When an external force is applied to a body, which is not free to move, there will be a relative displacement of the particles. Due to the property of elasticity, the particles tend to regain their original position.
The external forces may produce the change in length, volume and shape of the body. This external force which produces these changes in the body is called deforming force. A body which experiences such a force is called the deformed body.
When the deforming force is removed, the body regains its original state due to the force developed within the body. This force is called restoring force. The property of a material to regain its original state when the deforming force is removed is called elasticity.
The bodies which possess this property are called elastic bodies. Bodies which do not exhibit the property of elasticity are called plastic. The study of mechanical properties helps us to select the material for specific purposes. For example, springs are made of steel because steel is highly elastic.