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ELASTICITY Synopsis : 1. Deforming force : The external force acting on a body on account of which its size or shape or both change is defined as the deforming force. 2. Restoring force : The force which restores the size and shape of the body when deformation forces are removed is called restoring force. Deforming force and restoring force are not action reaction pair. Restoring force opposes the change in the size and shape of a body. 3. Rigid body : A body whose shape and size cannot be changed however large the applied force is called rigid body. There is no perfectly rigid body in nature. 4. Elasticity : The property of a body by virtue of which it regains its original size and shape immediately after the deformation forces are removed is called elasticity. Elasticity is a molecular phenomenon. It is because of cohesive forces. 5. Elastic body : A body which shows elastic behaviour is called elastic body. E.g. steel, rubber. Quartz is very nearly perfectly elastic body. 6. Plastic body : A body which does not show elastic behaviour is called plastic body. E.g. putty, clay, mud, wax, lead, dough, chewing gum, butter wax etc. 7. Out of the given materials, a body in which it is more difficult to produce strain is more elastic. OR The body which requires greater deforming force to produce a certain change in dimension is more elastic. a) steel is more elastic than rubber b) glass is more elastic than rubber c) water is more elastic than air d) springs are made of steel but not of copper because steel is more elastic than copper. 8. By the process of hammering or rolling the body elasticity increases. 9. By the process of annealing, the elastic property of a body is reduced. 10. For invar steel (Fe-64%, Ni-36%) the elastic pro