Solids subject to tension or to compression generally obey Hooke's law. This states that the applied force per unit area (the stress) is proportional to the resultant fractional change in length (the strain). The constant of proportionality is the elastic modulus which is characteristic of the material in question. Hooke's law arises from the form of the forces between atoms which are to a good approximation harmonic for small displacements (see tutorial 12 on harmonic motion).