understand and use the terms load, extension, compression and limit of proportionality

📐 Stress and Strain – A-Level Physics 9702

Key Terms

• Load – The external force applied to a material (symbol $F$).
• Extension – The increase in length of a material when stretched (symbol $ΔL$).
• Compression – The shortening of a material when pushed together.
• Limit of Proportionality – The maximum stress for which strain remains directly proportional to stress (Hooke’s law holds).

What is Stress?

Stress is the internal force per unit area that resists deformation. Mathematically: $$\sigma = \frac{F}{A}$$ where $\sigma$ is stress, $F$ is the applied load, and $A$ is the cross‑sectional area.

Think of a rubber band being pulled: the tighter you pull, the higher the stress inside the band.

What is Strain?

Strain measures how much a material deforms relative to its original length. Mathematically: $$\varepsilon = \frac{ΔL}{L_0}$$ where $ΔL$ is the change in length and $L_0$ is the original length.

Imagine a spring that stretches when you hang a weight from it – the longer it gets, the greater the strain.

Load, Extension & Compression – Everyday Examples

1. Load: The weight of a book on a table. The book exerts a downward force on the table.
2. Extension: A fishing line pulled taut. The line elongates as you increase the pull.
3. Compression: A stack of books pressing down on the shelf below. The shelf experiences a shortening force.

Hooke’s Law & the Limit of Proportionality

For many elastic materials, stress and strain are linearly related up to a certain point:

$$\sigma = E\,\varepsilon$$

where $E$ is the Young’s modulus. The limit of proportionality is the maximum stress where this linear relationship holds. Beyond this, the material may yield or break.

Analogy: A rubber band stretches linearly until it reaches its maximum stretch; after that it snaps.

Units & Conversion Table

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