All the rigid bodies are to some extent elastic. This means that their dimensions can be changed by pulling, pushing, twisting or by compressing them. e.g. When a helical spring is gently pulled at its both ends, the length of helical spring increases slightly. When we remove the forces applied at the ends of spring, it regains its original size and shape. Similarly when a rubber cord is pulled at both the ends, its length increases as long as it is pulled. As soon as the forces applied at the two ends of rubber cord are removed, rubber cord regains its original size and shape.
            When a body is subjected to a set of balanced forces; which do not cause any type of motion, produce change in size, shape or both. Some bodies undergo change in dimensions (size, shape or both) easily, while some bodies are reluctant to undergo any change in dimensions. Some bodies show permanent change in their size, shape or both, after removal of applied force, whereas some bodies preserve their original size, shape or both after removal of applied force.

 Deforming forces: The system of external forces which tend to change the dimension (shape, size or both) of a body are called deforming forces.

Restoring forces: The system of internal forces which tend to bring the body back to the original dimension (shape, size or both) are called restoring forces. 

Elasticity: The property by the virtue of which a body tends to regain its original configuration after the removal of deforming forces is called elasticity.

Elastic body: Bodies/ materials which return exactly to its original dimensions after removal of deforming forces are called perfectly elastic bodies/materials.
            e.g., quartz

Plasticity: Plasticity is the property of a material to undergo a permanent deformation even after removal of external deforming forces.

Plastic bodies: The bodies which do not return at all its original dimensions after removal of deforming force are called plastic (inelastic) bodies/materials.
            e.g., Plasticine, putty (perfectly plastic body), lead, clay etc.

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