Edexcel GCSE Combined Science CP15 Forces and matter: a complete overview of elasticity, Hooke's law and stored energy

What CP15 actually demands

Forces and matter, on Physics Paper 2, is a focused topic about how forces deform objects, centred on Hooke's law and the spring core practical. The examiners reward correct use of $F = ke$, a clear understanding of the limit of proportionality, and the method of the spring practical.

This guide walks through the topic and ties together the matching dot-point page, which has its own practice questions.

Deformation and Hooke's law

Changing the shape of an object needs more than one force. Elastic deformation returns to the original shape; inelastic does not. Hooke's law says extension is proportional to force:

where $k$ is the spring constant (N/m) and $e$ is the extension (m). A stiffer spring has a larger $k$.

The limit of proportionality and stored energy

A force-extension graph is a straight line through the origin until the limit of proportionality, beyond which it curves. Work done stretching a spring is stored as elastic potential energy, equal to the area under the graph, and released when the spring returns to shape.

How CP15 is examined

• Calculations. Using and rearranging $F = ke$.
• Core practical. Describing how to investigate force and extension for a spring.
• Graphs. Interpreting the force-extension graph and the limit of proportionality.
• Energy. Linking the work done stretching a spring to its stored elastic potential energy.

Check your knowledge

A mix of recall and calculation questions covering CP15. Attempt them under timed conditions, then check against the solutions.

1. State Hooke's law. (1 mark)
2. Write the equation linking force, spring constant and extension. (1 mark)
3. A 20 N force stretches a spring by 0.5 m. Calculate the spring constant. (2 marks)
4. State the difference between elastic and inelastic deformation. (2 marks)
5. What is the limit of proportionality? (1 mark)
6. What shape is a force-extension graph while a spring obeys Hooke's law? (1 mark)
7. What is stored in a stretched spring? (1 mark)
8. A spring constant is 30 N/m and a force of 6 N is applied. Calculate the extension. (2 marks)