# A student compares two materials: a steel spring and a strip of polythene. The steel spring obeys Hooke's Law up to a certain point. The polythene strip stretches easily under a small force and does not return to its original length when the force is removed. Explain the behaviour of each material, using the terms elastic deformation, plastic deformation, and limit of proportionality in your answer.

> AQA A-Level Physics (7408) — 3.4.2 Materials · Explain · 5 marks

## Mark scheme (5 marks)

1. The steel spring undergoes elastic deformation (below the limit of proportionality)
2. The extension of the steel spring is proportional to the force applied (up to the limit of proportionality / it obeys Hooke's Law)
3. The limit of proportionality is the point beyond which extension is no longer proportional to the applied force
4. The polythene strip undergoes plastic deformation
5. Plastic deformation means the material is permanently deformed / the deformation is irreversible after the force is removed

## Key terms

- [elastic deformation](https://www.gradenine.co.uk/glossary/elastic-deformation)
- [plastic deformation](https://www.gradenine.co.uk/glossary/plastic-deformation)
- [limit of proportionality](https://www.gradenine.co.uk/glossary/limit-of-proportionality)

## Related

- [Revision notes for AQA A-Level Physics (7408)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-compares-two-materials-a-49382c46) · Published by Druglandscape Ltd.