$$F = ke \qquad k = \frac{F}{e}\ \ (\text{N/m})$$ $F$ = force, $e$ = extension, $k$ = spring constant. Valid up to the elastic limit.

• Springs in series: $\dfrac{1}{k} = \dfrac{1}{k_1} + \dfrac{1}{k_2}$. Springs in parallel: $k = k_1 + k_2$.
• Energy stored in a stretched spring: $E = \tfrac{1}{2}Fe = \tfrac{1}{2}ke^2$ (area under the force-extension graph).