NCERT Book Class 11 Physics Chapter 6 System of Particles and Rotational Motion

CHAPTER SIX

SYSTEMS OF PARTICLES AND ROTATIONAL MOTION

6.1 INTRODUCTION
In the earlier chapters we primarily considered the motion
of a single particle. (A particle is ideally represented as a
6.1 Introduction point mass having no size.) We applied the results of our
6.2 Centre of mass study even to the motion of bodies of finite size, assuming
6.3 Motion of centre of mass that motion of such bodies can be described in terms of the
6.4 Linear momentum of a motion of a particle.
system of particles Any real body which we encounter in daily life has a
6.5 Vector product of two vectors finite size. In dealing with the motion of extended bodies
6.6 Angular velocity and its (bodies of finite size) often the idealised model of a particle is
relation with linear velocity inadequate. In this chapter we shall try to go beyond this
6.7 Torque and angular inadequacy. We shall attempt to build an understanding of
momentum the motion of extended bodies. An extended body, in the
6.8 Equilibrium of a rigid body first place, is a system of particles. We shall begin with the
6.9 Moment of inertia consideration of motion of the system as a whole. The centre
6.10 Kinematics of rotational of mass of a system of particles will be a key concept here.
motion about a fixed axis We shall discuss the motion of the centre of mass of a system
of particles and usefulness of this concept in understanding
6.11 Dynamics of rotational
motion about a fixed axis the motion of extended bodies.
A large class of problems with extended bodies can be
6.12 Angular momentum in case
of rotation about a fixed solved by considering them to be rigid bodies. Ideally a
axis rigid body is a body with a perfectly definite and
unchanging shape. The distances between all pairs of
Summary
particles of such a body do not change. It is evident from
Points to Ponder
this definition of a rigid body that no real body is truly rigid,
Exercises
since real bodies deform under the influence of forces. But in
many situations the deformations are negligible. In a number
of situations involving bodies such as wheels, tops, steel
beams, molecules and planets on the other hand, we can ignore
that they warp (twist out of shape), bend or vibrate and treat
them as rigid.

6.1.1 What kind of motion can a rigid body have?
Let us try to explore this question by taking some examples
of the motion of rigid bodies. Let us begin with a rectangular