Elasticity
Elasticity - the ability of a solid to recover its shape when the deforming forces are removed. The deformation of an elastic material obeys Hooke's law, which states that deformation is proportional to the applied stress up to a certain point. This point is called the elastic limit. Beyond this point additional stresses will cause permanent deformation. All materials have some degree of elasticity, but rubber, for example, is 'more elastic' then metal or wood. The elasticity is a consequence of forces acting between atoms and molecules in any material. These forces keep individual atoms at constant distance in positions at which the potential energy of atoms (or molecules) is minimal. They are called equilibrium positions. If we apply an external force to a given material, we have moved atoms from their equilibrium positions. After removing the force, atoms in elastic materials return to their original positions. We will describe elasticity from a macroscopic point of view, not going into details of crystal structure. The main law governing elasticity of materials is Hooke's law (2).