Elasticity: It is the property by which a body offers resistance to external forces tending to change its volume or shape or both and it will regain its original form when the deforming force is removed.
Perfectly Rigid body: A body is said to be perfectly rigid when it is impossible to alter its shape by the application of force. But no body is perfectly rigid.
Perfectly Elastic Body: When a body is acted upon by a force or a system of forces, it undergoes changes in its shape or size. If the body recovers its original dimension completely after withdraw of the applied force, then the body is said to be perfectly elastic but no body is perfectly elastic.
Stress:
When a force is applied on a body, internal forces are generated. These internal forces react in a direction opposite to the forces applied and tend to bring the body to its original state; the restoring force per unit area of the body is known as stress.
Stress = Force/Area
If the force is inclined to the surface, then its component, perpendicular to the surface, measured per unit area, is called normal stress and the component acting along the surface, per unit area, is called tangential or shearing stress.
(i) Normal Stress: Restoring force per unit area perpendicular to the surface as called normal stress.
(ii) Tangential Stress: Restoring force per unit area parallel to the surface is called tangential stress