Sol. : When external force acts on the body, the body tends to undergo some deformation. If external force is removed and body comes back to its original shape and size (that the deformation disappears completely), the body is called as elastic body. This property, by virtue of which certain materials return back to their original position after the removal of the external force, is called elasticity.
The body will regain its previous shape and size only when the deformation caused by the external force, is within certain limit. Thus there is limiting value of force up to and within which, deformation disappears completely on the removal of the force. The value of stress corresponding to limiting force is called as the elastic limit of the material.
If the external force is large so that the stress exceeds elastic limit, the material loses to some extent the property of its elasticity. If force is removed now, the material will not return to the origin shape of it and size and there will be residual deformation in material.