Elasticity

When an external force is applied on an object, its shape or size (or both) change, i.e. deformation takes place. The extent of deformation depends on the material and shape of the body and the external force.

When the deforming forces are withdrawn, the body tries to regain its original shape and size. For example, a spring loaded with a mass or a force applied on the string of a bow or pressing of a rubber ball.

If you apply a force on the string of the bow to pull it, you will observe that its shape changes. But on releasing the string, the bow regains its original shape and size.

The property of matter to regain its original shape and size after removal of the deforming forces is called elasticity.

Molecular Theory of Elasticity

A solid is composed of a large number of atoms arranged in a definite order. Each atom is acted upon by forces due to neighbouring atoms. Due to inter-atomic forces, solid takes such a shape that each atom remains in a stable equilibrium.

When the body is deformed, the atoms are displaced from their original positions and the inter-atomic distances change. If in deformation, the separation increases beyond their equilibrium separation (R > R₀), strong attractive forces are developed. However, if inter–atomic separation decreases (R < R₀), strong repulsive forces develop. These forces, called restoring forces, drive atoms to their original positions.