A force cannot only change the speed and direction of an object but also its shape.

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Elastic potential energy

An elastic object is able to store energy in that way. If it’s stretched it stores elastic potential energy. A catapult is a good example of that.

An elastic object is also able to store elastic potential energy when it’s squashed.

When an elastic object changes shape and stores this energy the process is known as work: work has been done on the object.

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Hooke’s Law

If an elastic object, like a spring, is stretched it increases in length. This increase is known as its extension. The extension of an elastic object and the force applied to it are directly proportional to one another. This is known as Hooke’s Law.

F = k x e

• – F is the force in newtons (N)
• – k is the spring constant in newtons per metre (N/m)
• – e is the extension in metres (m)

The spring constant (k) changes depending on the object and material. It’s the elasticity of the spring. The larger the value the more force that’s needed to be exerted to extend the spring (the stiffer the spring is). If a graph of force is plotted against exertion then you get a straight line.

Hooke’s Law is only correct if the elastic limit, which is the limit of proportionality, is not exceeded. If a spring is stretched past its elastic limit then it won’t be able to return to its original length once the load has been removed and Hooke’s Law is no longer correct.