Extension

Extension is the increase in length of a material (such as a spring or wire) when a stretching force (load) is applied. It is the difference between the stretched length and the original (natural) length.

Equation

Extension = stretched length - natural length.

Hooke’s law

For a spring within its limit of proportionality: F = k x e, where F is the applied force, k is the spring constant and e is the extension.

The limit of proportionality is the point beyond which the extension is no longer proportional to the force. Beyond this point, the spring does not return to its original length when the force is removed.

Spring constant

The spring constant k (measured in N/m) tells you how stiff the spring is. A larger k means a stiffer spring (more force is needed for the same extension).

In exam questions

Know how to read the spring constant from a force-extension graph (k = gradient). Know that the area under a force-extension graph represents the elastic potential energy stored in the spring.