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10 rotation of a rigid object about a fixed axis


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10 rotation of a rigid object about a fixed axis

  1. 1. P U Z Z L E RDid you know that the CD inside thisplayer spins at different speeds, depend-ing on which song is playing? Why wouldsuch a strange characteristic be incor-porated into the design of every CDplayer? (George Semple)c h a p t e r Rotation of a Rigid Object About a Fixed Axis Chapter Outline 10.1 Angular Displacement, Velocity, 10.5 Calculation of Moments of and Acceleration Inertia 10.2 Rotational Kinematics: Rotational 10.6 Torque Motion with Constant Angular 10.7 Relationship Between Torque Acceleration and Angular Acceleration 10.3 Angular and Linear Quantities 10.8 Work, Power, and Energy in 10.4 Rotational Energy Rotational Motion292
  2. 2. 10.1 Angular Displacement, Velocity, and Acceleration 293W hen an extended object, such as a wheel, rotates about its axis, the motion cannot be analyzed by treating the object as a particle because at any given time different parts of the object have different linear velocities and linearaccelerations. For this reason, it is convenient to consider an extended object as alarge number of particles, each of which has its own linear velocity and linearacceleration. In dealing with a rotating object, analysis is greatly simplified by assuming thatthe object is rigid. A rigid object is one that is nondeformable — that is, it is an Rigid objectobject in which the separations between all pairs of particles remain constant. Allreal bodies are deformable to some extent; however, our rigid-object model is use-ful in many situations in which deformation is negligible. In this chapter, we treat the rotation of a rigid object about a fixed axis, whichis commonly referred to as pure rotational motion.10.1 ANGULAR DISPLACEMENT, VELOCITY, AND ACCELERATIONFigure 10.1 illustrates a planar (flat), rigid object of arbitrary shape confined tothe xy plane and rotating about a fixed axis through O. The axis is perpendicular yto the plane of the figure, and O is the origin of an xy coordinate system. Let uslook at the motion of only one of the millions of “particles” making up this object. PA particle at P is at a fixed distance r from the origin and rotates about it in a circle r sof radius r. (In fact, every particle on the object undergoes circular motion about θ xO.) It is convenient to represent the position of P with its polar coordinates (r, ␪), Owhere r is the distance from the origin to P and ␪ is measured counterclockwise fromsome preferred direction — in this case, the positive x axis. In this representation,the only coordinate that changes in time is the angle ␪ ; r remains constant. (Incartesian coordinates, both x and y vary in time.) As the particle moves along thecircle from the positive x axis (␪ ϭ 0) to P, it moves through an arc of length s,which is related to the angular position ␪ through the relationship Figure 10.1 A rigid object rotat- ing about a fixed axis through O perpendicular to the plane of the s ϭ r␪ (10.1a) figure. (In other words, the axis of rotation is the z axis.) A particle at s P rotates in a circle of radius r cen- ␪ϭ (10.1b) r tered at O. It is important to note the units of ␪ in Equation 10.1b. Because ␪ is the ratioof an arc length and the radius of the circle, it is a pure number. However, we com-monly give ␪ the artificial unit radian (rad), where one radian is the angle subtended by an arc length equal to the radius of the Radian arc.Because the circumference of a circle is 2␲r, it follows from Equation 10.1b that360° corresponds to an angle of 2␲r/r rad ϭ 2␲ rad (one revolution). Hence,1 rad ϭ 360°/2␲ Ϸ 57.3°. To convert an angle in degrees to an angle in radians,we use the fact that 2␲ rad ϭ 360°: ␲ ␪ (rad) ϭ ␪ (deg) 180°For example, 60° equals ␲/3 rad, and 45° equals ␲/4 rad.
  3. 3. 294 CHAPTER 10 Rotation of a Rigid Object About a Fixed Axis y Q ,tf r P, ti θf In a short track event, such as a 200-m or θi 400-m sprint, the runners begin from stag- x gered positions on the track. Why don’t O they all begin from the same line?Figure 10.2 A particle on a rotat-ing rigid object moves from P to Qalong the arc of a circle. In thetime interval ⌬t ϭ t f Ϫ t i , the ra- As the particle in question on our rigid object travels from position P to positiondius vector sweeps out an angle Q in a time ⌬t as shown in Figure 10.2, the radius vector sweeps out an angle ⌬␪ ϭ ␪f⌬␪ ϭ ␪f Ϫ ␪i . Ϫ ␪i . This quantity ⌬␪ is defined as the angular displacement of the particle: ⌬␪ ϭ ␪f Ϫ ␪i (10.2) We define the average angular speed ␻ (omega) as the ratio of this angular dis- placement to the time interval ⌬t: ␪f Ϫ ␪i ⌬␪ Average angular speed ␻ϵ ϭ (10.3) tf Ϫ ti ⌬t In analogy to linear speed, the instantaneous angular speed ␻ is defined as the limit of the ratio ⌬␪/⌬t as ⌬t approaches zero: ⌬␪ d␪ Instantaneous angular speed ␻ ϵ lim ϭ (10.4) ⌬t:0 ⌬t dt Angular speed has units of radians per second (rad/s), or rather secondϪ1 (sϪ1) because radians are not dimensional. We take ␻ to be positive when ␪ is in- creasing (counterclockwise motion) and negative when ␪ is decreasing (clockwise motion). If the instantaneous angular speed of an object changes from ␻i to ␻f in the time interval ⌬t, the object has an angular acceleration. The average angular ac- celeration ␣ (alpha) of a rotating object is defined as the ratio of the change in the angular speed to the time interval ⌬t : ␻f Ϫ ␻i ⌬␻ Average angular acceleration ␣ϵ ϭ (10.5) tf Ϫ ti ⌬t
  4. 4. 10.1 Angular Displacement, Velocity, and Acceleration 295 ω Figure 10.3 The right-hand rule for deter- ω mining the direction of the angular velocity vector. In analogy to linear acceleration, the instantaneous angular acceleration isdefined as the limit of the ratio ⌬␻/⌬t as ⌬t approaches zero: ⌬␻ d␻ ␣ ϵ lim ϭ (10.6) Instantaneous angular ⌬t:0 ⌬t dt accelerationAngular acceleration has units of radians per second squared (rad/s2 ), or just sec-ondϪ2 (sϪ2 ). Note that ␣ is positive when the rate of counterclockwise rotation isincreasing or when the rate of clockwise rotation is decreasing. When rotating about a fixed axis, every particle on a rigid object rotatesthrough the same angle and has the same angular speed and the same an-gular acceleration. That is, the quantities ␪, ␻, and ␣ characterize the rotationalmotion of the entire rigid object. Using these quantities, we can greatly simplifythe analysis of rigid-body rotation. Angular position (␪), angular speed (␻), and angular acceleration (␣) areanalogous to linear position (x), linear speed (v), and linear acceleration (a). Thevariables ␪, ␻, and ␣ differ dimensionally from the variables x, v, and a only by afactor having the unit of length. We have not specified any direction for ␻ and ␣. Strictly speaking, thesevariables are the magnitudes of the angular velocity and the angular accelera-tion vectors ␻ and ␣, respectively, and they should always be positive. Becausewe are considering rotation about a fixed axis, however, we can indicate the di-rections of the vectors by assigning a positive or negative sign to ␻ and ␣, as dis-cussed earlier with regard to Equations 10.4 and 10.6. For rotation about a fixedaxis, the only direction that uniquely specifies the rotational motion is the di-rection along the axis of rotation. Therefore, the directions of ␻ and ␣ arealong this axis. If an object rotates in the xy plane as in Figure 10.1, the direc-tion of ␻ is out of the plane of the diagram when the rotation is counterclock-wise and into the plane of the diagram when the rotation is clockwise. To illus-trate this convention, it is convenient to use the right-hand rule demonstrated inFigure 10.3. When the four fingers of the right hand are wrapped in the direc-tion of rotation, the extended right thumb points in the direction of ␻. The di-rection of ␣ follows from its definition d␻/dt. It is the same as the direction of␻ if the angular speed is increasing in time, and it is antiparallel to ␻ if the an-gular speed is decreasing in time.
  5. 5. 296 CHAPTER 10 Rotation of a Rigid Object About a Fixed Axis Quick Quiz 10.1 Describe a situation in which ␻ Ͻ 0 and ␻ and ␣ are antiparallel. 10.2 ROTATIONAL KINEMATICS: ROTATIONAL MOTION WITH CONSTANT ANGULAR ACCELERATION In our study of linear motion, we found that the simplest form of accelerated mo- 7.2 tion to analyze is motion under constant linear acceleration. Likewise, for rota- tional motion about a fixed axis, the simplest accelerated motion to analyze is mo- tion under constant angular acceleration. Therefore, we next develop kinematic relationships for this type of motion. If we write Equation 10.6 in the form d␻ ϭ ␣ dt, and let ti ϭ 0 and tf ϭ t, we can integrate this expression directly: ␻f ϭ ␻i ϩ ␣t (for constant ␣) (10.7) Substituting Equation 10.7 into Equation 10.4 and integrating once more we obtainRotational kinematic equations ␪f ϭ ␪i ϩ ␻ i t ϩ 1 ␣t 2 2 (for constant ␣) (10.8) If we eliminate t from Equations 10.7 and 10.8, we obtain ␻f 2 ϭ ␻i 2 ϩ 2␣(␪f Ϫ ␪i) (for constant ␣) (10.9) Notice that these kinematic expressions for rotational motion under constant an- gular acceleration are of the same form as those for linear motion under constant linear acceleration with the substitutions x : ␪, v : ␻, and a : ␣. Table 10.1 compares the kinematic equations for rotational and linear motion. EXAMPLE 10.1 Rotating Wheel A wheel rotates with a constant angular acceleration of Solution Because the angular acceleration and the angu- 3.50 rad/s2. If the angular speed of the wheel is 2.00 rad/s at lar speed are both positive, we can be sure our answer must ti ϭ 0, (a) through what angle does the wheel rotate in 2.00 s? be greater than 2.00 rad/s. ␻f ϭ ␻i ϩ ␣t ϭ 2.00 rad/s ϩ (3.50 rad/s2)(2.00 s) Solution We can use Figure 10.2 to represent the wheel, and so we do not need a new drawing. This is a straightfor- ϭ 9.00 rad/s ward application of an equation from Table 10.1: We could also obtain this result using Equation 10.9 and the ␪f Ϫ ␪i ϭ ␻i t ϩ 1␣t 2 ϭ (2.00 rad/s)(2.00 s) 2 results of part (a). Try it! You also may want to see if you can formulate the linear motion analog to this problem. ϩ 1 (3.50 rad/s2)(2.00 s)2 2 ϭ 11.0 rad ϭ (11.0 rad)(57.3°/rad) ϭ 630° Exercise Find the angle through which the wheel rotates between t ϭ 2.00 s and t ϭ 3.00 s. 630° ϭ ϭ 1.75 rev 360°/rev Answer 10.8 rad. (b) What is the angular speed at t ϭ 2.00 s?
  6. 6. 10.3 Angular and Linear Quantities 297 TABLE 10.1 Kinematic Equations for Rotational and Linear Motion Under Constant Acceleration Rotational Motion About a Fixed Axis Linear Motion ␻f ϭ ␻ i ϩ ␣t vf ϭ vi ϩ at ␪f ϭ ␪i ϩ ␻ i t ϩ 1 2 ␣t 2 xf ϭ xi ϩ v i t ϩ 1 2 at 2 ␻f ϭ ␻ i ϩ 2␣(␪f Ϫ ␪i ) 2 2 vf ϭ v i ϩ 2a(xf Ϫ xi ) 2 2 y10.3 ANGULAR AND LINEAR QUANTITIESIn this section we derive some useful relationships between the angular speed and vacceleration of a rotating rigid object and the linear speed and acceleration of an Parbitrary point in the object. To do so, we must keep in mind that when a rigid ob-ject rotates about a fixed axis, as in Figure 10.4, every particle of the object moves rin a circle whose center is the axis of rotation. θ We can relate the angular speed of the rotating object to the tangential speed x Oof a point P on the object. Because point P moves in a circle, the linear velocityvector v is always tangent to the circular path and hence is called tangential velocity.The magnitude of the tangential velocity of the point P is by definition the tangen-tial speed v ϭ ds/dt, where s is the distance traveled by this point measured along Figure 10.4 As a rigid object ro-the circular path. Recalling that s ϭ r ␪ (Eq. 10.1a) and noting that r is constant, tates about the fixed axis throughwe obtain O, the point P has a linear velocity v that is always tangent to the circu- ds d␪ vϭ ϭr lar path of radius r. dt dtBecause d␪/dt ϭ ␻ (see Eq. 10.4), we can say Relationship between linear and v ϭ r␻ (10.10) angular speedThat is, the tangential speed of a point on a rotating rigid object equals the per-pendicular distance of that point from the axis of rotation multiplied by the angu-lar speed. Therefore, although every point on the rigid object has the same angu-lar speed, not every point has the same linear speed because r is not the same forall points on the object. Equation 10.10 shows that the linear speed of a point onthe rotating object increases as one moves outward from the center of rotation, aswe would intuitively expect. The outer end of a swinging baseball bat moves muchfaster than the handle. QuickLab We can relate the angular acceleration of the rotating rigid object to the tan- Spin a tennis ball or basketball andgential acceleration of the point P by taking the time derivative of v: watch it gradually slow down and stop. Estimate ␣ and at as accurately dv d␻ as you can. at ϭ ϭr dt dt Relationship between linear and at ϭ r ␣ (10.11) angular accelerationThat is, the tangential component of the linear acceleration of a point on a rotat-ing rigid object equals the point’s distance from the axis of rotation multiplied bythe angular acceleration.
  7. 7. 298 CHAPTER 10 Rotation of a Rigid Object About a Fixed Axis In Section 4.4 we found that a point rotating in a circular path undergoes a centripetal, or radial, acceleration ar of magnitude v 2/r directed toward the center of rotation (Fig. 10.5). Because v ϭ r␻ for a point P on a rotating object, we can y express the radial acceleration of that point as at v2 P ar ϭ ϭ r ␻2 (10.12) a r ar The total linear acceleration vector of the point is a ϭ at ϩ ar . (at describes the change in how fast the point is moving, and ar represents the change in its di- x rection of travel.) Because a is a vector having a radial and a tangential compo- O nent, the magnitude of a for the point P on the rotating rigid object is aϭ √a t 2 ϩ a r2 ϭ √r 2␣ 2 ϩ r 2␻ 4 ϭ r √␣ 2 ϩ ␻ 4 (10.13)Figure 10.5 As a rigid object ro-tates about a fixed axis through O,the point P experiences a tangen- Quick Quiz 10.2tial component of linear accelera-tion at and a radial component of When a wheel of radius R rotates about a fixed axis, do all points on the wheel have (a) thelinear acceleration ar . The total lin- same angular speed and (b) the same linear speed? If the angular speed is constant andear acceleration of this point is a ϭ equal to ␻, describe the linear speeds and linear accelerations of the points located ata t ϩ ar . (c) r ϭ 0, (d) r ϭ R/2, and (e) r ϭ R, all measured from the center of the wheel. EXAMPLE 10.2 CD Player ΂ ΃ On a compact disc, audio information is stored in a series of 1 pits and flat areas on the surface of the disc. The information ϭ (56.5 rad/s) 2␲ rev/rad (60 s/min) is stored digitally, and the alternations between pits and flat areas on the surface represent binary ones and zeroes to be ϭ 5.4 ϫ 10 2 rev/min read by the compact disc player and converted back to sound waves. The pits and flat areas are detected by a system consist- ing of a laser and lenses. The length of a certain number of ones and zeroes is the same everywhere on the disc, whether the information is near the center of the disc or near its outer edge. In order that this length of ones and zeroes al- ways passes by the laser – lens system in the same time period, the linear speed of the disc surface at the location of the lens must be constant. This requires, according to Equation 10.10, 23 mm that the angular speed vary as the laser – lens system moves ra- dially along the disc. In a typical compact disc player, the disc spins counterclockwise (Fig. 10.6), and the constant speed of the surface at the point of the laser – lens system is 1.3 m/s. (a) Find the angular speed of the disc in revolutions per minute when information is being read from the innermost first track (r ϭ 23 mm) and the outermost final track (r ϭ 58 mm 58 mm). Solution Using Equation 10.10, we can find the angular speed; this will give us the required linear speed at the posi- tion of the inner track, v 1.3 m/s ␻i ϭ ϭ ϭ 56.5 rad/s ri 2.3 ϫ 10 Ϫ2 m Figure 10.6 A compact disc.
  8. 8. 10.4 Rotational Energy 299 For the outer track, (c) What total length of track moves past the objective lens during this time? v 1.3 m/s ␻f ϭ ϭ ϭ 22.4 rad/s rf 5.8 ϫ 10 Ϫ2 m Solution Because we know the (constant) linear velocity and the time interval, this is a straightforward calculation: ϭ 2.1 ϫ 10 2 rev/min x f ϭ v i t ϭ (1.3 m/s)(4 473 s) ϭ 5.8 ϫ 10 3 m The player adjusts the angular speed ␻ of the disc within this range so that information moves past the objective lens at a More than 3.6 miles of track spins past the objective lens! constant rate. These angular velocity values are positive be- cause the direction of rotation is counterclockwise. (d) What is the angular acceleration of the CD over the 4 473-s time interval? Assume that ␣ is constant. (b) The maximum playing time of a standard music CD is 74 minutes and 33 seconds. How many revolutions does the Solution We have several choices for approaching this disc make during that time? problem. Let us use the most direct approach by utilizing Equation 10.5, which is based on the definition of the term Solution We know that the angular speed is always de- we are seeking. We should obtain a negative number for the creasing, and we assume that it is decreasing steadily, with ␣ angular acceleration because the disc spins more and more constant. The time interval t is (74 min)(60 s/min) ϩ slowly in the positive direction as time goes on. Our answer 33 s ϭ 4 473 s. We are looking for the angular position ␪f , should also be fairly small because it takes such a long time — where we set the initial angular position ␪i ϭ 0. We can use more than an hour — for the change in angular speed to be Equation 10.3, replacing the average angular speed ␻ with its accomplished: mathematical equivalent (␻i ϩ ␻ f )/2: ␻f Ϫ ␻i 22.4 rad/s Ϫ 56.5 rad/s ␪f ϭ ␪i ϩ 1 (␻i ϩ ␻f )t 2 ␣ϭ ϭ t 4 473 s ϭ 0 ϩ 1 (540 rev/min ϩ 210 rev/min) 2 (1 min/60 s)(4 473 s) ϭ Ϫ7.6 ϫ 10 Ϫ3 rad/s2 The disc experiences a very gradual decrease in its rotation ϭ 2.8 ϫ 10 4 rev rate, as expected. web 10.4 ROTATIONAL ENERGY If you want to learn more about the physics of CD players, visit the Special Interest Let us now look at the kinetic energy of a rotating rigid object, considering the ob- Group on CD Applications and Technology at www.sigcat.org7.3 ject as a collection of particles and assuming it rotates about a fixed z axis with an angular speed ␻ (Fig. 10.7). Each particle has kinetic energy determined by its mass and linear speed. If the mass of the ith particle is mi and its linear speed is vi , y its kinetic energy is vi K i ϭ 1m iv i 2 2 To proceed further, we must recall that although every particle in the rigid object mi has the same angular speed ␻, the individual linear speeds depend on the distance ri ri from the axis of rotation according to the expression vi ϭ ri ␻ (see Eq. 10.10). θ The total kinetic energy of the rotating rigid object is the sum of the kinetic ener- x O gies of the individual particles: K R ϭ ⌺ K i ϭ ⌺ 1m i v i 2 ϭ 1 ⌺ m i r i 2␻ 2 2 2 i i i Figure 10.7 A rigid object rotat- We can write this expression in the form ing about a z axis with angular speed ␻. The kinetic energy of KR ϭ 1 2 ΂⌺ m r ΃␻ i i i 2 2 (10.14) the particle of mass mi is 1 m iv i 2. 2 The total kinetic energy of the ob- ject is called its rotational where we have factored ␻ 2 from the sum because it is common to every particle. kinetic energy.
  9. 9. 300 CHAPTER 10 Rotation of a Rigid Object About a Fixed Axis We simplify this expression by defining the quantity in parentheses as the moment of inertia I:Moment of inertia I ϵ ⌺ mi ri 2 (10.15) i From the definition of moment of inertia, we see that it has dimensions of ML2 (kgи m2 in SI units).1 With this notation, Equation 10.14 becomesRotational kinetic energy KR ϭ 1I␻2 2 (10.16) Although we commonly refer to the quantity 2I␻ 2 as rotational kinetic energy, 1 it is not a new form of energy. It is ordinary kinetic energy because it is derived from a sum over individual kinetic energies of the particles contained in the rigid object. However, the mathematical form of the kinetic energy given by Equation 10.16 is a convenient one when we are dealing with rotational motion, provided we know how to calculate I. It is important that you recognize the analogy between kinetic energy associ- ated with linear motion 1mv 2 and rotational kinetic energy 1 I␻ 2. The quantities I 2 2 and ␻ in rotational motion are analogous to m and v in linear motion, respectively. (In fact, I takes the place of m every time we compare a linear-motion equation with its rotational counterpart.) The moment of inertia is a measure of the resis- tance of an object to changes in its rotational motion, just as mass is a measure of the tendency of an object to resist changes in its linear motion. Note, however, that mass is an intrinsic property of an object, whereas I depends on the physical arrangement of that mass. Can you think of a situation in which an object’s mo- ment of inertia changes even though its mass does not? EXAMPLE 10.3 The Oxygen Molecule Consider an oxygen molecule (O2 ) rotating in the xy plane about the z axis. The axis passes through the center of the ϭ 1.95 ϫ 10 Ϫ46 kgиm2 molecule, perpendicular to its length. The mass of each oxy- gen atom is 2.66 ϫ 10Ϫ26 kg, and at room temperature the This is a very small number, consistent with the minuscule average separation between the two atoms is d ϭ 1.21 ϫ masses and distances involved. 10Ϫ10 m (the atoms are treated as point masses). (a) Calcu- (b) If the angular speed of the molecule about the z axis is late the moment of inertia of the molecule about the z axis. 4.60 ϫ 1012 rad/s, what is its rotational kinetic energy? Solution This is a straightforward application of the def- Solution We apply the result we just calculated for the mo- inition of I. Because each atom is a distance d/2 from the z ment of inertia in the formula for K R : axis, the moment of inertia about the axis is KR ϭ 1 I␻2 2 ΂2΃ ΂2΃ d 2 d 2 I ϭ ⌺ mi ri 2 ϭ m ϩm ϭ 1md 2 2 ϭ 1(1.95 ϫ 10Ϫ46 kgиm2)(4.60 ϫ 1012 rad/s)2 2 i ϭ 1(2.66 ϫ 10 Ϫ26 kg)(1.21 ϫ 10 Ϫ10 m)2 ϭ 2.06 ϫ 10 Ϫ21 J 2 1 Civil engineers use moment of inertia to characterize the elastic properties (rigidity) of such struc- tures as loaded beams. Hence, it is often useful even in a nonrotational context.
  10. 10. 10.5 Calculation of Moments of Inertia 301 EXAMPLE 10.4 Four Rotating Masses Four tiny spheres are fastened to the corners of a frame of Therefore, the rotational kinetic energy about the y axis is negligible mass lying in the xy plane (Fig. 10.8). We shall as- sume that the spheres’ radii are small compared with the di- K R ϭ 1Iy ␻ 2 ϭ 1(2Ma 2)␻ 2 ϭ Ma 2␻ 2 2 2 mensions of the frame. (a) If the system rotates about the y axis with an angular speed ␻, find the moment of inertia and The fact that the two spheres of mass m do not enter into this the rotational kinetic energy about this axis. result makes sense because they have no motion about the axis of rotation; hence, they have no rotational kinetic en- Solution First, note that the two spheres of mass m, which ergy. By similar logic, we expect the moment of inertia about lie on the y axis, do not contribute to Iy (that is, ri ϭ 0 for the x axis to be Ix ϭ 2mb 2 with a rotational kinetic energy these spheres about this axis). Applying Equation 10.15, we about that axis of K R ϭ mb 2␻ 2. obtain (b) Suppose the system rotates in the xy plane about an Iy ϭ ⌺ m i ri 2 ϭ Ma 2 ϩ Ma 2 ϭ 2Ma 2 axis through O (the z axis). Calculate the moment of inertia i and rotational kinetic energy about this axis. y Solution Because ri in Equation 10.15 is the perpendicular distance to the axis of rotation, we obtain m I z ϭ ⌺m i r i 2 ϭ Ma 2 ϩ Ma 2 ϩ mb 2 ϩ mb 2 ϭ 2Ma 2 ϩ 2mb 2 i b K R ϭ 1Iz ␻ 2 ϭ 1(2Ma 2 ϩ 2mb 2)␻ 2 ϭ (Ma 2 ϩ mb 2)␻ 2 2 2 M a a M x O Comparing the results for parts (a) and (b), we conclude that the moment of inertia and therefore the rotational ki- b netic energy associated with a given angular speed depend on the axis of rotation. In part (b), we expect the result to in- clude all four spheres and distances because all four spheres m are rotating in the xy plane. Furthermore, the fact that the ro- Figure 10.8 The four spheres are at a fixed separation as shown. tational kinetic energy in part (a) is smaller than that in part The moment of inertia of the system depends on the axis about (b) indicates that it would take less effort (work) to set the which it is evaluated. system into rotation about the y axis than about the z axis. 10.5 CALCULATION OF MOMENTS OF INERTIA We can evaluate the moment of inertia of an extended rigid object by imagining7.5 the object divided into many small volume elements, each of which has mass ⌬m. We use the definition I ϭ ⌺ r i 2 ⌬m i and take the limit of this sum as ⌬m : 0. In i this limit, the sum becomes an integral over the whole object: I ϭ lim ⌺ ri 2 ⌬mi ϭ ⌬m i :0 i ͵ r 2 dm (10.17) It is usually easier to calculate moments of inertia in terms of the volume of the elements rather than their mass, and we can easily make that change by using Equation 1.1, ␳ ϭ m/V, where ␳ is the density of the object and V is its volume. We want this expression in its differential form ␳ ϭ dm/dV because the volumes we are dealing with are very small. Solving for dm ϭ ␳ dV and substituting the result
  11. 11. 302 CHAPTER 10 Rotation of a Rigid Object About a Fixed Axis into Equation 10.17 gives Iϭ ͵␳r 2 dV If the object is homogeneous, then ␳ is constant and the integral can be evaluated for a known geometry. If ␳ is not constant, then its variation with position must be known to complete the integration. The density given by ␳ ϭ m/V sometimes is referred to as volume density for the obvious reason that it relates to volume. Often we use other ways of expressing density. For instance, when dealing with a sheet of uniform thickness t, we can de- fine a surface density ␴ ϭ ␳t, which signifies mass per unit area. Finally, when mass is distributed along a uniform rod of cross-sectional area A, we sometimes use linear density ␭ ϭ M/L ϭ ␳A, which is the mass per unit length. EXAMPLE 10.5 Uniform Hoop Find the moment of inertia of a uniform hoop of mass M and y radius R about an axis perpendicular to the plane of the hoop and passing through its center (Fig. 10.9). dm Solution All mass elements dm are the same distance r ϭ R from the axis, and so, applying Equation 10.17, we obtain x for the moment of inertia about the z axis through O: O ͵ ͵ R Iz ϭ r 2 dm ϭ R 2 dm ϭ MR 2 Note that this moment of inertia is the same as that of a sin- gle particle of mass M located a distance R from the axis of rotation. Figure 10.9 The mass elements dm of a uniform hoop are all the same distance from O. Quick Quiz 10.3 (a) Based on what you have learned from Example 10.5, what do you expect to find for the moment of inertia of two particles, each of mass M/2, located anywhere on a circle of ra- dius R around the axis of rotation? (b) How about the moment of inertia of four particles, each of mass M/4, again located a distance R from the rotation axis? EXAMPLE 10.6 Uniform Rigid Rod Calculate the moment of inertia of a uniform rigid rod of Substituting this expression for dm into Equation 10.17, with length L and mass M (Fig. 10.10) about an axis perpendicu- r ϭ x, we obtain ͵ ͵ ͵ lar to the rod (the y axis) and passing through its center of L/2 L/2 M M mass. Iy ϭ r 2 dm ϭ x2 dx ϭ x 2 dx ϪL/2 L L ϪL/2 Solution The shaded length element dx has a mass dm ΄ x3 ΅ M 3 L/2 equal to the mass per unit length ␭ multiplied by dx : ϭ ϭ 1 12 ML 2 L ϪL/2 M dm ϭ ␭ dx ϭ dx L
  12. 12. 10.5 Calculation of Moments of Inertia 303 y′ y dx x O x Figure 10.10 A uniform rigid rod of length L. The moment of in- L ertia about the y axis is less than that about the yЈ axis. The latter axis is examined in Example 10.8. EXAMPLE 10.7 Uniform Solid Cylinder A uniform solid cylinder has a radius R, mass M, and length cylindrical shells, each of which has radius r, thickness dr, and L. Calculate its moment of inertia about its central axis (the z length L, as shown in Figure 10.11. The volume dV of each axis in Fig. 10.11). shell is its cross-sectional area multiplied by its length: dV ϭ dAи L ϭ (2␲ r dr)L. If the mass per unit volume is ␳, then the Solution It is convenient to divide the cylinder into many mass of this differential volume element is dm ϭ ␳dV ϭ ␳2␲ rL dr. Substituting this expression for dm into Equation z 10.17, we obtain dr r Iz ϭ ͵ r 2 dm ϭ 2␲␳L ͵0 R r 3 dr ϭ 1␲␳LR 4 2 Because the total volume of the cylinder is ␲R 2L, we see that R ␳ ϭ M/V ϭ M/␲R 2L. Substituting this value for ␳ into the above result gives L (1) Iz ϭ 1 2 MR 2 Note that this result does not depend on L, the length of the cylinder. In other words, it applies equally well to a long cylin- der and a flat disc. Also note that this is exactly half the value Figure 10.11 Calculating I about the z axis for a uniform solid we would expect were all the mass concentrated at the outer cylinder. edge of the cylinder or disc. (See Example 10.5.) Table 10.2 gives the moments of inertia for a number of bodies about specificaxes. The moments of inertia of rigid bodies with simple geometry (high symme-try) are relatively easy to calculate provided the rotation axis coincides with an axisof symmetry. The calculation of moments of inertia about an arbitrary axis can becumbersome, however, even for a highly symmetric object. Fortunately, use of animportant theorem, called the parallel-axis theorem, often simplifies the calcula-tion. Suppose the moment of inertia about an axis through the center of mass ofan object is ICM . The parallel-axis theorem states that the moment of inertia aboutany axis parallel to and a distance D away from this axis is I ϭ ICM ϩ MD 2 (10.18) Parallel-axis theorem
  13. 13. 304 CHAPTER 10 Rotation of a Rigid Object About a Fixed Axis TABLE 10.2 Moments of Inertia of Homogeneous Rigid Bodies with Different Geometries Hoop or Hollow cylinder cylindrical shell I CM = 1 M(R 12 + R 22) R1 I CM = MR 2 R 2 R2 Solid cylinder Rectangular plate or disk R I CM = 1 M(a 2 + b 2) I CM = 1 MR 2 12 2 b a Long thin rod Long thin with rotation axis rod with through center rotation axis through end I CM = 1 ML 2 L L 12 I = 1 ML 2 3 Solid sphere Thin spherical shell I CM = 2 MR 2 5 I CM = 2 MR 2 3 R R Proof of the Parallel-Axis Theorem (Optional). Suppose that an object rotates in the xy plane about the z axis, as shown in Figure 10.12, and that the coordinates of the center of mass are x CM , y CM . Let the mass element dm have coordinates x, y. Because this element is a distance r ϭ √x 2 ϩ y 2 from the z axis, the moment of in- ertia about the z axis is Iϭ ͵ r 2 dm ϭ ͵ (x 2 ϩ y 2) dm However, we can relate the coordinates x, y of the mass element dm to the coordi- nates of this same element located in a coordinate system having the object’s cen- ter of mass as its origin. If the coordinates of the center of mass are x CM , y CM in the original coordinate system centered on O, then from Figure 10.12a we see that the relationships between the unprimed and primed coordinates are x ϭ xЈ ϩ x CM
  14. 14. 10.5 Calculation of Moments of Inertia 305 y dm x, y z Axis through y′ r Rotation CM axis y y CM xCM, yCM O CM yCM D x O x xCM x′ x (a) (b)Figure 10.12 (a) The parallel-axis theorem: If the moment of inertia about an axis perpendic-ular to the figure through the center of mass is ICM , then the moment of inertia about the z axisis Iz ϭ I CM ϩ MD 2. (b) Perspective drawing showing the z axis (the axis of rotation) and the par-allel axis through the CM.and y ϭ yЈ ϩ y CM . Therefore, Iϭ ͵ [(xЈ ϩ x CM)2 ϩ (yЈ ϩ y CM)2] dm ϭ ͵ [(xЈ)2 ϩ (yЈ)2] dm ϩ 2x CM ͵ xЈ dm ϩ 2y CM ͵ yЈ dm ϩ (x CM2 ϩ y CM 2) ͵ dmThe first integral is, by definition, the moment of inertia about an axis that is par-allel to the z axis and passes through the center of mass. The second two integralsare zero because, by definition of the center of mass, ͵ xЈ dm ϭ ͵ yЈ dm ϭ 0. Thelast integral is simply MD 2 because ͵ dm ϭ M and D 2 ϭ x CM2 ϩ y CM2. Therefore,we conclude that I ϭ ICM ϩ MD 2 EXAMPLE 10.8 Applying the Parallel-Axis Theorem ΂L΃ Consider once again the uniform rigid rod of mass M and 2 length L shown in Figure 10.10. Find the moment of inertia I ϭ ICM ϩ MD 2 ϭ 12 ML 2 ϩ M 1 ϭ 1 3 ML 2 2 of the rod about an axis perpendicular to the rod through one end (the yЈaxis in Fig. 10.10). So, it is four times more difficult to change the rotation of a rod spinning about its end than it is to change the motion of Solution Intuitively, we expect the moment of inertia to one spinning about its center. be greater than ICM ϭ 12ML 2 because it should be more diffi- 1 cult to change the rotational motion of a rod spinning about Exercise Calculate the moment of inertia of the rod about an axis at one end than one that is spinning about its center. a perpendicular axis through the point x ϭ L/4. Because the distance between the center-of-mass axis and the yЈ axis is D ϭ L/2, the parallel-axis theorem gives Answer I ϭ 48 ML 2. 7
  15. 15. 306 CHAPTER 10 Rotation of a Rigid Object About a Fixed Axis 10.6 TORQUE Why are a door’s doorknob and hinges placed near opposite edges of the door? 7.6 This question actually has an answer based on common sense ideas. The harder we push against the door and the farther we are from the hinges, the more likely we are to open or close the door. When a force is exerted on a rigid object pivoted about an axis, the object tends to rotate about that axis. The tendency of a force to rotate an object about some axis is measured by a vector quantity called torque ␶ (tau). Consider the wrench pivoted on the axis through O in Figure 10.13. The ap- plied force F acts at an angle ␾ to the horizontal. We define the magnitude of the torque associated with the force F by the expressionDefinition of torque ␶ ϵ r F sin ␾ ϭ Fd (10.19) where r is the distance between the pivot point and the point of application of F and d is the perpendicular distance from the pivot point to the line of action of F. (The line of action of a force is an imaginary line extending out both ends of the vector representing the force. The dashed line extending from the tail of F in Fig- ure 10.13 is part of the line of action of F.) From the right triangle in Figure 10.13 that has the wrench as its hypotenuse, we see that d ϭ r sin ␾. This quantity d isMoment arm called the moment arm (or lever arm) of F. It is very important that you recognize that torque is defined only when a reference axis is specified. Torque is the product of a force and the moment arm of that force, and moment arm is defined only in terms of an axis of rotation. In Figure 10.13, the only component of F that tends to cause rotation is F sin ␾, the component perpendicular to r. The horizontal component F cos ␾, because it passes through O, has no tendency to produce rotation. From the definition of torque, we see that the rotating tendency increases as F increases and as d in- creases. This explains the observation that it is easier to close a door if we push at the doorknob rather than at a point close to the hinge. We also want to apply our push as close to perpendicular to the door as we can. Pushing sideways on the doorknob will not cause the door to rotate. If two or more forces are acting on a rigid object, as shown in Figure 10.14, each tends to produce rotation about the pivot at O. In this example, F2 tends to F1 F sin φ F r d1 φ F cos φ O O φ Line of d2 d action Figure 10.13 The force F has a F2 greater rotating tendency about O as F increases and as the moment Figure 10.14 The force F1 tends arm d increases. It is the compo- to rotate the object counterclock- nent F sin ␾ that tends to rotate the wise about O, and F2 tends to ro- wrench about O. tate it clockwise.
  16. 16. 10.7 Relationship Between Torque and Angular Acceleration 307 rotate the object clockwise, and F1 tends to rotate it counterclockwise. We use the convention that the sign of the torque resulting from a force is positive if the turn- ing tendency of the force is counterclockwise and is negative if the turning ten- dency is clockwise. For example, in Figure 10.14, the torque resulting from F1 , which has a moment arm d 1 , is positive and equal to ϩ F1 d 1 ; the torque from F2 is negative and equal to Ϫ F2 d 2 . Hence, the net torque about O is ⌺ ␶ ϭ ␶1 ϩ ␶2 ϭ F 1d 1 Ϫ F 2d 2 Torque should not be confused with force. Forces can cause a change in lin- ear motion, as described by Newton’s second law. Forces can also cause a change in rotational motion, but the effectiveness of the forces in causing this change de- pends on both the forces and the moment arms of the forces, in the combination that we call torque. Torque has units of force times length — newton и meters in SI units — and should be reported in these units. Do not confuse torque and work, which have the same units but are very different concepts. EXAMPLE 10.9 The Net Torque on a Cylinder A one-piece cylinder is shaped as shown in Figure 10.15, with Solution The torque due to F1 is Ϫ R 1 F1 (the sign is nega- a core section protruding from the larger drum. The cylinder tive because the torque tends to produce clockwise rotation). is free to rotate around the central axis shown in the drawing. The torque due to F2 is ϩ R 2 F 2 (the sign is positive because A rope wrapped around the drum, which has radius R 1 , ex- the torque tends to produce counterclockwise rotation). erts a force F1 to the right on the cylinder. A rope wrapped Therefore, the net torque about the rotation axis is around the core, which has radius R 2 , exerts a force F2 down- ward on the cylinder. (a) What is the net torque acting on the ⌺ ␶ ϭ ␶1 ϩ ␶2 ϭ ϪR 1F 1 ϩ R 2F 2 cylinder about the rotation axis (which is the z axis in Figure 10.15)? We can make a quick check by noting that if the two forces are of equal magnitude, the net torque is negative because y R 1 Ͼ R 2 . Starting from rest with both forces acting on it, the cylinder would rotate clockwise because F 1 would be more ef- fective at turning it than would F 2 . F1 R1 (b) Suppose F 1 ϭ 5.0 N, R 1 ϭ 1.0 m, F 2 ϭ 15.0 N, and R 2 ϭ 0.50 m. What is the net torque about the rotation axis, and which way does the cylinder rotate from rest? R2 x O ⌺ ␶ ϭ Ϫ(5.0 N)(1.0 m) ϩ (15.0 N)(0.50 m) ϭ 2.5 Nиm z Because the net torque is positive, if the cylinder starts from rest, it will commence rotating counterclockwise with increas- F2 ing angular velocity. (If the cylinder’s initial rotation is clock- Figure 10.15 A solid cylinder pivoted about the z axis through O. wise, it will slow to a stop and then rotate counterclockwise The moment arm of F1 is R 1 , and the moment arm of F2 is R 2 . with increasing angular speed.) 10.7 RELATIONSHIP BETWEEN TORQUE AND ANGULAR ACCELERATION In this section we show that the angular acceleration of a rigid object rotating7.6 about a fixed axis is proportional to the net torque acting about that axis. Before discussing the more complex case of rigid-body rotation, however, it is instructive
  17. 17. 308 CHAPTER 10 Rotation of a Rigid Object About a Fixed Axis first to discuss the case of a particle rotating about some fixed point under the in- fluence of an external force. Consider a particle of mass m rotating in a circle of radius r under the influ- ence of a tangential force Ft and a radial force Fr , as shown in Figure 10.16. (As we learned in Chapter 6, the radial force must be present to keep the particle moving in its circular path.) The tangential force provides a tangential acceleration at , and Ft ϭ ma t The torque about the center of the circle due to Ft isFigure 10.16 A particle rotating ␶ ϭ F t r ϭ (ma t )rin a circle under the influence of a Because the tangential acceleration is related to the angular acceleration throughtangential force Ft . A force Fr inthe radial direction also must be the relationship at ϭ r␣ (see Eq. 10.11), the torque can be expressed aspresent to maintain the circular ␶ ϭ (mr ␣)r ϭ (mr 2)␣motion. Recall from Equation 10.15 that mr 2 is the moment of inertia of the rotating parti- cle about the z axis passing through the origin, so that Relationship between torque and ␶ ϭ I␣ (10.20) angular acceleration That is, the torque acting on the particle is proportional to its angular accel- eration, and the proportionality constant is the moment of inertia. It is important to note that ␶ ϭ I␣ is the rotational analog of Newton’s second law of motion, F ϭ ma. Now let us extend this discussion to a rigid object of arbitrary shape rotating about a fixed axis, as shown in Figure 10.17. The object can be regarded as an infi- y nite number of mass elements dm of infinitesimal size. If we impose a cartesian co- d Ft ordinate system on the object, then each mass element rotates in a circle about the origin, and each has a tangential acceleration at produced by an external tangen- dm tial force d Ft . For any given element, we know from Newton’s second law that dFt ϭ (dm)a t r The torque d␶ associated with the force dFt acts about the origin and is given by x O d ␶ ϭ r dF t ϭ (r dm)a t Because at ϭ r ␣, the expression for d␶ becomesFigure 10.17 A rigid object ro- d ␶ ϭ (r dm)r ␣ ϭ (r 2 dm)␣tating about an axis through O.Each mass element dm rotates It is important to recognize that although each mass element of the rigid ob-about O with the same angular ac- ject may have a different linear acceleration at , they all have the same angular ac-celeration ␣, and the net torque on celeration ␣. With this in mind, we can integrate the above expression to obtainthe object is proportional to ␣. the net torque about O due to the external forces: ⌺␶ ϭ ͵ (r 2 dm)␣ ϭ ␣ ͵ r 2 dm where ␣ can be taken outside the integral because it is common to all mass ele- ments. From Equation 10.17, we know that ͵ r 2 dm is the moment of inertia of the object about the rotation axis through O, and so the expression for ⌺␶ becomes Torque is proportional to angular ⌺ ␶ ϭ I␣ (10.21) acceleration Note that this is the same relationship we found for a particle rotating in a circle (see Eq. 10.20). So, again we see that the net torque about the rotation axis is pro-
  18. 18. 10.7 Relationship Between Torque and Angular Acceleration 309portional to the angular acceleration of the object, with the proportionality factorbeing I, a quantity that depends upon the axis of rotation and upon the size andshape of the object. In view of the complex nature of the system, it is interesting tonote that the relationship ⌺␶ ϭ I␣ is strikingly simple and in complete agreementwith experimental observations. The simplicity of the result lies in the manner inwhich the motion is described. Although each point on a rigid object rotating about a fixed axis may not expe- Every point has the same ␻ and ␣ rience the same force, linear acceleration, or linear speed, each point experi- ences the same angular acceleration and angular speed at any instant. There- fore, at any instant the rotating rigid object as a whole is characterized by QuickLab specific values for angular acceleration, net torque, and angular speed. Tip over a child’s tall tower of blocks. Try this several times. Does the tower “break” at the same place each time? Finally, note that the result ⌺␶ ϭ I␣ also applies when the forces acting on the What affects where the tower comes apart as it tips? If the tower weremass elements have radial components as well as tangential components. This is made of toy bricks that snap together,because the line of action of all radial components must pass through the axis of what would happen? (Refer to Con-rotation, and hence all radial components produce zero torque about that axis. ceptual Example 10.11.) EXAMPLE 10.10 Rotating Rod A uniform rod of length L and mass M is attached at one end compute the torque on the rod, we can assume that the gravi- to a frictionless pivot and is free to rotate about the pivot in tational force acts at the center of mass of the rod, as shown the vertical plane, as shown in Figure 10.18. The rod is re- in Figure 10.18. The torque due to this force about an axis leased from rest in the horizontal position. What is the initial through the pivot is angular acceleration of the rod and the initial linear accelera- tion of its right end? ␶ ϭ ⌴g ΂L΃ 2 Solution We cannot use our kinematic equations to find ␣ With ⌺␶ ϭ I␣, and I ϭ 1 ML 2 for this axis of rotation (see 3 or a because the torque exerted on the rod varies with its po- Table 10.2), we obtain sition, and so neither acceleration is constant. We have enough information to find the torque, however, which we ␶ ⌴g (L/2) 3g can then use in the torque – angular acceleration relationship ␣ϭ ϭ ϭ (Eq. 10.21) to find ␣ and then a. I 1͞3 ⌴L 2 2L The only force contributing to torque about an axis All points on the rod have this angular acceleration. through the pivot is the gravitational force Mg exerted on To find the linear acceleration of the right end of the rod, the rod. (The force exerted by the pivot on the rod has zero we use the relationship a t ϭ r␣ (Eq. 10.11), with r ϭ L: torque about the pivot because its moment arm is zero.) To a t ϭ L␣ ϭ 3 2g L/2 This result — that at Ͼ g for the free end of the rod — is rather interesting. It means that if we place a coin at the tip of the rod, hold the rod in the horizontal position, and then release the rod, the tip of the rod falls faster than the coin does! Pivot Mg Other points on the rod have a linear acceleration that is less than 3 g. For example, the middle of the rod has 2 Figure 10.18 The uniform rod is pivoted at the left end. an acceleration of 3 g. 4
  19. 19. 310 CHAPTER 10 Rotation of a Rigid Object About a Fixed Axis CONCEPTUAL EXAMPLE 10.11 Falling Smokestacks and Tumbling Blocks When a tall smokestack falls over, it often breaks somewhere along its length before it hits the ground, as shown in Figure 10.19. The same thing happens with a tall tower of children’s toy blocks. Why does this happen? Solution As the smokestack rotates around its base, each higher portion of the smokestack falls with an increasing tangential acceleration. (The tangential acceleration of a given point on the smokestack is proportional to the dis- tance of that portion from the base.) As the acceleration in- creases, higher portions of the smokestack experience an acceleration greater than that which could result from gravity alone; this is similar to the situation described in Example 10.10. This can happen only if these portions are being pulled downward by a force in addition to the gravi- tational force. The force that causes this to occur is the shear force from lower portions of the smokestack. Eventu- ally the shear force that provides this acceleration is greater than the smokestack can withstand, and the smokestack breaks. Figure 10.19 A falling smokestack. EXAMPLE 10.12 Angular Acceleration of a Wheel A wheel of radius R, mass M, and moment of inertia I is mounted on a frictionless, horizontal axle, as shown in Figure M 10.20. A light cord wrapped around the wheel supports an object of mass m. Calculate the angular acceleration of the wheel, the linear acceleration of the object, and the tension in the cord. O Solution The torque acting on the wheel about its axis of rotation is ␶ ϭ TR, where T is the force exerted by the R cord on the rim of the wheel. (The gravitational force ex- erted by the Earth on the wheel and the normal force ex- T erted by the axle on the wheel both pass through the axis of rotation and thus produce no torque.) Because ⌺␶ ϭ I␣, we obtain ⌺ ␶ ϭ I␣ ϭ TR T TR (1) ␣ϭ I Now let us apply Newton’s second law to the motion of the m object, taking the downward direction to be positive: ⌺ F y ϭ mg Ϫ T ϭ ma mg Ϫ T (2) aϭ m Equations (1) and (2) have three unknowns, ␣, a, and T. Be- mg cause the object and wheel are connected by a string that does not slip, the linear acceleration of the suspended object Figure 10.20 The tension in the cord produces a torque about is equal to the linear acceleration of a point on the rim of the the axle passing through O.
  20. 20. 10.7 Relationship Between Torque and Angular Acceleration 311wheel. Therefore, the angular acceleration of the wheel and gthis linear acceleration are related by a ϭ R␣. Using this fact aϭ 1 ϩ I/mR 2together with Equations (1) and (2), we obtain TR 2 mg Ϫ ⌻ (3) a ϭ R␣ ϭ ϭ ␣ϭ a ϭ g I m R R ϩ I/mR mg (4) Tϭ Exercise The wheel in Figure 10.20 is a solid disk of M ϭ mR 2 2.00 kg, R ϭ 30.0 cm, and I ϭ 0.090 0 kgи m2. The suspended 1ϩ I object has a mass of m ϭ 0.500 kg. Find the tension in the cord and the angular acceleration of the wheel.Substituting Equation (4) into Equation (2), and solving fora and ␣, we find that Answer 3.27 N; 10.9 rad/s2.EXAMPLE 10.13 Atwood’s Machine RevisitedTwo blocks having masses m1 and m 2 are connected to each Substituting Equation (6) into Equation (5), we haveother by a light cord that passes over two identical, friction- [(m 1 Ϫ m 2)g Ϫ (m 1 ϩ m 2)a]R ϭ 2I␣less pulleys, each having a moment of inertia I and radius R,as shown in Figure 10.21a. Find the acceleration of each Because ␣ ϭ a/R, this expression can be simplified toblock and the tensions T1 , T2 , and T3 in the cord. (Assume ano slipping between cord and pulleys.) (m 1 Ϫ m 2)g Ϫ (m 1 ϩ m 2)a ϭ 2I R2Solution We shall define the downward direction as posi-tive for m 1 and upward as the positive direction for m 2 . This (m 1 Ϫ m 2)gallows us to represent the acceleration of both masses by a (7) aϭ Isingle variable a and also enables us to relate a positive a to a m1 ϩ m2 ϩ 2 R2positive (counterclockwise) angular acceleration ␣. Let uswrite Newton’s second law of motion for each block, using This value of a can then be substituted into Equations (1)the free-body diagrams for the two blocks as shown in Figure10.21b: (1) m 1g Ϫ T1 ϭ m 1a (2) T3 Ϫ m 2g ϭ m 2a T2 T1 T3 Next, we must include the effect of the pulleys on the mo- T1 T3 +tion. Free-body diagrams for the pulleys are shown in Figure m1 m210.21c. The net torque about the axle for the pulley on the m1 m1gleft is (T1 Ϫ T2 )R, while the net torque for the pulley on the m2 m2gright is (T2 Ϫ T3 )R. Using the relation ⌺␶ ϭ I␣ for each pul- +ley and noting that each pulley has the same angular acceler- (a)ation ␣, we obtain (b) (3) (T1 Ϫ T2)R ϭ I␣ n1 T2 T2 n2 (4) (T2 Ϫ T3)R ϭ I␣ We now have four equations with four unknowns: a, T1 ,T2 , and T3 . These can be solved simultaneously. Adding T1 mpg mpgEquations (3) and (4) gives T3 (5) (T1 Ϫ T3)R ϭ 2I␣ (c)Adding Equations (1) and (2) gives Figure 10.21 (a) Another look at Atwood’s machine. (b) Free-body diagrams for the blocks. (c) Free-body diagrams for T3 Ϫ T1 ϩ m 1g Ϫ m 2g ϭ (m 1 ϩ m 2)a the pulleys, where m p g represents the force of gravity acting on each (6) T1 Ϫ T3 ϭ (m 1 Ϫ m 2)g Ϫ (m 1 ϩ m 2)a pulley.
  21. 21. 312 CHAPTER 10 Rotation of a Rigid Object About a Fixed Axis and (2) to give T1 and T3 . Finally, T2 can be found from pulleys accelerate counterclockwise. If m 1 Ͻ m 2 , then all the Equation (3) or Equation (4). Note that if m 1 Ͼ m 2 , the ac- values are negative and the motions are reversed. If m 1 ϭ m 2 , celeration is positive; this means that the left block acceler- then no acceleration occurs at all. You should compare these ates downward, the right block accelerates upward, and both results with those found in Example 5.9 on page 129. 10.8 WORK, POWER, AND ENERGY IN ROTATIONAL MOTION F In this section, we consider the relationship between the torque acting on a rigid object and its resulting rotational motion in order to generate expressions for the power and a rotational analog to the work – kinetic energy theorem. Consider the rigid object pivoted at O in Figure 10.22. Suppose a single external force F is ap- φ plied at P, where F lies in the plane of the page. The work done by F as the object ds rotates through an infinitesimal distance ds ϭ r d␪ in a time dt is P dθ dW ϭ Fؒds ϭ (F sin ␾)r d ␪ r O where F sin ␾ is the tangential component of F, or, in other words, the component of the force along the displacement. Note that the radial component of F does no work because it is perpendicular to the displacement.Figure 10.22 A rigid object ro-tates about an axis through O un- Because the magnitude of the torque due to F about O is defined as rF sin ␾der the action of an external force by Equation 10.19, we can write the work done for the infinitesimal rotation asF applied at P. dW ϭ ␶ d ␪ (10.22) The rate at which work is being done by F as the object rotates about the fixed axis is dW d␪ ϭ␶ dt dt Because dW/dt is the instantaneous power ᏼ (see Section 7.5) delivered by the force, and because d␪/dt ϭ ␻, this expression reduces to dW Power delivered to a rigid object ᏼϭ ϭ ␶␻ (10.23) dt This expression is analogous to ᏼ ϭ Fv in the case of linear motion, and the ex- pression dW ϭ ␶ d␪ is analogous to dW ϭ Fx dx. Work and Energy in Rotational Motion In studying linear motion, we found the energy concept — and, in particular, the work – kinetic energy theorem — extremely useful in describing the motion of a system. The energy concept can be equally useful in describing rotational motion. From what we learned of linear motion, we expect that when a symmetric object rotates about a fixed axis, the work done by external forces equals the change in the rotational energy. To show that this is in fact the case, let us begin with ⌺␶ ϭ I␣. Using the chain rule from the calculus, we can express the resultant torque as d␻ d␻ d ␪ d␻ ⌺ ␶ ϭ I␣ ϭ I dt ϭI d ␪ dt ϭI d␪ ␻