Elastic Behavior of Solids

A body is called an elastic body, and the underlying property is called elasticity, if it regains its original shape and size completely once the deforming force acting on it is removed. Physically, this happens because the internal restoring forces set up when the body is deformed are strong enough to pull every displaced molecule all the way back to its original position. Common elastic materials include metals, rubber and quartz.

A body is said to be perfectly elastic if this recovery of original shape and size is both complete and instantaneous the moment the deforming force is withdrawn. In practice, no real material achieves this ideal exactly, but quartz fibre comes closest, which is why it is used as the standard example of a near-ideal elastic solid.

The opposite behaviour is shown by a plastic body: here, the restoring forces are not strong enough to pull the displaced molecules all the way back, so the body does not regain its original shape or size and instead permanently retains the altered form it was deformed into — the underlying property is called plasticity. Clay, putty, plasticine and thick mud are everyday examples of plastic materials, with putty regarded as the best real-world approximation to a perfectly plastic body. …