In a previous chapter the motion of a rigid body was discussed. It was assumed at that stage that a rigid body is one in which the distance between any two points in it is invariable no matter what external forces may act on the body. In practice no body is completely rigid and the subject of elasticity is concerned with the behaviour of materials under the action of external forces. Since no body is perfectly rigid, then a body, when subjected to an external force, undergoes a change of shape or size or both and it is said to be in a state of strain. If the body regains completely its original shape and size when the external force or stress is removed the body is said to be perfectly elastic. On the other hand, if the body completely retains its altered shape and size when the external force is removed then the body is termed perfectly plastic. These terms have a very limited application, however, since materials which are perfectly elastic only regain their original shape and size if the external force is not excessively large. It is also probable that even a perfectly plastic material recovers partially if it is acted upon by only a small external force.
KeywordsDepression Mercury Torque Helium Rubber
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