# A bungee cord is made from a polymer material. When a person jumps and the cord stretches, the cord stores elastic potential energy and then returns the person upwards. Explain why the bungee cord is described as having a low Young modulus, and describe how the force needed to stretch the cord changes as extension increases beyond the limit of proportionality.

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

> A bungee cord stretches considerably under the weight of a jumper, storing energy and then recoiling to return the jumper upwards.

## Mark scheme (5 marks)

1. A low Young modulus means the material stretches a large amount (large strain) for a given applied stress
2. Young modulus = stress ÷ strain, so a low value means a large strain relative to the stress applied
3. Up to the limit of proportionality, force and extension are proportional (Hooke's law is obeyed)
4. Beyond the limit of proportionality the relationship is no longer proportional / Hooke's law is no longer obeyed
5. Beyond the limit of proportionality, a smaller additional force produces a larger increase in extension (the cord becomes easier to stretch / less force needed per unit extension)

## Key terms

- [Young modulus](https://www.gradenine.co.uk/glossary/young-modulus)
- [limit of proportionality](https://www.gradenine.co.uk/glossary/limit-of-proportionality)
- [elastic potential energy](https://www.gradenine.co.uk/glossary/elastic-potential-energy)

## 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-bungee-cord-is-made-from-d68b94d6) · Published by Druglandscape Ltd.