When an external force is applied on a body, it sometimes undergoes change in shape and size, such a force is called the deforming force.

The property of a body by virtue of which it regains its original shape and size after the removal of the deforming force is called elasticity. Rubber and metals are elastic in nature. A body which regains its shape and size after the removal of deforming force is called perfectly elastic body.

The property of a body by which it does not retain its original shape and size after the removal of deforming force is called plasticity. Clay, plastic sheets are examples of plastic.

In fact, no substance is perfectly elastic or plastic.

Elastic limit:

When the deforming force on a body is increased gradually, a stage is reached beyond which the body doesn’t come back to its original shape and size even the deforming force is removed. This limit is called elastic limit and the body behaves as a plastic body after it.

Stress:

When a deforming force is applied on a body, an internal restoring force is developed within the body which brings the body back to its original shape after the removal of the deforming force. The restoring force per unit area is called stress. Symbolically,

[latex]Stress = \frac{Restoring\ Force}{Area}[/latex]

In equilibrium condition, the restoring force is equal to the magnitude of the deforming force. Hence, stress may be defined as the applied force per unit area.

Mathematically,

Let, ‘F’ be the force applied on a body of area ‘A’, then, the stress is:

[latex]Stress = F/A[/latex].

Types of stress: