A steel cable used to support a suspension bridge is stretched by a large tensile force. Explain how stress, strain and the Young modulus are each used to describe the behaviour of the cable under this force, and state what the Young modulus tells an engineer about the cable material.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Engineers selecting materials for a suspension bridge must consider how the cables will respond to large tensile forces. Steel is chosen because of its mechanical properties under stress.
Model answer (5 marks)
Stress is the force applied divided by the cross‑sectional area of the cable.
Strain is the fractional extension – the change in length divided by the original length.
The Young modulus is the ratio of stress to strain (stress ÷ strain).
The Young modulus is a measure of the stiffness of the material.
A high Young modulus for steel means the cable will only deform (stretch) by a small amount under large stress, making it suitable for the bridge.
Strain is the fractional extension – the change in length divided by the original length.
The Young modulus is the ratio of stress to strain (stress ÷ strain).
The Young modulus is a measure of the stiffness of the material.
A high Young modulus for steel means the cable will only deform (stretch) by a small amount under large stress, making it suitable for the bridge.
Examiner tips
- Use the exact definitions of stress, strain and Young’s modulus. Show the relationship stress = E × strain. Explain that a high E indicates a stiff material. Link the stiffness to the bridge’s requirement for minimal deformation.
Common mistakes
- Confusing stress with strain or vice versa. Using the wrong formula for Young’s modulus (e.g. strain ÷ stress). Failing to mention that a high Young’s modulus means less deformation under load.
Mark scheme (5 marks)
- Stress is defined as the force applied divided by the cross-sectional area of the cable
- Strain is the fractional extension — the extension of the cable divided by its original length
- The Young modulus is the ratio of stress to strain (stress divided by strain)
- The Young modulus is a measure of the stiffness of the material
- A high Young modulus for steel means the cable will only deform (stretch) by a small amount under large stress, making it suitable for the bridge
Key terms in this question
stress · strain · Young modulus · tensile force
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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