Introduction
Rigid body is a body with a perfectly definite and unchanging shape . The distances between all pairs of particles of such a body do not change .
In pure translational motion at any instant of time all particles of the body have the same velocity .
In rotation of a rigid body about a fixed axis , every particle of the body moves in a circle , which lies in a plane perpendicular to the axis and has its centre on the axis .
The motion of a rigid body which is not pivoted or fixed in some way is either a pure translation or a combination of translation and rotation . The motion of a rigid body which is pivoted or fixed in some way is rotation .
Centre of mass
o Imaginary point where the whole mass of system can be assumed to be concentrated o The centre of mass of two bodies lies in a straight line . ( Here m1 & m2 are two bodies such that m1 is at a distance x1 from O , & m2 at a distance x2 from O . )