A rigid body can be defined as one which does not undergo any deformation when external
forces act on it. When forces are applied on a rigid body the distance between any two
particles of the body will remain unchanged however large the force may be. In actual
practice no body is perfectly rigid. For practical purposes solid bodies are taken as rigid bodies. When external force acts on it bring about a change in its Length, volume or shape. Such forces are called deforming force.