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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
A steel cable is described as stiff because it has a high spring constant – a large force is required to produce a small extension.
Up to the elastic limit the cable obeys Hooke’s Law, so the extension is proportional to the applied load.
If the load exceeds the elastic limit the cable undergoes plastic deformation: it does not return to its original length when the load is removed and has a permanent extension.
This permanent deformation weakens the cable and makes it unsafe, because it can no longer support the designed load correctly.
Up to the elastic limit the cable obeys Hooke’s Law, so the extension is proportional to the applied load.
If the load exceeds the elastic limit the cable undergoes plastic deformation: it does not return to its original length when the load is removed and has a permanent extension.
This permanent deformation weakens the cable and makes it unsafe, because it can no longer support the designed load correctly.
Examiner tips
- Use the term ‘stiff’ to refer to a high spring constant or large force for a small extension.
- Show the Hooke’s Law relationship up to the elastic limit.
- Explain that exceeding the elastic limit causes permanent (plastic) deformation and loss of load‑bearing capacity.
Common mistakes
- Confusing stiffness with strength – stiffness is about force‑extension, not load‑capacity.
- Saying the cable simply ‘breaks’ instead of describing plastic deformation and permanent extension.
- Omitting the consequence that the cable becomes unsafe and unreliable.
Mark scheme (5 marks)
- A stiff material requires a large force to produce a small extension (or has a high spring constant / force constant)
- Up to the elastic limit the cable obeys Hooke's Law (extension is proportional to force / load)
- Beyond the elastic limit the cable does not return to its original length / shape when the load is removed
- The cable is permanently (plastically) deformed / has a permanent extension
- 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 in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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