Statement: “the force needed to extend or compress a spring by some distance is proportional to that distance.” and this can be written as:

**F = -kx …………………………(1)**

Where F is the force, x is the length of extension/compression and k is a constant of proportionality known as the spring constant which is usually given in N/m. The negative sign is to signify that the restoring force due to the spring is in the opposite direction to the force which caused the displacement. Pulling down on a spring will cause an extension of the spring downward, which will, in turn, result in an upward force due to the spring.

Figure 1. Stress and strain to understand the Hook’s Law |

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