Hooke's Law

If a force $F$ acts on a spring, its length increases. The extension of the spring measured from its original position will be called $s$. In this case, a proportional correlation between the repelling force $F_R$, which is equal to $F$, and the extension can be found. This result can be mathematically formulated as $$\boxed{F_\mathrm{R} = -ks}$$ This linearity with the spring constant $k$ is called Hooke's law. The minus sign indicates the restoring character of the force.