Hooke’s law describes the opposing force exerted by a spring:

where k is the spring constant, and d is the distance the end of the spring is disturbed from its position at relaxation (assuming the other end is fixed).

This law is used not only for metal springs, but also in many other “spring-like” interactions in nature, including interactions between microscopic or subatomic particles. The simple linear nature of the law makes it very convenient to work with analytically.

However, this simple linear behavior should be a clue that in most instances the law is not precise; it is just an approximation. For metal springs, the law only holds in the region where the spring behaves elastically; outside of this region, the law breaks down.