THEORY:

Robert Hooke discovered that force was proportional to the extension of a spring in 1676, this could be applied to any material as long as it remained within its elastic limit. Hence, the equation was formed; where F = Force, k = spring constant and x = extension of the spring/material.

Consequently, if force F was exerted on the spring, it would extend by x; however if force 2F was exerted on the spring it would extend by 2x. Showing the relationship between the force and length is proportional. This occurs up until the materials elastic limit or otherwise known as its limit of proportionality; after this point the relationship between the force and length is no longer proportional, this is called the plastic region.