Introduction : Solid or Rigid Bodies -
The materials having a definite shape and Volume which it tends to retain are known as solid or rigid bodies.The atoms in a solid vibrate about fixed equilibrium positions. When an external force or load is applied on a body , its shape and size may change.If the body regains its original shape or length on the removal of the applied force or load , the body is said to be elastic and If the body donot regains its original shape or length on the removal of the applied force or load , the body is said to be Plastic.
Elastic Behaviour of Solids :
In a solid , atoms and molecules are arranged in such a way that each molecules is acted upon by forces due to the neighbouring molecules. These forces are known as intermolecular forces.
When no external force is applied on the body , each molecules of the solid is in its equilibrium position and the intermolecular forces between the molecules of the solid are maximum.
What happens to a spring when it is streched or compressed
Case 1. When a spring is slightly stretched by pulling it's ends , the length of the spring increase Means the spring is deformed.
The force acting on the spring to deform it is known as deforming force.
When the deforming force is removed , the spring comes to its original state. The force responsible to bring the spring to its original state is known as restoring force.
Case 2 : When deforming force is applied on a body so that it's length decreases, then the molecules of the body come closer to each other.
As a result of this , the spring is slightly compressed and the intermolecular forces get changed.
The restoring forces developed in the spring bring these displaced molecules to their respective equilibrium positions .
Elasticity :
The property of a solid body by virtue of which it regains its original configuration (i.e Shape and size ) when the external deforming force acting on it is removed is called Elasticity
Elastic :
If the body regains its original shape or length on the removal of the applied force or load , the body is said to be elastic.
Perfectly Elastic Body :
A body is said to be perfectly Elastic if it completely regains its original form , when the deforming force acting on it is removed. Since no material can regain completely its original form so the concept of perfectly Elastic Body is only an ideal concept.
Example : A quartz fibre is the nearest approach to the perfectly Elastic Body.
Plasticity :
The property of the material body by virtue of which it does not regain its original configuration when the external force acting on it is removed is called plasticity.
Plastic :
If the body donot regains its original shape or length on the removal of the applied force or load , the body is said to be Plastic.
Perfectly plastic body :
A body is said to be perfectly plastic if it does not regain its original form , even slightly , when the deforming force is removed .
Since every material partially regain its original form on the removal of deforming force , so the concept of perfectly plastic body is only an ideal concept.