Hooke's Law and Elastic Behaviour

Measuring extension

1. Measure the original length of spring with a ruler.
2. Clamp the ruler vertically, checked with a set square.
3. Read at eye level using a pointer, measuring between the same two points on the spring.
4. Add masses and measure new length for a range of masses.
5. extension = stretched length − original length.
6. Measure the extension with decreasing mass as well.
7. Repeat and average.

Force-extension graphs

• Hooke's law: the extension is directly proportional to force, up to limit of proportionality.
• Showing Hooke's law on a graph: the line is straight and the line passes through origin. Draw a straight line of best fit passing through origin.
• The limit of proportionality is the end of the straight section.
• Beyond it the line is curved, so force and extension are not proportional.
• Sketch: axes labelled extension and force, a straight line of positive gradient that passes through origin.
• A graph of length against force meets the length axis above zero, at the original length.

Elastic behaviour

• Elastic behaviour: the material returns to its original shape when the force is removed.
• Unloaded, the line returns to origin: no extension remains.
• Beyond the elastic limit: permanent deformation, not returning to original shape.
• Unloaded after the elastic limit, the line returns to the extension axis above the origin: a permanent extension remains.