Hooke's law

Hooke's law describes the elastic force under a small deformation:

$F=k\cdot x$

where $k$ is the spring constant (stiffness) in N/m, and $x$ — the extension (or compression) in metres. The larger $k$, the stiffer the spring.

The energy stored in a deformed spring is:

$E_p=\tfrac12\,k\,x^2$

Enter two of the three quantities ($F$, $k$, $x$) and the calculator works out the third as well as the elastic potential energy. The law holds within the elastic range (for too large an extension it stops being linear).