# Explain why a steel cable used to suspend a bridge is described as stiff, and describe what happens to the cable if the load applied to it exceeds its elastic limit.

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

> Steel cables used in suspension bridges must support very large loads without permanently changing shape. Engineers select materials with specific mechanical properties to ensure the bridge remains safe.

## Mark scheme (5 marks)

1. A stiff material requires a large force to produce a small extension (or has a high spring constant / force constant)
2. Up to the elastic limit the cable obeys Hooke's Law (extension is proportional to force / load)
3. Beyond the elastic limit the cable does not return to its original length / shape when the load is removed
4. The cable is permanently (plastically) deformed / has a permanent extension
5. This makes the cable unsafe / unreliable because it will no longer support the designed load correctly (or the structure of the cable is changed / weakened)

## Key terms

- [stiff](https://www.gradenine.co.uk/glossary/stiff)
- [elastic limit](https://www.gradenine.co.uk/glossary/elastic-limit)

## 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/explain-why-a-steel-cable-used-254093d6) · Published by Druglandscape Ltd.