Rigid body and Momentum:

Body: A body consists of a Number of particles, it has definite shape.

Rigid body:  A rigid body can be defined as the combination of a large number of particles which occupy fixed position with respect to another, before and after applying a load.

A rigid body can be defined as a body, which retains its shape and size even if subjected to some external forces. In reality, nobody is perfectly rigid. But for the simplicity, we take the bodies as rigid bodies.

Momentum: It is the total motion possessed by body. It is vector quantity. It can be expressed

Momentum(M) = mass of the body(m) × Velocity(V) Kg-m/sec