦ (2.462) Izx Izy Izz − Ii cos γi 0The direction cosines αi , βi , γi must also satisfy cos2 αi + cos2 βi + cos2 γi = 1 (2.463) For the ﬁrst principal mass moment I1 = 9.6077, we have ⎡ ⎤⎡ ⎤ ⎡ ⎤ 10 − 9.6 −2 0 cos α1 0 ⎣ −2 20 − 9.6 −2 ⎦ ⎣cos β1 ⎦ = ⎣0⎦ (2.464) 0 −2 30 − 9.6 cos γ1 0
114 2 Vibration Dynamicsor 0.3923 cos α1 − 2 cos β1 + 0 = 0 (2.465) 10.392 cos β1 − 2 cos α1 − 2 cos γ1 = 0 (2.466) 0 + 20.392 cos γ1 − 2 cos β1 = 0 (2.467)and we obtain α1 = 11.148 deg (2.468) β1 = 78.902 deg (2.469) γ1 = 88.917 deg (2.470) Using I2 = 20 for the second principal axis ⎡ ⎤⎡ ⎤ ⎡ ⎤ 10 − 20 −2 0 cos α2 0 ⎣ −2 20 − 20 −2 ⎦ ⎣cos β2 ⎦ = ⎣0⎦ (2.471) 0 −2 30 − 20 cos γ2 0we obtain α2 = 101.10 deg (2.472) β2 = 15.793 deg (2.473) γ2 = 78.902 deg (2.474) The third principal axis is for I3 = 30.392 ⎡ ⎤⎡ ⎤ ⎡ ⎤ 10 − 30.4 −2 0 cos α3 0 ⎣ −2 20 − 30.4 −2 ⎦ ⎣cos β3 ⎦ = ⎣0⎦ (2.475) 0 −2 30 − 30.4 cos γ3 0which leads to α3 = 88.917 deg (2.476) β3 = 101.10 deg (2.477) γ3 = 11.139 deg (2.478)Example 70 (Mass moment of rectangular bar) Consider a homogeneous rectangu-lar brick with mass m, length l, width w, and height h, as shown in Fig. 2.26. The local central coordinate frame is attached to the brick at its mass center. Themoments of inertia matrix of the brick can be found by the integral method. We
2.3 Rigid Body Dynamics 115Fig. 2.26 A homogeneousrectangular brickbegin by calculating Ixx : m Ixx = y 2 + z2 dm = y 2 + z2 ρ dv = y 2 + z2 dv B v lwh v m h/2 w/2 l/2 = y 2 + z2 dx dy dz lwh −h/2 −w/2 −l/2 m 2 = w + h2 (2.479) 12The mass moments Iyy and Izz can be calculated similarly m 2 Iyy = h + l2 (2.480) 12 m 2 Izz = l + w2 (2.481) 12 The coordinate frame is central and, therefore, the products of inertia must bezero. To show this, we examine Ixy : Ixy = Iyx = − xy dm = xyρ dv B v m h/2 w/2 l/2 = xy dx dy dz = 0 (2.482) lwh −h/2 −w/2 −l/2 Therefore, the moment of inertia matrix for the rigid rectangular brick in its cen-tral frame is ⎡m 2 ⎤ 12 (w + h ) 2 0 0 ⎢ ⎥ I =⎣ 0 12 (h + l ) m 2 2 0 ⎦ (2.483) 0 0 m 2 12 (l + w2 )Example 71 (Translation of the inertia matrix) The mass moment matrix of thebrick shown in Fig. 2.27, in the principal frame B(oxyz), is given in Eq. (2.483).The mass moment matrix in the non-principal frame B (ox y z ) can be found byapplying the parallel-axes transformation formula (2.451): B I = B I + mB rC B rC ˜ ˜T (2.484)
116 2 Vibration DynamicsFig. 2.27 A rigid rectangularbrick in the principal andnon-principal framesThe mass center is at ⎡ ⎤ l 1⎣ ⎦ B rC = w (2.485) 2 hand, therefore, ⎡ ⎤ 0 −h w 1⎣ B rC = ˜ h 0 −l ⎦ (2.486) 2 −w l 0which provides ⎡1 ⎤ 3 h m + 3 mw − 1 lmw − 1 hlm 2 1 2 4 4 ⎢ ⎥ B I = ⎣ − 1 lmw 4 3h m + 3l m 1 2 1 2 − 1 hmw 4 ⎦ (2.487) − 1 hlm 4 − 1 hmw 4 3 l m + 3 mw 1 2 1 2Example 72 (Principal rotation matrix) Consider a mass moment matrix in the bodyframe B1 as ⎡ ⎤ 2/3 −1/2 −1/2 1 I = ⎣−1/2 5/3 −1/4⎦ (2.488) −1/2 −1/4 5/3The eigenvalues and eigenvectors of 1 I are ⎡ ⎤ 2.351 I1 = 0.2413 ⎣ 1 ⎦ (2.489) 1 ⎡ ⎤ −0.851 I2 = 1.8421 ⎣ 1 ⎦ (2.490) 1 ⎡ ⎤ 0 I3 = 1.9167 ⎣−1⎦ (2.491) 1
2.3 Rigid Body Dynamics 117The normalized eigenvector matrix W is the transpose of the required transforma-tion matrix to make the inertia matrix diagonal ⎡ ⎤ | | | W = ⎣ w 1 w 2 w 3 ⎦ = 2 R1 T | | | ⎡ ⎤ 0.856 9 −0.515 6 0.0 = ⎣0.364 48 0.605 88 −0.707 11⎦ (2.492) 0.364 48 0.605 88 0.707 11We may verify that 2 I = 2 R1 1 I 2 R1 = W T1 I W T ⎡ ⎤ 0.2413 −1 × 10−4 0.0 = ⎣−1 × 10 −4 1.842 1 −1 × 10−19 ⎦ (2.493) 0.0 0.0 1.916 7Example 73 ( (Relative diagonal moments of inertia) By the deﬁnitions for themass moments (2.427), (2.428), and (2.429) it is seen that the inertia matrix is sym-metric, and 1 x 2 + y 2 + z2 dm = (Ixx + Iyy + Izz ) (2.494) B 2and also Ixx + Iyy ≥ Izz Iyy + Izz ≥ Ixx Izz + Ixx ≥ Iyy (2.495)Noting that (y − z)2 ≥ 0, it is evident that (y 2 + z2 ) ≥ 2yz, and therefore Ixx ≥ 2Iyz (2.496)and similarly Iyy ≥ 2Izx Izz ≥ 2Ixy (2.497)Example 74 (Coefﬁcients of the characteristic equation) The determinant(2.454) Ixx − λ Ixy Ixz Iyx Iyy − λ Iyz = 0 (2.498) Izx Izy Izz − λfor calculating the principal mass moments, leads to a third-degree equation for λ,called the characteristic equation: λ3 − a1 λ2 + a 2 λ − a3 = 0 (2.499)
118 2 Vibration DynamicsThe coefﬁcients of the characteristic equation are called the principal invariantsof [I ]. The coefﬁcients of the characteristic equation can directly be found from a1 = Ixx + Iyy + Izz = tr [I ] (2.500) a2 = Ixx Iyy + Iyy Izz + Izz Ixx − Ixy − Iyz − Izx 2 2 2 Ixx Ixy I Iyz I Ixz = + yy + xx Iyx Iyy Izy Izz Izx Izz 1 2 = a − tr I 2 (2.501) 2 1 a3 = Ixx Iyy Izz + Ixy Iyz Izx + Izy Iyx Ixz − (Ixx Iyz Izy + Iyy Izx Ixz + Izz Ixy Iyx ) = Ixx Iyy Izz + 2Ixy Iyz Izx − Ixx Iyz + Iyy Izx + Izz Ixy 2 2 2 = det [I ] (2.502)Example 75 (The principal mass moments are coordinate invariants) The rootsof the inertia characteristic equation are the principal mass moments. They are allreal but not necessarily different. The principal mass moments are extreme. Thatis, the principal mass moments determine the smallest and the largest values of Iii .Since the smallest and largest values of Iii do not depend on the choice of the bodycoordinate frame, the solution of the characteristic equation is not dependent of thecoordinate frame. In other words, if I1 , I2 , and I3 are the principal mass moments for B1 I , theprincipal mass moments for B2 I are also I1 , I2 , and I3 when B2 I = B2 RB1 B1 I B2 RB1 TWe conclude that I1 , I2 , and I3 are coordinate invariants of the matrix [I ], andtherefore any quantity that depends on I1 , I2 , and I3 is also coordinate invariant.Because the mass matrix [I ] has rank 3, it has only three independent invariants andevery other invariant can be expressed in terms of I1 , I2 , and I3 . Since I1 , I2 , and I3 are the solutions of the characteristic equation of [I ] given in(2.499), we may write the determinant (2.454) in the form (λ − I1 )(λ − I2 )(λ − I3 ) = 0 (2.503)The expanded form of this equation is λ3 − (I1 + I2 + I3 )λ2 + (I1 I2 + I2 I3 + I3 I1 )a2 λ − I1 I2 I3 = 0 (2.504)By comparing (2.504) and (2.499) we conclude that a1 = Ixx + Iyy + Izz = I1 + I2 + I3 (2.505)
2.3 Rigid Body Dynamics 119 a2 = Ixx Iyy + Iyy Izz + Izz Ixx − Ixy − Iyz − Izx 2 2 2 = I1 I2 + I2 I3 + I3 I1 (2.506) a3 = Ixx Iyy Izz + 2Ixy Iyz Izx − Ixx Iyz + Iyy Izx + Izz Ixy 2 2 2 = I1 I2 I3 (2.507)Being able to express the coefﬁcients a1 , a2 , and a3 as functions of I1 , I2 , and I3 , wedetermine the coefﬁcients of the characteristic equation to be coordinate invariant.Example 76 (Short notation for the elements of inertia matrix) Taking advantageof the Kronecker delta δij (2.171) we may write the elements of the mass momentmatrix Iij in short: Iij = x1 + x2 + x3 δij − xi xj dm 2 2 2 (2.508) B Iij = r 2 δij − xi xj dm (2.509) B 3 Iij = xk xk δij − xi xj dm (2.510) B k=1where we utilized the following notation: x1 = x x2 = y x3 = z (2.511)Example 77 (Mass moment with respect to a plane, a line, and a point) Themass moment of a system of particles may be deﬁned with respect to a plane, a line,or a point as the sum of the products of the mass of the particles into the square ofthe perpendicular distance from the particle to the plane, the line, or the point. Fora continuous body, the sum would become a deﬁnite integral over the volume of thebody. The mass moments with respect to the xy-, yz-, and zx-plane are Iz 2 = z2 dm (2.512) B Iy 2 = y 2 dm (2.513) B Ix 2 = x 2 dm (2.514) BThe moments of inertia with respect to the x, y, and z axes are Ix = y 2 + z2 dm (2.515) B
120 2 Vibration Dynamics Iy = z2 + x 2 dm (2.516) B Iz = x 2 + y 2 dm (2.517) Band, therefore, Ix = Iy 2 + Iz 2 (2.518) Iy = Iz 2 + Ix 2 (2.519) Iz = Ix 2 + I y 2 (2.520)The moment of inertia with respect to the origin is Io = x 2 + y 2 + z2 dm = Ix 2 + Iy 2 + Iz2 B 1 = (Ix + Iy + Iz ) (2.521) 2Because the choice of the coordinate frame is arbitrary, the mass moment with re-spect to a line is the sum of the mass moments with respect to any two mutuallyorthogonal planes that pass through the line. The mass moment with respect to apoint has a similar meaning for three mutually orthogonal planes intersecting at thepoint.2.4 Lagrange MethodThe Lagrange method in deriving the equations of motion of vibrating systems hassome advantages over Newton–Euler due to its simplicity and generality, speciallyfor multi DOF systems.2.4.1 Lagrange Form of Newton EquationNewton’s equation of motion can be transformed to d ∂K ∂K − = Fr r = 1, 2, . . . , n (2.522) dt ˙ ∂ qr ∂qrwhere n ∂fi ∂gi ∂hi Fr = Fix + Fiy + Fiz (2.523) ∂q1 ∂q2 ∂qn i=1
2.4 Lagrange Method 121Equation (2.522) is called the Lagrange equation of motion, where K is the kineticenergy of the n DOF system, qr , r = 1, 2, . . . , n are the generalized coordinates ofthe system, Fi = [Fix Fiy Fiz ]T is the external force acting on the ith particle of thesystem, and Fr is the generalized force associated to qr . The functions fi , gi , hi ,are the relationships of x1 , yi , zi , based on the generalized coordinates of the systemxi = fi (qj , t), yi = gi (qj , t), zi = hi (qj , t).Proof Let mi be the mass of one of the particles of a system and let (xi , yi , zi )be its Cartesian coordinates in a globally ﬁxed coordinate frame. Assume that thecoordinates of every individual particle are functions of another set of coordinatesq1 , q2 , q3 , . . . , qn , and possibly time t: xi = fi (q1 , q2 , q3 , . . . , qn , t) (2.524) yi = gi (q1 , q2 , q3 , . . . , qn , t) (2.525) zi = hi (q1 , q2 , q3 , . . . , qn , t) (2.526) If Fxi , Fyi , Fzi are components of the total force acting on the particle mi , thenthe Newton equations of motion for the particle would be Fxi = mi xi ¨ (2.527) Fyi = mi yi ¨ (2.528) Fzi = mi zi ¨ (2.529)We multiply both sides of these equations by ∂fi /∂qr , ∂gi /∂qr , and ∂hi /∂qr , re-spectively, and add them up for all the particles to ﬁnd n n ∂fi ∂gi ∂hi ∂fi ∂gi ∂hi mi ¨ xi + yi ¨ + zi ¨ = Fxi + Fyi + Fzi (2.530) ∂qr ∂qr ∂qr ∂qr ∂qr ∂qr i=1 i=1where n is the total number of particles. Taking the time derivative of Eq. (2.524), ∂fi ∂fi ∂fi ∂fi ∂fi ˙ xi = q1 + ˙ q2 + ˙ q3 + · · · + ˙ qn + ˙ (2.531) ∂q1 ∂q2 ∂q3 ∂qn ∂twe ﬁnd ˙ ∂ xi ∂ ∂fi ∂fi ∂fi ∂fi ∂fi = q1 + ˙ q2 + · · · + ˙ qn + ˙ = (2.532) ˙ ∂ qr ˙ ∂ qr ∂q1 ∂q2 ∂qn ∂t ∂qrand, therefore, ∂fi ˙ ∂ xi d ˙ ∂ xi d ˙ ∂ xi ¨ xi = xi ¨ = ˙ xi − xi ˙ (2.533) ∂qr ˙ ∂ qr dt ˙ ∂ qr dt ˙ ∂ qr
122 2 Vibration DynamicsHowever, d ˙ ∂ xi d ∂fi ˙ xi = xi ˙ dt ˙ ∂ qr dt ∂qr ∂ 2 fi ∂ 2 fi ∂ 2 fi = xi ˙ q1 + · · · + ˙ qn + ˙ ∂q1 ∂qr ∂qn ∂qr ∂t∂qr ∂ ∂fi ∂fi ∂fi ∂fi = xi ˙ q1 + ˙ q2 + · · · + ˙ qn + ˙ ∂qr ∂q1 ∂q2 ∂qn ∂t ˙ ∂ xi = xi ˙ (2.534) ∂qrand we have ˙ ∂ xi d ˙ ∂ xi ˙ ∂ xi ¨ xi = ˙ xi − xi ˙ (2.535) ˙ ∂ qr dt ˙ ∂ qr ∂qrwhich is equal to ˙ xi d ∂ 1 2 ∂ 1 2 ¨ xi = ˙ x − ˙ x (2.536) ˙ qr ˙ dt ∂ qr 2 i ∂qr 2 iNow substituting (2.533) and (2.536) into the left-hand side of (2.530) yields n ∂fi ∂gi ∂hi ¨ mi xi + yi ¨ + zi ¨ ∂qr ∂qr ∂qr i=1 n d ∂ 1 2 1 2 1 2 = mi x + yi + zi ˙ ˙ ˙ ˙ dt ∂ qr 2 i 2 2 i=1 n ∂ 1 2 1 2 1 2 − mi ˙ ˙ ˙ x + yi + zi ∂qr 2 i 2 2 i=1 n 1 d ∂ = mi ˙ x 2 + yi2 + zi ˙ ˙2 2 ˙ dt ∂ qr i i=1 n 1 ∂ − mi ˙ x 2 + yi2 + zi ˙ ˙2 (2.537) 2 ∂qr i i=1where n 1 mi xi2 + yi2 + zi = K ˙ ˙ ˙2 (2.538) 2 i=1
2.4 Lagrange Method 123is the kinetic energy of the system. Therefore, the Newton equations of motion(2.527), (2.528), and (2.529) are converted to n d ∂K ∂K ∂fi ∂gi ∂hi − = Fxi + Fyi + Fzi (2.539) dt ˙ ∂ qr ∂qr ∂qr ∂qr ∂qr i=1 Because of (2.524), (2.525), and (2.526), the kinetic energy is a function ofq1 , q2 , q3 , . . . , qn and time t. The left-hand side of Eq. (2.539) includes the kineticenergy of the whole system and the right-hand side is a generalized force and showsthe effect of changing coordinates from xi to qj on the external forces. Let us assumethat a particular coordinate qr gets altered to qr + δqr while the other coordinatesq1 , q2 , q3 , . . . , qr−1 , qr+1 , . . . , qn and time t are unaltered. So, the coordinates of miare changed to ∂fi xi + δqr (2.540) ∂qr ∂gi yi + δqr (2.541) ∂qr ∂hi zi + δqr (2.542) ∂qrSuch a displacement is called a virtual displacement. The work done in this virtualdisplacement by all forces acting on the particles of the system is n ∂fi ∂gi ∂hi δW = Fxi + Fyi + Fzi δqr (2.543) ∂qr ∂qr ∂qr i=1Because the work done by internal forces appears in opposite pairs, only the workdone by external forces remains in Eq. (2.543). Let us denote the virtual work by δW = Fr (q1 , q2 , q3 , . . . , qn , t)δqr (2.544)Then we have d ∂K ∂K − = Fr (2.545) dt ˙ ∂ qr ∂qrwhere n ∂fi ∂gi ∂hi Fr = Fxi + Fyi + Fzi (2.546) ∂qr ∂qr ∂qr i=1Equation (2.545) is the Lagrange form of the equations of motion. This equationis true for all values of r from 1 to n. We thus have n second-order ordinary dif-ferential equations, in which q1 , q2 , q3 , . . . , qn are the dependent variables and t isthe independent variable. The generalized coordinates q1 , q2 , q3 , . . . , qn can be any
124 2 Vibration DynamicsFig. 2.28 A simplependulummeasurable parameters to provide the conﬁguration of the system. The number ofequations and the number of dependent variables are equal; therefore, the equationsare theoretically sufﬁcient to determine the motion of all mi .Example 78 (A simple pendulum) A pendulum is shown in Fig. 2.28. Using xand y for the Cartesian position of m, and using θ = q as the generalized coordinate,we have x = f (θ ) = l sin θ (2.547) y = g(θ ) = l cos θ (2.548) 1 1 ˙ ˙ ˙ K = m x 2 + y 2 = ml 2 θ 2 (2.549) 2 2and, therefore, d ∂K ∂K d − = ˙ ¨ ml 2 θ = ml 2 θ (2.550) dt ˙ ∂θ ∂θ dt The external force components, acting on m, are Fx = 0 (2.551) Fy = mg (2.552)and, therefore, ∂f ∂g Fθ = Fx + Fy = −mgl sin θ (2.553) ∂θ ∂θHence, the equation of motion for the pendulum is ¨ ml 2 θ = −mgl sin θ (2.554)Example 79 (A pendulum attached to an oscillating mass) Figure 2.29 illustratesa vibrating mass with a hanging pendulum. The pendulum can act as a vibrationabsorber if designed properly. Starting with coordinate relationships xM = f M = x (2.555)
2.4 Lagrange Method 125Fig. 2.29 A vibrating masswith a hanging pendulum y M = gM = 0 (2.556) xm = fm = x + l sin θ (2.557) ym = gm = l cos θ (2.558)we may ﬁnd the kinetic energy in terms of the generalized coordinates x and θ : 1 1 ˙2 K = M xM + yM + m xm + ym ˙2 ˙2 ˙2 2 2 1 1 ˙ ˙ ˙ ˙˙ = M x 2 + m x 2 + l 2 θ 2 + 2l x θ cos θ (2.559) 2 2Then the left-hand sides of the Lagrange equations are d ∂K ∂K − ¨ ˙ = (M + m)x + ml θ cos θ − ml θ 2 sin θ ¨ (2.560) dt ˙ ∂x ∂x d ∂K ∂K − ¨ = ml 2 θ + ml x cos θ ¨ (2.561) dt ˙ ∂θ ∂θThe external forces acting on M and m are FxM = −kx (2.562) FyM = 0 (2.563) Fxm = 0 (2.564) Fym = mg (2.565)Therefore, the generalized forces are ∂fM ∂gM ∂fm ∂gm Fx = FxM + FyM + Fxm + Fym = −kx (2.566) ∂x ∂x ∂x ∂x ∂fM ∂gM ∂fm ∂gm Fθ = FxM + FyM + Fxm + Fym = −mgl sin θ (2.567) ∂θ ∂θ ∂θ ∂θand ﬁnally the equations of motion are
126 2 Vibration Dynamics ¨ ˙ (M + m)x + ml θ cos θ − ml θ 2 sin θ = −kx ¨ (2.568) 2¨ ml θ + ml x cos θ = −mgl sin θ ¨ (2.569)2.4.2 Lagrangean MechanicsLet us assume that for some forces F = [Fix Fiy Fiz ]T there is a function V , calledpotential energy, such that the force is derivable from V F = −∇V (2.570)Such a force is called a potential or conservative force. Then the Lagrange equationof motion can be written as d ∂L ∂L − = Qr r = 1, 2, . . . , n (2.571) dt ˙ ∂ qr ∂qrwhere L=K −V (2.572)is the Lagrangean of the system and Qr is the nonpotential generalized force n ∂fi ∂gi ∂hi Qr = Fix + Fiy + Fiz (2.573) ∂q1 ∂q2 ∂qn i=1for which there is no potential function.Proof Assume that the external forces F = [Fxi Fyi Fzi ]T acting on the system areconservative: F = −∇V (2.574)The work done by these forces in an arbitrary virtual displacement δq1 , δq2 ,δq3 , . . . , δqn is ∂V ∂V ∂V ∂W = − δq1 − δq2 − · · · δqn (2.575) ∂q1 ∂q2 ∂qnand then the Lagrange equation becomes d ∂K ∂K ∂V − =− r = 1, 2, . . . , n (2.576) dt ˙ ∂ qr ∂qr ∂q1Introducing the Lagrangean function L = K − V converts the Lagrange equation ofa conservative system to d ∂L ∂L − = 0 r = 1, 2, . . . , n (2.577) dt ˙ ∂ qr ∂qr
2.4 Lagrange Method 127Fig. 2.30 An undampedthree DOF systemIf a force is not conservative, then the virtual work done by the force is n ∂fi ∂gi ∂hi δW = Fxi + Fyi + Fzi δqr ∂qr ∂qr ∂qr i=1 = Qr δqr (2.578)and the equation of motion would be d ∂L ∂L − = Qr r = 1, 2, . . . , n (2.579) dt ˙ ∂ qr ∂qrwhere Qr is the nonpotential generalized force doing work in a virtual displacementof the rth generalized coordinate qr . The Lagrangean L is also called the kinetic potential.Example 80 (An undamped three DOF system) Figure 2.30 illustrates an undampedthree DOF linear vibrating system. The kinetic and potential energies of the systemare 1 1 1 K = m1 x1 + m2 x2 + m3 x3 ˙2 ˙2 ˙2 (2.580) 2 2 2 1 1 1 1 V = k1 x1 + k2 (x1 − x2 )2 + k3 (x2 − x3 )2 + k4 x3 2 2 (2.581) 2 2 2 2Because there is no damping in the system, we may ﬁnd the Lagrangean L L=K −V (2.582)and use Eq. (2.644) with Qr = 0 ∂L = −k1 x1 − k2 (x1 − x2 ) (2.583) ∂x1 ∂L = k2 (x1 − x2 ) − k3 (x2 − x3 ) (2.584) ∂x2 ∂L = k3 (x2 − x3 ) − k4 x3 (2.585) ∂x3 ∂L = m1 x1 ˙ (2.586) ˙ ∂ x1
128 2 Vibration DynamicsFig. 2.31 A sphericalpendulum ∂L = m2 x2 ˙ (2.587) ˙ ∂ x2 ∂L = m3 x3 ˙ (2.588) ˙ ∂ x3to ﬁnd the equations of motion: m1 x1 + k1 x1 + k2 (x1 − x2 ) = 0 ¨ (2.589) m2 x2 − k2 (x1 − x2 ) + k3 (x2 − x3 ) = 0 ¨ (2.590) m3 x3 − k3 (x2 − x3 ) + k4 x3 = 0 ¨ (2.591)These equations can be rewritten in matrix form for simpler calculation: ⎡ ⎤⎡ ⎤ m1 0 0 ¨ x1 ⎣ 0 m2 0 ⎦ ⎣x2 ⎦ ¨ 0 0 m3 ¨ x3 ⎡ ⎤⎡ ⎤ k1 + k2 −k2 0 x1 + ⎣ −k2 k2 + k3 −k3 ⎦ ⎣x2 ⎦ = 0 (2.592) 0 −k3 k3 + k4 x3Example 81 (Spherical pendulum) A pendulum analogy is utilized in modeling ofmany dynamical problems. Figure 2.31 illustrates a spherical pendulum with massm and length l. The angles ϕ and θ may be used as describing coordinates of thesystem. The Cartesian coordinates of the mass as a function of the generalized coordi-nates are ⎡ ⎤ ⎡ ⎤ X r cos ϕ sin θ ⎣ Y ⎦ = ⎣ r sin θ sin ϕ ⎦ (2.593) Z −r cos θand, therefore, the kinetic and potential energies of the pendulum are 1 ˙ K = m l 2 θ 2 + l 2 ϕ 2 sin2 θ ˙ (2.594) 2
2.4 Lagrange Method 129Fig. 2.32 A controlledcompound pendulum V = −mgl cos θ (2.595)The kinetic potential function of this system is 1 ˙ L = m l 2 θ 2 + l 2 ϕ 2 sin2 θ + mgl cos θ ˙ (2.596) 2which leads to the following equations of motion: g ¨ ˙ θ − ϕ 2 sin θ cos θ + sin θ = 0 (2.597) l ¨ ˙˙ ϕ sin2 θ + 2ϕ θ sin θ cos θ = 0 (2.598)Example 82 (Controlled compound pendulum) A massive arm is attached to a ceil-ing at a pin joint O as illustrated in Fig. 2.32. Assume that there is viscous frictionin the joint where an ideal motor can apply a torque Q to move the arm. The rotorof an ideal motor has no mass moment by assumption. The kinetic and potential energies of the manipulator are 1 1 ˙ ˙ K = I θ 2 = IC + ml 2 θ 2 (2.599) 2 2 V = −mg cos θ (2.600)where m is the mass and I is the mass moment of the pendulum about O. TheLagrangean of the manipulator is 1 ˙ L = K − V = I θ 2 + mg cos θ (2.601) 2and, therefore, the equation of motion of the pendulum is d ∂L ∂L M= − ¨ = I θ + mgl sin θ (2.602) dt ˙ ∂θ ∂θ The generalized force M is the contribution of the motor torque Q and the vis- ˙cous friction torque −cθ . Hence, the equation of motion of the manipulator is ¨ ˙ Q = I θ + cθ + mgl sin θ (2.603)
130 2 Vibration DynamicsFig. 2.33 A uniform disc,rolling in a circular pathExample 83 (A rolling disc in a circular path) Figure 2.33 illustrates a uniformdisc with mass m and radius r. The disc is rolling without slip in a circular path withradius R. The disc may have a free oscillation around θ = 0. To ﬁnd the equation of motion, we employ the Lagrange method. The energiesof the system are 1 2 1 K = mvC + Ic ω2 2 2 1 1 1 2 ˙ = m(R − r)2 θ 2 + ˙ ˙ mr (ϕ − θ)2 (2.604) 2 2 2 V = −mg(R − r) cos θ (2.605)When there is no slip, there is a constraint between θ and ϕ: Rθ = rϕ (2.606)which can be used to eliminate ϕ from K: 3 ˙ K = m(R − r)2 θ 2 (2.607) 4Based on the partial derivatives d ∂L 3 ¨ = m(R − r)2 θ (2.608) dt ˙ ∂θ 2 ∂L = −mg(R − r) sin θ (2.609) ∂θwe ﬁnd the equation of motion for the oscillating disc: 3 ¨ (R − r)θ + g sin θ = 0 (2.610) 2When the oscillation is very small, we may substitute the oscillating disc with anequivalent mass–spring system. When θ is very small, this equation is equivalent toa mass–spring system with me = 3(R − r) and ke = 2g.Example 84 (A double pendulum) Figure 2.34 illustrates a double pendulum.There are two massless rods with lengths l1 and l2 , and two point masses m1 andm2 . The variables θ1 and θ2 can be used as the generalized coordinates to express
2.4 Lagrange Method 131Fig. 2.34 A doublependulumthe system conﬁguration. To calculate the Lagrangean of the system and ﬁnd theequations of motion, we start by deﬁning the global position of the masses: x1 = l1 sin θ1 (2.611) y1 = −l1 cos θ1 (2.612) x2 = l1 sin θ1 + l2 sin θ2 (2.613) y2 = −l1 cos θ1 − l2 cos θ2 (2.614)The time derivatives of the coordinates are ˙ ˙ x1 = l1 θ1 cos θ1 (2.615) ˙ ˙ y1 = l1 θ1 sin θ1 (2.616) ˙ ˙ ˙ x2 = l1 θ1 cos θ1 + l2 θ2 cos θ2 (2.617) ˙ ˙ ˙ y2 = l1 θ1 sin θ1 + l2 θ2 sin θ2 (2.618)and, therefore, the squares of the masses’ velocities are 2 ˙2 v 1 = x 1 + y 1 = l1 θ 1 2 ˙2 ˙2 (2.619) 2 ˙2 2 ˙2 ˙ ˙ 2 v2 = ˙2 x2 + y2 ˙2 = l1 θ 1 + l2 θ 2 + 2l1 l2 θ1 θ2 cos(θ1 − θ2 ) (2.620)The kinetic energy of the pendulum is then equal to 1 1 K = m1 v1 + m2 v2 2 2 2 2 1 2 ˙2 1 2 ˙2 2 ˙2 ˙ ˙ = m1 l1 θ1 + m2 l1 θ1 + l2 θ2 + 2l1 l2 θ1 θ2 cos(θ1 − θ2 ) (2.621) 2 2
132 2 Vibration DynamicsFig. 2.35 A chain pendulum The potential energy of the pendulum is equal to the sum of the potentials of eachmass: V = m1 gy1 + m2 gy2 = −m1 gl1 cos θ1 − m2 g(l1 cos θ1 + l2 cos θ2 ) (2.622)The kinetic and potential energies constitute the following Lagrangean: L=K −V 1 2 ˙2 1 2 ˙2 2 ˙2 ˙ ˙ = m1 l1 θ1 + m2 l1 θ1 + l2 θ2 + 2l1 l2 θ1 θ2 cos(θ1 − θ2 ) 2 2 + m1 gl1 cos θ1 + m2 g(l1 cos θ1 + l2 cos θ2 ) (2.623)Employing Lagrange method (2.644) we ﬁnd the following equations of motion: d ∂L ∂L 2 ¨ − ¨ = (m1 + m2 )l1 θ1 + m2 l1 l2 θ2 cos(θ1 − θ2 ) dt ˙ ∂ θ1 ∂θ1 ˙2 − m2 l1 l2 θ2 sin(θ1 − θ2 ) + (m1 + m2 )l1 g sin θ1 = 0 (2.624) d ∂L ∂L 2¨ − ¨ = m2 l2 θ2 + m2 l1 l2 θ1 cos(θ1 − θ2 ) dt ˙ ∂ θ2 ∂θ2 ˙2 + m2 l1 l2 θ1 sin(θ1 − θ2 ) + m2 l2 g sin θ2 = 0 (2.625)Example 85 (Chain pendulum) Consider an n-chain-pendulum as shown inFig. 2.35. Each pendulum has a massless length li with a concentrated point massmi , and a generalized angular coordinate θi measured from the vertical direction. The xi and yi components of the mass mi are i i xi = lj sin θj yi = − lj cos θj (2.626) j =1 j =1
2.4 Lagrange Method 133We ﬁnd their time derivatives: i i ˙ xi = ˙ lj θj cos θj yi = ˙ ˙ lj θj sin θj (2.627) j =1 j =1 ˙ ˙and the square of xi and yi : i i n n ˙ xi2 = ˙ lj θj cos θj ˙ lk θk cos θk = ˙ ˙ lj lk θj θk cos θj cos θk (2.628) j =1 k=1 j =1 k=1 i i i i yi2 = ˙ ˙ lj θj sin θj ˙ lk θk sin θk = ˙ ˙ lj lk θj θk sin θj sin θk (2.629) j =1 k=1 j =1 k=1to calculate the velocity vi of the mass mi : vi2 = xi2 + yi2 ˙ ˙ i i = ˙ ˙ lj lk θj θk (cos θj cos θk + sin θj sin θk ) j =1 k=1 i i = ˙ ˙ lj lk θj θk cos(θj − θk ) j =1 k=1 i i i 2˙ ˙ ˙ = lr θr2 + 2 lj lk θj θk cos(θj − θk ) (2.630) r=1 j =1 k=j +1Now, we calculate the kinetic energy, K, of the chain: n 1 K= mi vi2 2 i=1 n i i i 1 2˙ = mi lr θr2 + 2 ˙ ˙ lj lk θj θk cos(θj − θk ) 2 i=1 r=1 j =1 k=j +1 n i n i i 1 2˙ = mi lr θr2 + ˙ ˙ mi lj lk θj θk cos(θj − θk ) (2.631) 2 i=1 r=1 i=1 j =1 k=j +1The potential energy of the ith pendulum is related to mi , i Vi = mi gyi = −mi g lj cos θj (2.632) j =1
134 2 Vibration Dynamicsand, therefore, the potential energy of the chain is n n i V= mi gyi = − mi glj cos θj (2.633) i=1 i=1 j =1To ﬁnd the equations of motion for the chain, we may use the Lagrangean L L=K −V (2.634)and apply the Lagrange equation d ∂L ∂L − =0 s = 1, 2, . . . , n (2.635) dt ˙ ∂ qs ∂qsor d ∂K ∂K ∂V − + = 0 s = 1, 2, . . . , n (2.636) dt ˙ ∂ qs ∂qs ∂qsExample 86 (Mechanical energy) If a system of masses mi are moving in a poten-tial force ﬁeld Fmi = −∇i V (2.637)their Newton equations of motion will be ¨ mi ri = −∇i V i = 1, 2, . . . , n (2.638) ˙The inner product of equations of motion with ri and adding the equations n n ˙ ¨ mi ri · ri = − ˙ ri · ∇i V (2.639) i=1 i=1and then integrating over time n n 1 ˙ ˙ mi ri · ri = − ri · ∇i V (2.640) 2 i=1 i=1shows that n ∂V ∂V ∂V K =− xi + yi + zi = −V + E (2.641) ∂xi ∂yi ∂zi i=1where E is the constant of integration. E is called the mechanical energy of thesystem and is equal to kinetic plus potential energies: E =K +V (2.642)
2.5 Dissipation Function 1352.5 Dissipation FunctionThe Lagrange equation, d ∂K ∂K − = Fr r = 1, 2, . . . , n (2.643) dt ˙ ∂ qr ∂qror d ∂L ∂L − = Qr r = 1, 2, . . . , n (2.644) dt ˙ ∂ qr ∂qras introduced in Eqs. (2.522) and (2.571) can both be applied to ﬁnd the equationsof motion of a vibrating system. However, for small and linear vibrations, we mayuse a simpler and more practical Lagrange equation, thus: d ∂K ∂K ∂D ∂V − + + = fr r = 1, 2, . . . , n (2.645) dt ˙ ∂ qr ∂qr ˙ ∂ qr ∂qrwhere K is the kinetic energy, V is the potential energy, and D is the dissipationfunction of the system n n 1 1 ˙ ˙ K = qT [m]q = ˙ ˙ qi mij qj (2.646) 2 2 i=1 j =1 n n 1 1 V = qT [k]q = qi kij qj (2.647) 2 2 i=1 j =1 n n 1 1 ˙ ˙ D = qT [c]q = ˙ ˙ qi cij qj (2.648) 2 2 i=1 j =1and fr is the applied force on the mass mr .Proof Consider a one DOF mass–spring–damper vibrating system. When viscousdamping is the only type of damping in the system, we may employ a functionknown as the Rayleigh dissipation function 1 D = cx 2 ˙ (2.649) 2to ﬁnd the damping force fc by differentiation: ∂D fc = − (2.650) ˙ ∂xRemembering that the elastic force fk can be found from a potential energy V ∂V fk = − (2.651) ∂x
136 2 Vibration DynamicsFig. 2.36 A one DOF forcedmass–spring–damper systemthen the generalized force F can be separated to ∂D ∂V F = fc + fk + f = − − +f (2.652) ˙ ∂x ∂xwhere f is the non-conservative applied force on mass m. Substituting (2.652) in(2.643) d ∂K ∂K ∂D ∂V − =− − +f (2.653) dt ˙ ∂x ∂x ˙ ∂x ∂xgives us the Lagrange equation for a viscous damped vibrating system. d ∂K ∂K ∂D ∂V − + + =f (2.654) dt ˙ ∂x ∂x ˙ ∂x ∂x For vibrating systems with n DOF, the kinetic energy K, potential energy V ,and dissipating function D are as (2.646)–(2.648). Applying the Lagrange equationto the n DOF system would result n second-order differential equations (2.645).Example 87 (A one DOF forced mass–spring–damper system) Figure 2.36 illus-trates a single DOF mass–spring–damper system with an external force f appliedon the mass m. The kinetic and potential energies of the system, when it is in motion,are 1 K = mx 2 ˙ (2.655) 2 1 V = kx 2 (2.656) 2and its dissipation function is 1 ˙ D = cx 2 (2.657) 2
2.5 Dissipation Function 137Substituting (2.655)–(2.657) into the Lagrange equation (2.645) provides us withthe equation of motion: d (mx) + cx + kx = f ˙ ˙ (2.658) dtbecause ∂K ∂K ∂D ∂V = mx ˙ =0 = cx ˙ = kx (2.659) ˙ ∂x ∂x ˙ ∂x ∂xExample 88 (An eccentric excited one DOF system) An eccentric excited one DOFsystem is shown in Fig. 3.30 with mass m supported by a suspension made up ofa spring k and a damper c. There is also a mass me at a distance e that is rotatingwith an angular velocity ω. We may ﬁnd the equation of motion by applying theLagrange method. The kinetic energy of the system is 1 1 1 K = (m − me )x 2 + me (x + eω cos ωt)2 + me (−eω sin ωt)2 ˙ ˙ (2.660) 2 2 2 ˙because the velocity of the main vibrating mass m − me is x, and the velocity of theeccentric mass me has two components x + eω cos ωt and −eω sin ωt. The potential ˙energy and dissipation function of the system are 1 1 V = kx 2 D = cx 2 ˙ (2.661) 2 2Applying the Lagrange equation (2.645), ∂K = mx + me eω cos ωt ˙ (2.662) ∂x˙ d ∂K = mx − me eω2 sin ωt ¨ (2.663) dt ˙ ∂x ∂D = cx ˙ (2.664) ˙ ∂x ∂V = kx (2.665) ∂xprovides us with the equation of motion mx + cx + kx = me eω2 sin ωt ¨ ˙ (2.666)which is the same as Eq. (3.267).Example 89 (An eccentric base excited vibrating system) Figure 2.37 illustratesa one DOF eccentric base excited vibrating system. A mass m is mounted on aneccentric excited base by a spring k and a damper c. The base has a mass mb with
138 2 Vibration DynamicsFig. 2.37 A one DOFeccentric base excitedvibrating systeman attached unbalance mass me at a distance e. The mass me is rotating with anangular velocity ω. We may derive the equation of motion of the system by applying Lagrangemethod. The required functions are 1 1 K = mx 2 + (mb − me )y 2 ˙ ˙ 2 2 1 1 + me (y − eω cos ωt)2 + me (eω sin ωt)2 ˙ (2.667) 2 2 1 V = k(x − y)2 (2.668) 2 1 D = c(x − y)2 ˙ ˙ (2.669) 2Applying the Lagrange method (2.645) provides us with the equations mx + c(x − y) + k(x − y) = 0 ¨ ˙ ˙ (2.670) mb y + me eω2 sin ωt − c(x − y) − k(x − y) = 0 ¨ ˙ ˙ (2.671)because ∂K = mx ˙ (2.672) ∂x˙ d ∂K = mx ¨ (2.673) dt ˙ ∂x ∂D = c(x − y) ˙ ˙ (2.674) ˙ ∂x ∂V = k(x − y) (2.675) ∂x ∂K = mb y − me eω cos ωt ˙ (2.676) ˙ ∂y d ∂K = mb y + me eω2 sin ωt ¨ (2.677) dt ˙ ∂y
2.5 Dissipation Function 139 ∂D = −c(x − y) ˙ ˙ (2.678) ˙ ∂y ∂V = −k(x − y) (2.679) ∂yUsing z = x − y, we may combine Eqs. (2.670) and (2.671) to ﬁnd the equation ofrelative motion mmb mme z + c˙ + kz = ¨ z eω2 sin ωt (2.680) mb + m mb + mwhich is equal to z + 2ξ ωn z + ωn z = εeω2 sin ωt ¨ ˙ 2 (2.681) me ε= (2.682) mbExample 90 (Generalized forces) 1. Elastic force: An elastic force is a recoverable force from an elastic body that has changed itsinternal energy. An elastic body is a body for which any produced work is stored inthe body in the form of internal energy, and is recoverable. Therefore, the variationof the internal potential energy of the body, V = V (q, t) would be n δV = −δW = Qi δqi (2.683) i=1where qi is the generalized coordinate of the particle i of the body, δW is the virtualwork of the generalized elastic force Q: ∂V Qi = − (2.684) ∂qi 2. Dissipation force: A dissipative force between two bodies is proportional to and in opposite direc-tion of the relative velocity vector v between two bodies: vi Qi = −ci fi (vi ) (2.685) viThe coefﬁcient ci is a constant, fi (vi ) is the velocity function of the force, and viis the magnitude of the relative velocity: 3 vi = 2 vij (2.686) j =1
140 2 Vibration DynamicsThe virtual work of the dissipation force is n1 δW = Qi δqi (2.687) i=1 n1 ∂vk Qi = − ck fk (vk ) (2.688) ˙ ∂ qi k=1where n1 is the total number of dissipation forces. By introducing the dissipationfunction D as n1 vi D= ck fk (zk ) dz (2.689) i=1 0we have ∂D Qi = − (2.690) ˙ ∂ qiThe dissipation power P of the dissipation force Qi is n n ∂D P= Qi qi = ˙ ˙ qi (2.691) ˙ ∂ qi i=1 i=12.6 QuadraturesIf [m] is an n × n square matrix and x is an n × 1 vector, then S is a scalar functioncalled quadrature and is deﬁned by S = xT [m]x (2.692)The derivative of the quadrature S with respect to the vector x is ∂S = [m] + [m]T x (2.693) ∂xKinetic energy K, potential energy V , and dissipation function D are quadratures 1 ˙ K = xT [m]˙x (2.694) 2 1 V = xT [k]x (2.695) 2 1 ˙ D = xT [c]˙ x (2.696) 2
2.6 Quadratures 141and, therefore, ∂K 1 = [m] + [m]T x ˙ (2.697) ˙ ∂x 2 ∂V 1 = [k] + [k]T x (2.698) ∂x 2 ∂D 1 ˙ = [c] + [c]T x (2.699) ˙ ∂x 2Employing quadrature derivatives and the Lagrange method, d ∂K ∂K ∂D ∂V + + + =F (2.700) ˙ dt ∂ x ∂x ˙ ∂x ∂x δW = FT ∂x (2.701)the equation of motion for a linear n degree-of-freedom vibrating system becomes [m]¨ + [c]˙ + [k]x = F x x (2.702)where [m], [c], [k] are symmetric matrices: 1 [m] = [m] + [m]T (2.703) 2 1 [c] = [c] + [c]T (2.704) 2 1 [k] = [k] + [k]T (2.705) 2Quadratures are also called Hermitian forms.Proof Let us deﬁne a general asymmetric quadrature as S = xT [a]y = xi aij yj (2.706) i jIf the quadrature is symmetric, then x = y and S = xT [a]x = xi aij xj (2.707) i jThe vectors x and y may be functions of n generalized coordinates qi and time t: x = x(q1 , q2 , . . . , qn , t) (2.708) y = y(q1 , q2 , . . . , qn , t) (2.709) T q = q1 q2 · · · qn (2.710)
142 2 Vibration DynamicsThe derivative of x with respect to q is a square matrix ⎡ ∂x1 ∂x2 ∂xn ⎤ ∂q1 ∂q1 ··· ∂q1 ⎢ ∂x1 ⎥ ∂x ⎢ ∂q ∂x2 ··· ...⎥ = ⎢ 2 ∂q2 ⎥ (2.711) ∂q ⎢ . . . ⎣ ··· ... ⎥ ···⎦ ∂x1 ∂xn ∂qn ··· ··· ∂qnwhich can also be expressed by ⎡ ∂x ⎤ ∂q1 ⎢ ∂x ⎥ ∂x ⎢ ∂q ⎥ = ⎢ 2⎥ (2.712) ∂q ⎢ . . . ⎥ ⎣ ⎦ ∂x ∂qnor ∂x ∂x1 ∂x2 ∂xn = ∂q ∂q ... ∂q (2.713) ∂qThe derivative of S with respect to an element of qk is ∂S ∂ = xi aij yj ∂qk ∂qk i j ∂xi ∂yj = aij yj + xi aij ∂qk ∂qk i j i j ∂xi ∂yj = aij yj + aij xi ∂qk ∂qk j i i j ∂xi ∂yi = aij yj + aj i xj (2.714) ∂qk ∂qk j i j iand, hence, the derivative of S with respect to q is ∂S ∂x ∂y T = [a]y + [a] x (2.715) ∂q ∂q ∂q If S is a symmetric quadrature, then ∂S ∂ T ∂x ∂x T = x [a]x = [a]x + [a] x (2.716) ∂q ∂q ∂q ∂q
2.6 Quadratures 143and if q = x, then the derivative of a symmetric S with respect to x is ∂S ∂ T ∂x ∂x = x [a]x = [a]x + [a]T x ∂x ∂x ∂x ∂x = [a]x + [a]T x = [a] + [a]T x (2.717) If [a] is a symmetric matrix, then [a] + [a]T = 2[a] (2.718)however, if [a] is not a symmetric matrix, then [a] = [a] + [a]T is a symmetricmatrix because a ij = aij + aj i = aj i + aij = a j i (2.719)and, therefore, [a] = [a]T (2.720) Kinetic energy K, potential energy V , and dissipation function D can be ex-pressed by quadratures: 1 ˙ K = xT [m]˙x (2.721) 2 1 V = xT [k]x (2.722) 2 1 ˙ D = xT [c]˙ x (2.723) 2Substituting K, V , and D in the Lagrange equation provides us with the equationsof motion: d ∂K ∂K ∂D ∂V F= + + + ˙ dt ∂ x ∂x ˙ ∂x ∂x 1 d ∂ T 1 ∂ T 1 ∂ T = ˙ x [m]˙ + x ˙ x [c]˙ + x x [k]x ˙ 2 dt ∂ x ˙ 2 ∂x 2 ∂x 1 d = ˙ ˙ [m] + [m]T x + [c] + [c]T x + [k] + [k]T x 2 dt 1 1 1 = ¨ ˙ [m] + [m]T x + [c] + [c]T x+ [k] + [k]T x 2 2 2 = [m]¨ + [c]˙ + [k]x x x (2.724)where 1 [m] = [m] + [m]T (2.725) 2
144 2 Vibration DynamicsFig. 2.38 A quarter carmodel with driver 1 [c] = [k] + [k]T (2.726) 2 1 [k] = [c] + [c]T (2.727) 2 From now on, we assume that every equation of motion is found from the La-grange method to have symmetric coefﬁcient matrices. Hence, we show the equa-tions of motion, thus: [m]¨ + [c]˙ + [k]x = F x x (2.728)and use [m], [c], [k] as a substitute for [m], [c], [k]: [m] ≡ [m] (2.729) [c] ≡ [c] (2.730) [k] ≡ [k] (2.731)Symmetric matrices are equal to their transpose: [m] ≡ [m]T (2.732) [c] ≡ [c] T (2.733) [k] ≡ [k]T (2.734)Example 91 (A quarter car model with driver’s chair) Figure 2.38 illustrates aquarter car model plus a driver, which is modeled by a mass md over a linear cushionabove the sprung mass ms .
2.6 Quadratures 145 Assuming y=0 (2.735)we ﬁnd the free vibration equations of motion. The kinetic energy K of the systemcan be expressed by 1 1 1 K = mu xu + ms xs + md xd ˙2 ˙2 ˙2 2 2 2 ⎡ ⎤⎡ ⎤ mu 0 0 ˙ xu 1 = xu xs xd ⎣ 0 ms 0 ⎦ ⎣ xs ⎦ ˙ ˙ ˙ ˙ 2 0 0 md ˙ xd 1 ˙ = xT [m]˙ x (2.736) 2and the potential energy V can be expressed as 1 1 1 V = ku (xu )2 + ks (xs − xu )2 + kd (xd − xs )2 2 2 2 ⎡ ⎤⎡ ⎤ ku + ks −ks 0 xu 1 = xu xs xd ⎣ −ks ks + kd −kd ⎦ ⎣ xs ⎦ 2 0 −k k x d d d 1 = xT [k]x (2.737) 2Similarly, the dissipation function D can be expressed by 1 1 1 D = cu (xu )2 + cs (xs − xu )2 + cd (xd − xs )2 ˙ ˙ ˙ ˙ ˙ 2 2 2 ⎡ ⎤⎡ ⎤ cu + cs −cs 0 ˙ xu 1 = xu xs xd ⎣ −cs ˙ ˙ ˙ cs + cd −cd ⎦ ⎣ xs ⎦ ˙ 2 0 −c c ˙ x d d d 1 ˙ = xT [c]˙ x (2.738) 2Employing the quadrature derivative method, we may ﬁnd the derivatives of K, V ,and D with respect to their variable vectors: ⎡ ⎤ ˙ xu ∂K 1 1 T ⎣ ⎦ ˙ = [m] + [m] x = [k] + [k] T ˙ xs ∂x ˙ 2 2 ˙ x d ⎡ ⎤⎡ ⎤ mu 0 0 ˙ xu =⎣ 0 ms 0 ⎦ ⎣ xs ⎦ ˙ (2.739) 0 0 md ˙ xd
146 2 Vibration Dynamics ⎡ ⎤ xu ∂V 1 1 = [k] + [k]T x = [k] + [k]T ⎣ xs ⎦ ∂x 2 2 xd ⎡ ⎤⎡ ⎤ ku + ks −ks 0 xu = ⎣ −ks ks + kd −kd ⎦ ⎣ xs ⎦ (2.740) 0 −kd kd xd ∂D 1 ˙ = [c] + [c]T x ˙ ∂x 2 ⎡ ⎤ ˙ xu 1 = [c] + [c]T ⎣ xs ⎦ ˙ 2 ˙ x d ⎡ ⎤⎡ ⎤ cu + cs −cs 0 ˙ xu = ⎣ −cs cs + cd −cd ⎦ ⎣ xs ⎦ ˙ (2.741) 0 −cd cd ˙ xdTherefore, we ﬁnd the system’s free vibration equations of motion: [m]¨ + [c]˙ + [k]x = 0 x x (2.742) ⎡ ⎤⎡ ⎤ ⎡ ⎤⎡ ⎤ mu 0 0 ¨ xu cu + cs −cs 0 ˙ xu ⎣0 ms 0 ⎦ ⎣ xs ⎦ + ⎣ −cs ¨ cs + cd −cd ⎦ ⎣ xs ⎦ ˙ 0 0 md ¨ xd 0 −cd cd ˙ xd ⎡ ⎤⎡ ⎤ ku + ks −ks 0 xu + ⎣ −ks ks + kd −kd ⎦ ⎣ xs ⎦ = 0 (2.743) 0 −kd kd xdExample 92 (Different [m], [c], and [k] arrangements) Mass, damping, and stiff-ness matrices [m], [c], [k] for a vibrating system may be arranged in different formswith the same overall kinetic energy K, potential energy V , and dissipation functionD. For example, the potential energy V for the quarter car model that is shown inFig. 2.38 may be expressed by different [k]: 1 1 1 V = ku (xu )2 + ks (xs − xu )2 + kd (xd − xs )2 (2.744) 2 2 2 ⎡ ⎤ ku + ks −ks 0 1 V = xT ⎣ −ks ks + kd −kd ⎦ x (2.745) 2 0 −k kd d ⎡ ⎤ k + ks −2ks 0 1 T⎣ u V= x 0 ks + kd −2kd ⎦ x (2.746) 2 0 0 kd
2.6 Quadratures 147 ⎡ ⎤ k + ks 0 0 1 T⎣ u V= x −2ks ks + kd 0 ⎦x (2.747) 2 0 −2kd kd The matrices [m], [c], and [k], in K, D, and V , may not be symmetric; however, ˙ ˙the matrices [m], [c], and [k] in ∂K/∂ x, ∂D/∂ x, ∂V /∂x are always symmetric. When a matrix [a] is diagonal, it is symmetric and [a] = [a] (2.748)A diagonal matrix cannot be written in different forms. The matrix [m] in Exam-ple 91 is diagonal and, hence, K has only one form, (2.736).Example 93 (Quadratic form and sum of squares) We can write the sum of xi2 in thequadratic form, n xi2 = xT x = xT I x (2.749) i=1where xT = x1 x2 x3 · · · xn (2.750)and I is an n × n identity matrix. If we are looking for the sum of squares around amean value x0 , then n n n n 1 (xi − x0 )2 = xi2 − nx0 = xT x − 2 xi xi n i=1 i=1 i=1 i=1 1 T = xT x − x 1n 1T x n n 1 = xT I − 1n 1T x n (2.751) nwhere 1T = 1 1 n ··· 1 (2.752)Example 94 (Positive deﬁnite matrix) A matrix [a] is called positive deﬁnite ifxT [a]x > 0 for all x = 0. A matrix [a] is called positive semideﬁnite if xT [a]x ≥ 0for all x. The kinetic energy is positive deﬁnite, and this means we cannot have K = 0 ˙unless x = 0. The potential energy is positive semideﬁnite and this means that wehave V ≥ 0 as long as x > 0; however, it is possible to have a especial x0 > 0 atwhich V = 0. A positive deﬁnite matrix, such as the mass matrix [m], satisﬁes Silvester’s crite-rion, which is that the determinant of [m] and determinant of all the diagonal minors
148 2 Vibration Dynamicsmust be positive: m11 m12 ··· m1n m21 m22 ··· m2n Δn = . .. .. . >0 . . . . . . mn1 mn2 ··· mnn m11 m12 ··· m1,n−1 m21 m22 ··· m2,n−1 Δn−1 = . .. .. . >0 . . . . . . mn1 mn2 ··· mn−1,n−1 m11 m12 · · · Δ2 = >0 Δ1 = m11 > 0 (2.753) m21 m22Example 95 (Symmetric matrices) Employing the Lagrange method guaranteesthat the coefﬁcient matrices of equations of motion of linear vibrating systems aresymmetric. A matrix [A] is symmetric if the columns and rows of [A] are inter-changeable, so [A] is equal to its transpose: [A] = [A]T (2.754) The characteristic equation of a symmetric matrix [A] is a polynomial for whichall the roots are real. Therefore, the eigenvalues of [A] are real and distinct and [A]is diagonalizable. Any two eigenvectors that come from distinct eigenvalues of the symmetric ma-trix [A] are orthogonal.Example 96 (Linearization of energies) The kinetic energy of a system with n par-ticles is n 3n 1 1 K= mi xi2 + yi2 + zi = ˙ ˙ ˙2 ˙i mi u2 (2.755) 2 2 i=1 i=1Expressing the conﬁguration coordinate ui in terms of generalized coordinates qj ,we have n ∂ui ∂ui ˙ ui = qs + ˙ s = 1, 2, . . . , N (2.756) ∂qs ∂t s=1
2.6 Quadratures 149Therefore, the kinetic energy in terms of generalized coordinates is N n 2 1 ∂ui ∂ui K= mi qs + ˙ 2 ∂qs ∂t i=1 s=1 n n n 1 = aj k qj qk + ˙ ˙ bj qj + c ˙ (2.757) 2 j =1 k=1 j =1where N ∂ui ∂ui aj k = mi (2.758) ∂qj ∂qk i=1 N ∂ui ∂ui bj = mi (2.759) ∂qj ∂t i=1 N 2 1 ∂ui c= mi (2.760) 2 ∂t i=1where n 2 n n ∂ui ∂ui ∂ui ∂ui ∂ui ∂ui qs + ˙ = qj + ˙ qk + ˙ ∂qs ∂t ∂qj ∂t ∂qk ∂t s=1 j =1 k=1 n n ∂ui ∂ui = ˙ ˙ qj qk ∂qj ∂qk j =1 k=1 n 2 ∂ui ∂ui ∂ui +2 ˙ qj + (2.761) ∂qj ∂t ∂t j =1Using these expressions, we may show the kinetic energy of the dynamic system: K = K0 + K1 + K2 (2.762)where N 2 1 ∂ui K0 = mi (2.763) 2 ∂t i=1 n N ∂ui ∂ui K1 = mi ˙ qj (2.764) ∂qj ∂t j =1 i=1
150 2 Vibration Dynamics n n N 1 ∂ui ∂ui K2 = mi ˙ ˙ qj qk (2.765) 2 ∂qj ∂qk j =1 k=1 i=1where N N ∂ui ∂ui ˙ ˙ ∂ ui ∂ ui akj = aj k = mi = mi (2.766) ∂qk ∂qj ˙ ˙ ∂ qk ∂ qj i=1 i=1If the coordinates ui do not depend explicitly on time t, then ∂ui /∂t = 0, and wehave n n n n 1 1 ∂ 2 K(0) K = K2 = aj k qj qk = ˙ ˙ ˙ ˙ qj qk 2 2 ∂qj ∂qi j =1 k=1 j =1 k=1 n n 1 = ˙ ˙ mij qj qk (2.767) 2 j =1 k=1 The kinetic energy is a scalar quantity and, because of (2.755), must be positivedeﬁnite. The ﬁrst term of (2.757) is a positive quadratic form. The third term of(2.757) is also a nonnegative quantity, as indicated by (2.760). The second term of ˙(2.755) can be negative for some qj and t. However, because of (2.755), the sum ofall three terms of (2.757) must be positive: 1 ˙ ˙ K = qT [m]q (2.768) 2 The generalized coordinate qi represent deviations from equilibrium. The poten-tial energy V is a continuous function of generalized coordinates qi and, hence, itsexpansion is n n n ∂V (0) 1 ∂ 2 V (0) V (q) = V (0) + qi + qi qj + · · · (2.769) ∂qi 2 ∂qj ∂qi i=1 j =1 i=1where ∂V (0)/∂qi and ∂ 2 V (0)/(∂qj ∂qi ) are the values of ∂V /∂qi and ∂ 2 V /(∂qj ∂qi )at q = 0, respectively. By assuming V (0) = 0, and knowing that the ﬁrst derivativeof V is zero at equilibrium ∂V = 0 i = 1, 2, 3, . . . , n (2.770) ∂qiwe have the second-order approximation n n 1 V (q) = kij qi qj (2.771) 2 j =1 i=1
2.7 Variational Dynamics 151 ∂ 2 V (0) kij = (2.772) ∂qj ∂qiwhere kij are the elastic coefﬁcients. The second-order approximation of V is zeroonly at q = 0. The expression (2.771) can also be written as a quadrature: 1 V = qT [k]q (2.773) 22.7 Variational Dynamics ˙Consider a function f of x(t), x(t), and t: f = f (x, x, t) ˙ (2.774)The unknown variable x(t), which is a function of the independent variable t, iscalled a path. Let us assume that the path is connecting the ﬁxed points x0 and xfduring a given time t = tf − t0 . So x = x(t) satisﬁes the boundary conditions x(t0 ) = x0 x(tf ) = xf (2.775)The time integral of the function f over x0 ≤ x ≤ xf is J (x) such that its valuedepends on the path x(t): tf J (x) = ˙ f (x, x, t) dt (2.776) t0where J (x) is called an objective function or a functional. The particular path x(t) that minimizes J (x) must satisfy the following equation: ∂f d ∂f − =0 (2.777) ∂x dt ∂ x˙This equation is the Lagrange or Euler–Lagrange differential equation and is ingeneral of second order.Proof To show that a path x = x (t) is a minimizing path for the functional J (x) = tf ˙ t0 f (x, x, t) dt with boundary conditions (2.775), we need to show that J (x) ≥ J x (2.778)for all continuous paths x(t). Any path x(t) satisfying the boundary conditions(2.775) is called an admissible path. To see that x (t) is the optimal path, we mayexamine the integral J for every admissible path. Let us deﬁne an admissible pathby superposing another admissible path y(t) onto x , x(t) = x + y(t) (2.779)
152 2 Vibration Dynamicswhere y(t0 ) = y(tf ) = 0 (2.780)and is a small parameter 1 (2.781)Substituting x(t) in J of Eq. (2.776) and subtracting from J (x ) provides us withΔJ : ΔJ = J x + y(t) − J x tf tf = f x + y, x ˙ + y, t dt − ˙ ˙ f x , x , t dt (2.782) t0 t0Let us expand f (x + y, x ˙ + y, t) about (x , x ) ˙ ˙ f x + y, x ˙ + y, t ˙ ∂f ∂f = f x ,x ,t + ˙ y +y ˙ ∂x ∂x˙ ∂ 2f ∂ 2f ∂ 2f + 2 y2 + 2y y ˙ + y2 2 ˙ dt + O 3 (2.783) ∂x 2 ∂x ∂ x˙ ∂x ˙and ﬁnd ΔJ = V1 + 2 V2 + O 3 (2.784)where tf ∂f ∂f V1 = y +y ˙ dt (2.785) t0 ∂x ∂x˙ tf ∂ 2f ∂ 2f ∂ 2f V2 = y2 + 2y y ˙ + y2 2 ˙ (2.786) t0 ∂x 2 ∂x ∂ x˙ ∂x ˙The ﬁrst integral, V1 , is called the ﬁrst variation of J , and the second integral, V2 , iscalled the second variation of J . All the higher variations are combined and shownas O( 3 ). If x is the minimizing path, then it is necessary that ΔJ ≥ 0 for everyadmissible y(t). If we divide ΔJ by and make → 0, then we ﬁnd a necessarycondition for x to be the optimal path as V1 = 0. This condition is equivalent to tf ∂f ∂f y +y ˙ dt = 0 (2.787) t0 ∂x ∂x˙By integrating by parts, we may write tf tf tf ∂f ∂f d ∂f ˙ y dt = y − y dt (2.788) t0 ∂x˙ ∂x˙ t0 t0 dt ∂x˙
2.7 Variational Dynamics 153Because of y(t0 ) = y(tf ) = 0, the ﬁrst term on the right-hand side is zero. Therefore,the minimization integral condition (2.787) for every admissible y(t) reduces to tf ∂f d ∂f y − dt = 0 (2.789) t0 ∂x dt ∂ x˙The terms in parentheses are continuous functions of t, evaluated on the optimal pathx , and they do not involve y(t). Therefore, the only way for the bounded integralof the parentheses, ( ∂f − dt ∂f˙ ), multiplied by a nonzero function y(t) from t0 and ∂x d ∂xtf , to be zero is ∂f d ∂f − =0 (2.790) ∂x dt ∂ x˙Equation (2.790) is a necessary condition for x = x (t) to be a solution of the min-imization problem (2.776). This differential equation is called the Euler–Lagrangeor Lagrange equation. The second necessary condition to have x = x (t) as a min-imizing solution is that the second variation, evaluated on x (t), must be negative.Example 97 (Basic lemma) Consider two ﬁxed points x1 and x2 (> x1 ) and g(x)as a continuous function for x1 ≤ x ≤ x2 . If x2 f (x)g(x) dx = 0 (2.791) x1for every choice of the continuous and differentiable function f (x) for which f (x1 ) = f (x2 ) = 0 (2.792)then g(x) = 0 (2.793)identically in x1 ≤ x ≤ x2 . This result is called the basic lemma. To prove the lemma, let us assume that (2.793) does not hold. Therefore, supposethere is a particular x0 of x in x1 ≤ x0 ≤ x2 for which g(x0 ) = 0. At the moment,let us assume that g(x0 ) > 0. Because g(x) is continuous, there must be an intervalaround x0 such as x10 ≤ x0 ≤ x20 in which g(x) > 0 everywhere. However, (2.791)cannot then hold for every permissible choice of f (x). A similar contradiction isreached if we assume g(x0 ) < 0. Therefore, the lemma is correct. 2 2Example 98 (Lagrange equation for extremizing J = ˙ 1 x dt) The Lagrangeequation for extremizing the functional J , 2 J= ˙ x 2 dt (2.794) 1
154 2 Vibration Dynamicsis ∂f d ∂f − = −x = 0 ¨ (2.795) ∂x dt ∂ x ˙which shows that the optimal path is x = C1 t + C 2 (2.796)The boundary conditions x(1), x(2) provide C1 and C2 . For example, assumingboundary conditions x(1) = 0, x(2) = 3 provides us with x = 3t − 3 (2.797)Example 99 (Geodesics) The problem of determining the shortest path be-tween two given points at the same level of a quantitative characteristic is calledthe geodesic problem. An example of a geodesic problem is: What is the shortest arc lying on the surfaceof a sphere and connecting two given points? We can generalize the problem asfollows. Given two points on the surface of g(x, y, z) = 0 (2.798)what is the equation of the shortest arc lying on (2.798) and connecting the points?Let us express the equation of the surface in parametric form using parameters uand v: x = x(u, v) y = (u, v) z = z(u, v) (2.799)the differential of the arc length may be written as (ds)2 = (dx)2 + (dy)2 + (dz)2 = P (u, v)(du)2 + 2Q(u, v) du dv + R(u, v)(dv)2 (2.800)where 2 2 2 ∂x ∂y ∂z P (u, v) = + + (2.801) ∂u ∂u ∂u 2 2 2 ∂x ∂y ∂z R(u, v) = + + (2.802) ∂v ∂v ∂v ∂x ∂x ∂y ∂y ∂z ∂z Q(u, v) = + + (2.803) ∂u ∂v ∂u ∂v ∂u ∂vIf the curves u = const are orthogonal to the curves v = const, the quantity Q iszero. If the given ﬁxed points on the surface are (u1 , v1 ) and (u2 , v2 ) with u2 > u1and we express the arcs and points by v = v(u) v(u1 ) = v1 v(u2 ) = v2 (2.804)
2.7 Variational Dynamics 155then the length of the arc is given by u2 2 dv dv J= P (u, v) + 2Q(u, v) + R(u, v) (2.805) u1 du duOur problem, then, is to ﬁnd the function v(u) that renders the integral (2.805) aminimum. Employing the Lagrange equation, we ﬁnd dv 2 ∂R ∂P + 2 du ∂Q + dv du dv Q + R du dv ∂v ∂v ∂v − =0 (2.806) dv 2 du dv 2 2 P + 2Q du + R du dv P + 2Q du + R dv duIn the special case where P , Q, and R are explicitly functions of u alone, this lastresult becomes Q + R du dv = C1 (2.807) dv 2 P + 2Q du + R dv duIf the curves u = const are orthogonal to the curves v = const, we have √ P du v = C1 (2.808) R 2 − C1 R 2Still supposing that Q = 0 but having P and R as explicit functions of v alone, wehave √ R dv u = C1 (2.809) P 2 − C1 P 2 As a particular case let us consider the geodesic connecting two points on asphere with radius r. The most convenient parameters u and v for describing po-sition on the sphere surface are the colatitude θ and the longitude ϕ: x = r cos θ sin ϕ y = r sin θ sin ϕ z = r cos ϕ (2.810)where θ is the angle between the positive z-axis and the line drawn from the spherecenter to the designated point and ϕ is the angle between the (x, z)-plane and thehalf plane bounded by the z-axis and containing the designated point. Therefore, P = r 2 sin2 θ R=r Q=0 (2.811) dv cot θ ϕ = C1 = − sin−1 + C2 (2.812) r 2 r 2 sin θ 4 − C1 sin2 θ 2 C1 −1from which it follows that z r sin θ cos ϕ sin C2 − r sin θ sin ϕ cos C2 − =0 (2.813) r 2 C1 −1
156 2 Vibration DynamicsFig. 2.39 A curve joiningpoints P1 and P2 and africtionless sliding pointUsing (2.810) we ﬁnd that the sphere geodesic lies on the following plane, whichpasses through the center of the sphere: z x sin C2 − y cos C2 − =0 (2.814) r 2 C1 −1Therefore the shortest arc connecting two points on the surface of a sphere is theintersection of the sphere with the plane containing the given points and the centerof the sphere. Such an arc is called a great-circle arc.Example 100 (Brachistochrone problem) We may use the Lagrange equation andﬁnd the frictionless curve joining points P1 and P2 , as shown in Fig. 2.39, alongwhich a particle falling from rest due to gravity travels from the higher to the lowerpoint in minimum time. This is called the brachistochrone problem. If v is the velocity of the falling point along the curve, then the time requiredto fall an arc length ds is ds/v. Then the objective function to ﬁnd the curve ofminimum time is s2 ds J= (2.815) s1 vHowever, dy ds = 1 + y 2 dx y = (2.816) dxand according to the law of conservation of energy, we have v= 2gy (2.817)Therefore, the objective function simpliﬁes to x2 1+y 2 J= dx (2.818) x1 2gyApplying the Lagrange equations, we ﬁnd y 1 + y 2 = 2R (2.819)
2.7 Variational Dynamics 157where R is a constant. The optimal curve starting from y(0) = 0 can be expressedby the two parametric equations x = R(θ − sin θ ) y = R(1 − cos θ ) (2.820)The optimal curve is a cycloid. The name of the problem is derived from the Greek words “βραχισ τ oζ ,” mean-ing “shortest,” and “χρoνoζ ,” meaning “time.” The brachistochrone problem wasoriginally discussed in 1630 by Galileo Galilei (1564–1642) and was solved in 1696by Johann and Jacob Bernoulli.Example 101 (Lagrange multiplier) Assume f (x) is deﬁned on an open intervalx ∈ (a, b) and has continuous ﬁrst and second-order derivatives in some neighbor-hood of x0 ∈ (a, b). The point x0 is a local extremum of f (x) if df (x0 ) =0 (2.821) dxAssume that f (x) = 0, x ∈ Rn , and gi (x) = 0, i = 1, 2, . . . , j , are functions deﬁnedon an open region Rn and have continuous ﬁrst- and second-order derivatives in Rn .The necessary condition that x0 is an extremum of f (x) subject to the constraintsgi (x) = 0 is that there exist j Lagrange multipliers λi , i = 1, 2, . . . , j , such that ∇ s+ λi gi = 0 (2.822) For example, we can ﬁnd the minimum of f , f = 1 − x1 − x2 2 2 (2.823)subject to the constraint g g = x1 + x 2 − 1 = 0 2 (2.824)by ﬁnding the gradient of f + λg: ∇ 1 − x1 − x2 + λ x 1 + x2 − 1 2 2 2 =0 (2.825)which leads to ∂f = −2x1 + 2λx1 = 0 (2.826) ∂x1 ∂f = −2x2 + λ = 0 (2.827) ∂x2To ﬁnd the three unknowns x1 , x2 , and λ, we employ Eqs. (2.826), (2.827), and(2.824). There are two sets of solutions as follows: x1 = 0 √ x2 = 1 λ=2 (2.828) x1 = ±1/ 2 x2 = 1/2 λ=1
158 2 Vibration DynamicsFig. 2.40 Dido problem is toﬁnd a planar curve y(x) witha constant length l tomaximize the enclosed areaExample 102 (Dido problem) Consider a planar curve y(x) with a constant length lthat connects the points (−R, 0) and (R, 0) as shown in Fig. 2.40. The Dido problemis to ﬁnd the y(x) that maximized the enclosed area. The objective function of theDido problem is R J= y dx (2.829) −RHowever, the constant length provides us with a constraint equation: R R l= ds = 1 + y 2 dx (2.830) −R −R dy ds = 1 + y 2 dx y = (2.831) dxTherefore, using the Lagrange multiplier λ, the objective function with constraintwould be R J= f y, y , x + λg y, y , x dx (2.832) −R f =y g= 1+y 2 (2.833)The Lagrange equation for the constraint objective function (2.832) is ∂f d ∂f ∂g d ∂g − +λ − =0 (2.834) ∂y dx ∂y ∂y dx ∂yEquation (2.834) leads to 1 d y = (2.835) λ dx 1+y 2This differential equation must be solved to determine the maximizing curve y(x).First integration provides us with λy = x + C1 (2.836) 1+y 2
2.7 Variational Dynamics 159Solving this equation for y yields ±(x + C1 ) y = (2.837) λ2 − (x + C1 )and a second integration yields y = ± λ2 − (x + C1 ) + C2 (2.838)Satisfying the boundary conditions (−R, 0) and (R, 0), we have C1 = C 2 = 0 λ=R (2.839)which indicates that the function y(x) is x 2 + y 2 = R2 (2.840)It is a circle with center at O and radius R.Example 103 (Several independent variables) We now derive the differen-tial equations that must be satisﬁed by the twice-differentiable functions q1 (t),q2 (t), . . . , qn (t) that extremize the integral J : t2 J= ˙ ˙ ˙ f (q1 , q2 , . . . , qn , q1 , q2 , . . . , qn , t) dt (2.841) t1The functions q1 (t), q2 (t), . . . , qn (t) achieve given the values at the ﬁxed limits ofintegration t1 and t2 , where t1 < t2 . Let us show the optimal functions by qi (t), i = 1, 2, . . . , n. We may examinethe integral J for every admissible function. An admissible function may be deﬁnedby qi (t) = qi + yi (t) (2.842)where yi (t1 ) = yi (t2 ) = 0 (2.843)and is a small parameter, 1 (2.844) ˙Consider a function f = f (qi , qi , t). The variables qi (t) satisfy the boundary con-ditions q1 (t1 ) = q1 q(t2 ) = q2 (2.845)
160 2 Vibration DynamicsSubstituting qi (t) in J and subtracting from (2.841) yields ΔJ = J qi + yi (t) − J qi tf tf = f qi + yi , qi + yi , t dt − ˙ ˙ ˙ f qi , qi , t dt (2.846) t0 t0Let us expand f (x + y, x ˙ + y, t) about (x , x ): ˙ ˙ f qi + yi , q i + yi , t ˙ ˙ ∂f ∂f = f qi , qi , t + ˙ yi + yi ˙ ∂qi ˙ ∂ qi ∂ 2f ∂ 2f ∂ 2f + 2 yi2 + 2yi yj ˙ + yi2 2 ˙ dt ∂qi2 ˙ ∂qi ∂ qj ˙ ∂ qi +O 3 (2.847)and ﬁnd ΔJ = V1 + 2 V2 + O 3 (2.848)where tf ∂f ∂f V1 = yi + yj ˙ dt (2.849) t0 ∂qi ˙ ∂ qj tf ∂ 2f ∂ 2f ∂ 2f V2 = yi2 + 2yi yj ˙ + yi2 2 ˙ (2.850) t0 ∂qi2 ˙ ∂qi ∂ qj ˙ ∂ qiIf we divide ΔJ by and make → 0, then we ﬁnd a necessary condition V1 = 0for qi to be the optimal path. By integrating V1 by parts, we may write tf t2 t2 ∂f ∂f d ∂f ˙ y1 dt = y1 − y1 dt (2.851) t0 ˙ ∂ q1 ˙ ∂ q1 t1 t1 dt ˙ ∂ q1Since y1 (t1 ) = y2 (t2 ) = 0, the ﬁrst term on the right-hand side is zero and the inte-gral of V1 reduces to t2 ∂f d ∂f y1 − dt = 0 (2.852) t1 ˙ ∂q1 dt ∂ q1The terms in parentheses are continuous functions of t evaluated on the optimal pathx , and they do not involve y1 (t). So, the only way for the bounded integral of the ∂fparentheses, ( ∂q1 − dt ∂∂f1 ), multiplied by a nonzero function y1 (t) to be zero is if d ˙ qthe parentheses are zero. Therefore, the minimization integral condition for everyadmissible y1 (t) is ∂f d ∂f − =0 (2.853) ˙ ∂q1 dt ∂ q1
2.8 Key Symbols 161Using a similar treatment of the successive pairs of terms of (2.851), we derive thefollowing n conditions to minimize (2.841): ∂f d ∂f − = 0 i = 1, 2, . . . , n (2.854) ∂qi ˙ dt ∂ qiTherefore, when a deﬁnite integral is given which contains n functions to be deter-mined by the condition that the integral be stationary, we can vary these functionsindependently. So, the Euler–Lagrange equation can be formed for each functionseparately. This provides us with n differential equations.2.8 Key Symbolsa≡x ¨ accelerationa, b distance, Fourier series coefﬁcientsa, b, w, h lengtha accelerationA, B weight factor, coefﬁcients for frequency responsesc damping[c] damping matrixce equivalent dampingC mass centerd position vector of the body coordinate framedf inﬁnitesimal forcedm inﬁnitesimal massdm inﬁnitesimal momentE mechanical energy, Young modulus of elasticityf = 1/T cyclic frequency [Hz]f, F, f, F forceFC Coriolis forceg gravitational accelerationH heightI moment of inertia matrixI1 , I2 , I3 principal moment of inertiak stiffnesske equivalent stiffnesskij element of row i and column j of a stiffness matrix[k] stiffness matrixK kinetic energyl directional lineL moment of momentumL=K −V Lagrangeanm massme eccentric mass, equivalent mass
162 2 Vibration Dynamicsmij element of row i and column j of a mass matrixmk spring massms sprung mass[m] mass matrixn number of coils, number of decibels, number of noteN natural numbersp pitch of a coilP powerr frequency ratior position vectort timet0 initial timeT periodTn natural periodv ≡ x, v ˙ velocityV potential energyx, y, z, x displacementx0 initial displacement ˙x0 initial velocity ˙ ˙ ˙x, y, z velocity, time derivative of x, y, zx¨ accelerationX amplitude of xz relative displacementGreekα, β, γ angle, angle of spring with respect to displacementδ deﬂection, angleδs static deﬂectionε mass ratioθ angular motion coordinateω, ω, Ω angular frequencyϕ, Φ phase angleλ eigenvalue2.9 Exercises 1. Equation of motion of vibrating systems. Determine the equation of motion of the systems in Figs. 2.41(a) and (b) by (a) using the energy method (b) using the Newton method (c) using the Lagrange method
2.9 Exercises 163Fig. 2.41 Two undamped discrete vibrating systemsFig. 2.42 A frictioncoefﬁcient measurementdeviceFig. 2.43 A slab on asemi-cylinder 2. Friction coefﬁcient measurement. Figure 2.42 illustrates two rollers that turn in opposite direction with equal angular speed ω. A slab of size t × w × l and mass m is put on the rollers. A small disturbance or misplacement will cause the slab to oscillate about the equilibrium position. If the coefﬁcient of friction between the roller and the slab is μ, determine (a) the equation of motion of the slab (b) the natural frequency of the slab’s oscillation for a given μ (c) the value of μ for a measured frequency of oscillation ω 3. A slab on a semi-cylinder. Determine the equation of motion of the slab t × w × l in Figs. 2.43 if (a) the thickness of the slab is ignorable, t = 0 (b) the thickness of the slab is not ignorable, t = 0 4. Velocity dependent friction. The device in problem 2 is to measure the friction coefﬁcient between the slab and the rollers. Assume the friction force is f = f0 − Cv where v is the relative velocity of slab and rollers.
164 2 Vibration DynamicsFig. 2.44 Spring connectedpendulum ˙ (a) Determine the equation of motion of the slab if x and x are assumed small. ˙ (b) Determine the equation of motion of the slab if x is comparable with Rω. (c) Determine the equation of motion of the slab if one roller is turning with ˙ angular speed of 2ω, assuming x is very small. 5. Moving on x-axis. The displacement of a particle moving along the x-axis is given by x = 0.01t 4 − t 3 + 4.5t 2 − 10 t ≥ 0 (a) Determine t1 at which x becomes positive. For how long does x remain positive after t = t1 ? (b) (c) How long does it take for x to become positive for the second time? (d) When and where does the particle reach its maximum acceleration? (e) Derive an equation to calculate its acceleration when its speed is given. 6. Kinetic energy of a rigid link. Consider a straight and uniform bar as a rigid link of a manipulator. The link has a mass m. Show that the kinetic energy of the bar can be expressed as 1 K = m(v1 · v1 + v1 · v2 + v2 · v2 ) 6 where v1 and v2 are the velocity vectors of the end points of the link. 7. Ideal spring connected pendulum. Determine the kinetic and potential energies of the pendulum in Fig. 2.44, at an arbitrary angle θ . The free length of the spring is l = a − b. 8. General spring connected pendulum. Determine the potential energy of the pendulum in Fig. 2.44, at an angle θ , if: (a) The free length of the spring is l = a − 1.2b. (b) The free length of the spring is l = a − 0.8b. 9. Moving on a cycloid. A particle is moving on a planar curve with the following parametric expres- sion: x = r(ωt − sin ωt) y = r(1 − cos ωt)
2.9 Exercises 165Fig. 2.45 Spring connectedrectilinear oscillatorFig. 2.46 Mathematicalmodel for cushion suspension (a) Determine the speed of the particle at time t. (b) Show that the magnitude of acceleration of the particle is constant. (c) Determine the tangential and normal accelerations of the particle. (d) Using ds = v dt, determine the length of the path that the particle travels up to time t. (e) Check if the magnitude of acceleration of the particle is constant for the following path: x = a(ωt − sin ωt) y = b(1 − cos ωt)10. Spring connected rectilinear oscillator. Determine the kinetic and potential energies of the oscillator shown in Fig. 2.45. The free length of the spring is a. (a) Express your answers in terms of the variable angle θ . (b) Express your answers in terms of the variable distance x. (c) Determine the equation of motion for large and small θ . (d) Determine the equation of motion for large and small x.11. Cushion mathematical model. Figure 2.46 illustrates a mathematical model for cushion suspension. Such a model can be used to analyze the driver’s seat, or a rubbery pad suspension. (a) Derive the equations of motion for the variables x and z and using y as a known input function. (b) Eliminate z and derive a third-order equation for x.12. Relative frequency. Consider a body B that is moving along the x-axis with a constant velocity u and every T seconds emits small particles which move with a constant velocity c along the x-axis. If f denotes the frequency and λ the distance between two
166 2 Vibration DynamicsFig. 2.47 A two DOFvibrating system successively emitted particles, then we have 1 c−u f= = T λ Now suppose that an observer moves along the x-axis with velocity v. Let us show the number of particles per second that the observer meets by the relative frequency f and the time between meeting the two successive particles by the relative period T , where c−v f = λ Show that v−u f ≈f 1− c13. Equation of motion of a multiple DOF system. Figure 2.47 illustrates a two DOF vibrating system. (a) Determine the K, V , and D functions. (b) Determine the equations of motion using the Lagrange method. (c) Rewrite K, V , and D in quadrature form. (d) Determine the natural frequencies and mode shapes of the system.14. Absolute and relative coordinates. Figure 2.48 illustrates two similar double pendulums. We express the motion of the left one using absolute coordinates θ1 and θ2 , and express the motion of the right one with absolute coordinate θ1 and relative coordinate θ2 . (a) Determine the equation of motion of the absolute coordinate double pendu- lum. (b) Determine the equation of motion of the relative coordinate double pendu- lum. (c) Compare their mass and stiffness matrices.15. Static equilibrium position. Figure 2.49 illustrates a combination of pendulums and springs.
2.9 Exercises 167Fig. 2.48 Two similar doublependulums, expressed byabsolute and relativecoordinatesFig. 2.49 A combination ofpendulums and springsFig. 2.50 An elasticpendulum (a) Determine the value of equilibrium θ0 and static stretch of spring δ0 if we assemble the system and let it go slowly. (b) Determine the equation of motion of the system in terms of x measured from the shown equilibrium position. (c) Determine the equation of motion of the system in terms of y.16. Elastic pendulum. Figure 2.50 illustrates an elastic pendulum. Such a pendulum has two DOF. (a) Determine the equations of motion using the energy method. (b) Determine the equations of motion using the Lagrange method. (c) Determine the equations of motion using the Newton–Euler method. (d) Linearize the equations of motion.
168 2 Vibration DynamicsFig. 2.51 (a) A slender as apendulum with variabledensity. (b) A simplependulum with the samelengthFig. 2.52 Four connectedpendulumsFig. 2.53 Two springconnected heavy discs17. Variable density. Figure 2.51(a) illustrates a slender as a pendulum with variable density, and Fig. 2.51(b) illustrates a simple pendulum with the same length. Determine the equivalent mass me is the mass density ρ = m/ l is (a) ρ = C1 z (b) ρ = C2 (l − z) (c) ρ = C3 (z − 2 )2 l (d) ρ = C4 ( 2 − (z − 2 ))2 l l18. Four pendulums are connected as shown in Fig. 2.52. (a) Determine the kinetic energy K, linearize the equation and ﬁnd the mass matrix [m]. (b) Determine the potential energy V , linearize the equation and ﬁnd the stiff- ness matrix [k]. (c) Determine the equations of motion using K and V and determine the sym- metric matrices [m] and [k].19. Two spring connected heavy discs. The two spring connected disc system of Fig. 2.53 is linear for small θ1 and θ2 . Find the equations of motion by energy and Lagrange methods.
2.9 Exercises 169Fig. 2.54 Two connecteddisc systemFig. 2.55 A pendulum and apegFig. 2.56 A pendulum oncircular wall20. Two connected heavy discs. Determine the equations of motion of the two connected disc system of Fig. 2.54 by energy and Lagrange methods.21. A pendulum and a peg. Determine the equation of motion of the pendulum in Fig. 2.55 using only one variable θ or ϕ.22. A pendulum on circular wall. Determine the equation of motion of the pendulum in Fig. 2.56 using the variable θ .23. A wire of the shape y = f (x). Consider a wire in an arbitrary shape given by y = f (x) as is shown in Fig. 2.57. Determine the equation of motion of a bid with mass m that is sliding frictionless on the wire.24. A particle in a cone. A particle of mass m slides without friction inside an upside down conical shell of semivertical angle α, as is shown in Fig. 2.58. (a) Use the Euler equation M = dt L to determine the equations of motion of d the particle.
170 2 Vibration DynamicsFig. 2.57 A wire of theshape of y = f (x)Fig. 2.58 A particle of massm slides inside a conical shellFig. 2.59 A particle on a circular surface (b) Show that it is possible for the particle to move such that it is at a constant R with the cone axis. (c) Determine the angular speed of the particle for a uniform motion of part (b).25. A particle on a circular surface. Draw FBD of the particle in Figs. 2.59(a) and (b) for a = cR, c < 1, and determine their equation of motion. The spring is linear and applies a tangential force on m.26. Falling on a spring. A solid mass m falls on a spring as shown in Fig. 2.60(a) or with a spring as shown Fig. 2.60(b). The spring exerts a stiffness force Fs . Determine the maximum compression xMax of the springs if: (a) The restitution coefﬁcient e = 0 and Fs = kx. (b) The restitution coefﬁcient e = 0 and Fs = kx 3 . (c) The restitution coefﬁcient e = 1 and Fs = kx 3 . We deﬁne a restitution coefﬁcient e by v2 − v1 = e(v1 − v2 ), 0 ≤ e ≤ 1, where v1 , v2 are the speed of the two particles before impact and v1 , v2 are their speeds after impact. The case e = 1 indicates an inelastic collision in which the particles stick to each other after impact, and the case e = 0 is called the plastic collision in which the energy conserves in impact.
2.9 Exercises 171Fig. 2.60 A solid mass mfalls on or with a springFig. 2.61 Pendulums withﬂexible supportFig. 2.62 Heavy pendulums,with and without a revolutejoint27. Pendulum with ﬂexible support. Figures 2.61(a) and (b) illustrate two pendulums with ﬂexible supports in directions of x and y, respectively. Determine the equations of motion for: (a) A pendulum with a ﬂexible support in the x-direction of Fig. 2.61(a). (b) A pendulum with a ﬂexible support in the y-direction of Fig. 2.61(b).28. Heavy pendulum. Figure 2.62(a) illustrates a heavy disc with mass m and radius R suspended by a massless rod of length l. Figure 2.62(b) illustrates another heavy disc with mass m and radius R that is attached to a massless rod of length l by a friction- less revolute joint at its center.
172 2 Vibration Dynamics (a) Derive the equations of motion for the pendulums in (a) and (b). (b) Linearize the equations of motion. Is it possible to compare the periods of the oscillations? (c) Assume that the disc of Fig. 2.62(b) has an angular velocity of ω when θ = 0. Determine the equation of motion, linearize the equation, and deter- mine the period of oscillation.