A rigid body is defined as a body which does not change in shape or size when acted upon by external
forces. In other words, it is assumed that bodies under consideration are rigid when the distance between
any two parts in the body remains constant even on the application of external forces.
In actual practice, nobody is perfectly rigid. Each and everybody when acted upon by an external
force undergoes certain deformations in the shape or size of the body. However, in the case of most bodies,
the deformation is too small to be ignored and the bodies are considered to be rigid bodies.
Rigidity is a relative term. For example, bodies made of steel or cast iron are more rigid than
rubber and cork is more rigid than rubber. Similarly wood is more rigid when compared to chalk.