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DYNAMICS: PART OF RIGID BODY MECHANICS
 In Chapter 1 of the book we had defined Mechanics as that branch of
physical science which is concerned with the resultant effect of forces on
bodies, both in a state of rest or in motion. Mechanics is subdivided into
three branches viz. Mechanics of Rigid Bodies, Mechanics of Deformable
Bodies and Mechanics of Fluids.
 Mechanics of Rigid Bodies is the branch of Mechanics where the body is
assumed to be perfectly rigid i.e there is no relative movement between the
particles forming the body and also there is no deformation of the body
under the action of the forces. Though engineering structures and
machines do have slight deformation under the action of loads, still they
are treated as rigid bodies since the deformations are very small and they
can be neglected. We can thus apply the conditions of equilibrium to such
static bodies and the equations of motions to moving bodies.
 Mechanics of Rigid Bodies is further divided into Statics and Dynamics.
Statics is that part of Rigid Body Mechanics which analysis bodies at rest.
On the other hand, Dynamics is that part of Rigid Body Mechanics which
analyses motion of moving bodies. By the application of motion analysis we
wish to gather information about the following parameters of a moving
body.
1) the path traced by moving body
2) the rate at which the body moves
3) the time aspect of motion
4) the position occupied by the body during motion
5) the forces acting on the body which are responsible for the motion
6) the relation between the forces and the motion so produced
Introduction to Dynamics

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The study of Dynamics herein would help us in answering all the above
questions about a moving body.
KINEMATICS AND KINETICS
 Dynamics is further subdivided into Kinematics and Kinetics. Bodies
acquire motion due to the action of forces on it. The Kinematics part of
Dynamics is concerned only with the study of motion of the body without
consideration of the forces which have caused the motion of the body.
Kinematics therefore analyses only the geometry of the motion.
 Kinetics is that part of Dynamics which relates the forces acting on the
body to the motion of the body. By knowing the forces acting on the body
we can predict the resultant motion of the body using the various laws and
principles of Kinetics.

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PARTICLE DYNAMICS
 The motion analysis of a body is referred to as Particle Dynamics when the
body is idealized as a particle.
 Particle Dynamics may be Kinematic Analysis or could be Kinetic Analysis
of motion. When we idealise a body as a particle it does not mean that we
are dealing with minute point like object. It rather means that the size and
shape of the body is of no consequence in the analysis of the motion.
 For example if we wish to analyse the motion of a ship traveling between
two ports, kilometers apart, then the shape and size of the ship is of no
relevance in Motion Analysis. Hence motion of the ship can be
characterized by motion of its mass centre. Whenever a body is treated as a
particle, all the forces acting on the body are assumed to be concurrent at
the mass centre of the body.
 Any rotation of the body is also neglected. For example if we analyse the
motion of an airplane, treating it as a particle is justifiable, as long as we
are interested to know the position, rate of motion or the path described by
an airplane.
 However if the plane experiences turbulent motion due to low air pressure
pocket then the motion analysis at such an instant cannot be carried out
treating the airplane as a particle. In such situations Rigid Body Dynamics
has an important role to play.
RIGID BODY DYNAMICS
 In Rigid Body Dynamics, motion analysis involves the shape and size of the
body. Since the size is involved, the forces, usually of non-concurrent type
act at different locations on the body and they tend to cause rotation of the
body apart from moving the body. The relation between the forces and the
motion so produced is studied in Kinetic Analysis of Rigid Bodies. The
relations between the motions of different particles forming the rigid body
are studied in Kinematics of Rigid Bodies.
 In this book, the Particle Dynamics has been dealt in Chapters 9, 10, 11
and 12. Chapter 9 involves Kinematics of Particles whereas Chapters 10, 11
and 12 deal with Kinetics of Particles.
 Kinematics of Rigid Body Dynamics is dealt with in Chapters 13. Kinetics of
rigid Bodies is not dealt with us it is beyond the scope of this book.

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REFERENCE FRAME
 For any motion analysis, we need to take a reference frame i.e an origin
with a set of co-ordinate axes, for measurement of motion parameters. The
reference frame could be fixed or moving.
 Newtonian frame of reference also known as inertial reference frame is a set
of co-ordinates axes fixed or moving with uniform velocity. Newton's laws
are valid for such a reference frame.
 For most of our engineering applications, the Newtonian reference frame is
fixed at the earth, assuming it to be stationary. This assumption will not
introduce any significant error in the calculations, since earth's angular
velocity or angular acceleration is very small as compared to the moving
bodies on which we work However for motion analysis of planets, satellites
or rockets, we are required to fix the Newtonian reference frame on a fixed
star like the Sun.

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EXERCISE
Theory Questions
Q. 1. List any ten applications where knowledge of Dynamics would be
essential.
Q. 2. What are different branches of dynamics? Explain with suitable
examples.
Q. 3. What is meant by motion analysis? Explain with an illustration.
Q. 4. Explain kinematic and kinetics with suitable example.
Q. 5. When is the motion analysis referred to as Particle. Dynamics and when
as Rigid Body Dynamics.
Q. 6. Write a short note on "Newtonian reference frame".
UNIVERSITY QUESTIONS
1. Find out forces in FB and BE using method of section and other members
by method of joints. (8 Marks)
AF = FB = FE = ED = 2m
CE = 3m
2. Determine the Centre of gravity of the shaded area. (8 Marks)

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3. A particle moves in a straight line with a velocity-time diagram shown in
figure. If S=-25m at t=0, draw displacement – time and acceleration time
diagrams for 0 to 24 seconds. (6 Marks)