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# Elasticity

The property by virtue of which a body tends to regain its original shape and size after removal of the deforming force is known as elasticity.

When an external force is applied on a body, which is not free to move, the shape and size of the body change. The force applied is called deforming force.

When the deforming forces are removed, the body tends to regain its original shape and size due to a force developed within the body.

The force developed within the body, which is equal and opposite to deforming force is called restoring force.

Bodies, which completely regain their original size and shape after the removal of the deforming force, are called elastic bodies. Bodies which change the shape and size on the application of force and which do not regain their original condition on removal of the deforming forces are said to be plastic bodies.

Bodies which do not change their shape and size on application of force are called rigid bodies.

The property by virtue of which a body tends to regain its original shape and size after removal of the deforming force is known as elasticity.

## Stress

When an external force is acting on an elastic body, it causes deformation (change in shape or in size or both). At the same time, due to elastic property, a force is developed within the material, which is equal and opposite to the applied force, to bring the body to its original shape and size. This force is ‘restoring force’.

## Stress =Force (F)/Area (A)

The stress is defined as the restoring force acting on unit area.

Since the applied force and the restoring force are equal in magnitude, the ‘stress’ is measured as the applied force acting per unit area.

The unit for stress is newton per square metre with symbol N m-2 or ‘pascal’ with symbol ‘Pa’.

## Tensile stress

When it tends to increase the length in the direction of the force, it is tensile stress

## Compressive stress

When the applied force tends to compress the body, the stress is compressive stress.

## Shear stress/ tangential stress

When it acts parallel to the surface of a body, the stress is tangential stress.

## Hooke’s law

Within the elastic limits, the strain produced in a body is directly proportional to the stress which causes it.

i.e., strain ∝ stress

or stress ∝ strain

stress/ strain= a constant

This constant is called ‘modulus of elasticity’.