In mechanics of rigid bodies, it is assumed that there is no relative displacement between different parts of the body when a system of force acts on it. But really this does not happen. There must be some relative displacement between different parts of the body. This change depends upon the strength of the material of the body. If a system of forces, in equilibrium, acts on a non-rigid body, the body changes its shape or size or both. This can be written as “the body undergoes a deformation” or “a deformation is produced in the body” or “the body is deformed”. The system of forces is called a system of deforming forces. When a system of deforming forces in equilibrium, is applied on a non-rigid body, it (the body) is deformed, and when all the forces are removed, there are two cases: 1. The body tends to regain its original shape and size. 2. The body tends to retain