What is rigid body ? Explain rigid body. 

The distance between all pairs of particles of system (body) do not change then this body is known as rigid body.

Rigid body is a special case of system of particles.

A rigid body is an ideal concept while solved body is a real.

Rigid body is a body with a perfectly definite and unchanging shape. A solid body can be deformed while a rigid body cannot. For many particle purposes a solid body can be treated as a rigid body.

For example : Wheel, tops, steel beams, molecules and planets. For a body it can be ignore that they warp, bend or vibrate and treat them as rigid.