Elastic materials, and objects such as springs, change shape when a force is exerted on them:
A change in shape like this is called deformation. In general, the greater the force exerted, the greater the amount of deformation. This is why an elastic band gets longer the harder you pull it, and why a rubber ball squashes more the harder you squeeze it.
Remember that if you pull or squeeze too hard, the object may not return to its original size and shape afterwards, and it may even snap. Until you reach this point, a special case called Hooke’s Law applies.
The extension of a material or a spring is its increase in length when pulled. Hooke’s Law says that the extension of an elastic object is directly proportional to the force applied to it. In other words:
You can investigate Hooke’s Law using a spring:
For mass added, calculate the extension (new length – length at start). You can then plot a force-extension graph:
The graph should be a straight line that passes through the origin (0,0). The diagram shows an example of this.
In a force-extension graph:
Using the graph, calculate the work done to extend the spring from 0 m to 0.10 m.
The area under the line is a triangle:
area = ½ × base × height
= ½ × 0.10 × 5 = 0.25 J