Hooke's Law

The length of an unstretched material is called its natural length. When the material is stretched, the change in length is called the extension. This extension will occur with metal wires, helical springs and any other material that will stretch without immediately snapping or deforming.

We know that forces can make an object change shape. We know that with some objects the amount it changes shape is directly proportional to how much force is applied. These objects are said to obey Hooke's Law:

The extension is directly proportional to the applied force (or load)

The Elastic Limit

All objects have an elastic limit; this is the point at which they will not return to their original shape when released.

Hooke's Law no longer applies when an object reachers the elastic limit. We can see this on a graph of extension against load. The first part of the graph shows Hooke's law being obeyed - it produces a straight line graph.

When the force becomes great enough, the graph starts to curve, (this is the elastic limit). The fact that the line is no longer straight shows that Hooke's law no longer applies.

Springs and wires will obey Hooke's Law and produce a straight-line graph when extended. However, elastic bands do not and produce a curved line when stretched.

You should know:

that the extension of a wire or spring changes when you apply a force to it
that Hooke's Law describes what happens to a spring or wire before it has been stretched to its elastic limit
how to identify the elastic limit on a graph
how rubber bands stretch when a force is applied to them.