Elastic behaviour of materials

In a solid, interatomic forces of attraction and repulsion hold every pair of neighbouring atoms at a particular equilibrium separation -- their mean positions. When an external DEFORMING FORCE acts on the solid, it pulls some atoms further apart or pushes others closer together than their equilibrium separation. The moment that deforming force is removed, the very same interatomic forces of attraction and repulsion that held the atoms in place to begin with act to restore every atom back to its original mean position. A body that does this -- that regains its exact original size and shape once the deforming force is taken away -- is called an ELASTIC body, and the property itself is called ELASTICITY. Rubber, metals, and steel ropes are everyday examples of materials that behave (at least over some range of applied force) elastically. Not every solid does this, however: a body that does NOT regain its original shape and size once the deforming force is removed is called a PLASTIC body, and the property is called PLASTICITY -- glass is the textbook's example of a material that is essentially plastic (it shatters or stays permanently deformed rather than springing back). It is worth noting that 'elastic' and 'plastic' describe the behaviour of a given material only up to …