Question

Explain the definition of a "rigid body". Why is this approximation useful?

Answer 1

A rigid body is defined as a body on which the distance between two points never changes whatever be the force applied on it.

The rigid body approximation is useful when we want to ignore or abstract away issues of shape deformation. This is so even though many of the forces (normal force, friction) do occur due to these deformations. As long as these forces do not change the shapes of the bodies to a significant extent, the rigid body approximation is still useful for exploring the geometry of what is happening.