When a force is applied to a material, it deforms. This
means that the particles of the material are displaced
from their original positions. Provided the force does not
exceed a critical value, the displacements are reversible;
the particles of the material return to their original positions when the force is removed, and no permanent deformation results. This is called elastic behavior.
The laws of elastic deformation are illustrated by the
following example. Consider a right cylindrical block of
height h and cross-sectional area A, subjected to a force
F which acts to extend the block by the amount h
(Fig. 3.1). Experiments show that for elastic deformation h is directly proportional to the applied force and
to the un stretched dimension of the block, but is
inversely proportional to the cross-section of the block
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