Hooke's law
Tensional stress of a uniform bar

A rod of any elastic material may be viewed as a linear spring. The rod has length L and cross-sectional area A. Its tensile stress σ is linearly proportional to its fractional extension or strain ε by the modulus of elasticity E:

The modulus of elasticity may often be considered constant. In turn,

(that is, the fractional change in length), and since

it follows that:

The change in length may be expressed as
combination:two_responses