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.)