Hooke's law

Hooke's law and elastic energy

Tension in an elastic string or spring is $T=\frac{\lambda x}{l}$, where $\lambda$ is the modulus of elasticity, $l$ the natural length and $x$ the extension. Stretching it stores elastic potential energy $\frac{\lambda x^2}{2l}$, which joins kinetic and gravitational energy in the work–energy principle.