# Solids under stress

> Hooke's law, the spring constant, stress, strain and the Young modulus, elastic and plastic behaviour, and energy stored in a stretched material.

Canonical: https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.materials
Last updated: 2026-08-01

Flagship subject: topics and specification statements are authored to the WJEC specification. No question in it has been through human review yet.

- Subject: [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- Specification reference: Unit 1.5
- Difficulty: 3/5
- Assessment objectives: AO1, AO2
- Status: authored · checked · 2026-08-01

## Specification coverage

| Reference | Statement | AOs |
| --- | --- | --- |
| Unit 1.5(a) | define stress, strain and the Young modulus and relate them for an elastic material | AO1 |
| Unit 1.5(b) | describe Hooke law and the spring constant for an elastic body | AO1 |
| Unit 1.5(c) | identify elastic, plastic and fracture behaviour including the limit of proportionality and elastic limit | AO1, AO2 |
| Unit 1.5(d) | calculate elastic strain energy from a force-extension graph | AO2 |
| Unit 1.5(e) | interpret force-extension and stress-strain graphs to identify proportional limit and yield | AO2, AO3 |
| Unit 1.5(f) | derive and use strain energy and strain energy density relations | AO2 |

## Summary

Hooke's law, the spring constant, stress, strain and the Young modulus, elastic and plastic behaviour, and energy stored in a stretched material.

## What earns marks

- Stress = F/A, strain = Δl/l, Young modulus E = stress/strain.
- Elastic strain energy = ½Fx = area under a force–extension graph.
- Brittle materials fracture without plastic deformation; ductile ones show a large plastic region.
- The limit of proportionality precedes the elastic limit — they are not the same point.

## Where marks are lost

- Using diameter instead of radius in the cross-sectional area.
- Reading the Young modulus off a force–extension rather than a stress–strain graph.
- Confusing the elastic limit with the yield point.

## Related

- [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- [Solids under stress lesson](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.materials)