Q.Distinguish between elastic behaviour and plastic behaviour of a material, giving one everyday example of a nearly perfectly elastic body and one example of a strongly plastic (non-elastic) body.

A body is elastic if it fully recovers its original size and shape once a deforming force is removed (e.g. a stretched rubber band or a bent, released steel ruler), and plastic if it stays permanently deformed instead (e.g. squeezed wet clay or dough); no real material is perfectly one or the other. Stress is the internal restoring force set up per unit cross-sectional area of a body in response to a deforming force, $\text{Stress} = F/A$ (SI unit $\text{Pa}$); strain is the resulting fractional change in a dimension of the body (a dimensionless ratio). Three kinds of each arise depending on how the force is applied: tensile/compressive (longitudinal) stress-strain changes length, shearing (tangential) stress-strain changes shape at constant volume, and hydraulic (volume) stress-strain changes volume at constant shape.