A solid has definite shape and size. In order to change (or deform) the shape or size of a body, a force is required. If you stretch a helical spring by gently pulling its ends, the length of the spring increases slightly. When you leave the ends of the spring, it regains its original size and shape. The property of a body, by virtue of which it tends to regain its original size and shape when the applied force is removed, is known as elasticity and the deformation caused is known as elastic deformation. However, if you apply force to a lump of putty or mud, they have no gross tendency to regain their previous shape, and they get permanently deformed. Such substances are called plastic and this property is called plasticity. Putty and mud are close to ideal plastics.
Solids and their mechanical properties
Mechanical Properties of solids describe characteristics such as their strength and resistance to deformation.
It describes the ability of an object to withstand the stress applied to that object. Objects also resist changing their shape.
For example:- Objects such as clay can be easily deformed so they have less resistance to deformation but objects like iron don’t change their shapes easily. When heated they change their shapes which means they have very high resistance to deformation.
Elastic Behaviour of Solids:
- These are bonded together by interatomic or intermolecular forces and stay in a stable equilibrium position.
- When a solid is deformed, the atoms or molecules are displaced from their equilibrium positions causing a change in the interatomic(or intermolecular) distances.
- When the deforming force is removed, the interatomic forces tend to drive them back to their original positions. Thus the body regains its original shape and size.
For example:- A spring, If we stretch a spring it changes its shape and when the external force is removed spring comes back to its original position.
Another similar example is the Rubber band.
Perfectly elastic bodies:
Those bodies which regain its original configuration immediately and completely after the removal of deforming force are called perfectly elastic bodies. The nearest approach to perfectly elastic bodies is quartz fiber.
If a body does not regain its original size and shape completely and immediately after the removal of deforming force, it is said to be a plastic body and this property is called plasticity.
Perfectly plastic bodies:
That body which does not regain its original configuration at all on the removal of the deforming force, it is said to be a plastic body and the property is known as plasticity. Putty and paraffin wax are nearly perfectly plastic bodies.