A student is investigating how a spring stretches when different forces are applied to it. The student notices that at first the extension increases steadily with force, but beyond a certain point the spring does not return to its original length when the force is removed. Explain what is happening to the spring at each stage, using the terms elastic deformation, plastic deformation, limit of proportionality, and Hooke's Law.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student hangs increasing weights from a spring and records the extension each time. For the first several weights, the extension increases by the same amount for each extra weight added. After a certain point, the spring stretches much more for each extra weight and, when the weights are removed, the spring is permanently longer than it was at the start.
Model answer (5 marks)
In the first stage the extension is directly proportional to the force applied – the spring obeys Hooke’s Law.
The spring undergoes elastic deformation, so it returns to its original length when the force is removed.
The point at which the behaviour changes is called the limit of proportionality (elastic limit).
Beyond this limit the spring undergoes plastic deformation.
After plastic deformation the spring does not return to its original shape or length when the force is removed.
The spring undergoes elastic deformation, so it returns to its original length when the force is removed.
The point at which the behaviour changes is called the limit of proportionality (elastic limit).
Beyond this limit the spring undergoes plastic deformation.
After plastic deformation the spring does not return to its original shape or length when the force is removed.
Examiner tips
- Use the exact terms – Hooke’s Law, elastic deformation, plastic deformation, limit of proportionality.
- Show the progression from proportional to non‑proportional behaviour.
- Keep each point short and to the point to match the 5‑mark structure.
Common mistakes
- Confusing elastic limit with yield point or mis‑labeling the stages.
- Using vague phrases like ‘stretched’ instead of ‘elastic/plastic deformation’.
- Omitting the term ‘limit of proportionality’ or ‘Hooke’s Law’.
Mark scheme (5 marks)
- In the first stage, the extension is directly proportional to the force applied / the spring obeys Hooke's Law
- In the first stage, the spring undergoes elastic deformation, so it returns to its original length when the force is removed
- The point at which the behaviour changes is called the limit of proportionality / elastic limit, beyond which extension is no longer proportional to force
- Beyond the limit of proportionality / elastic limit, the spring undergoes plastic deformation
- After plastic deformation, the spring does not return to its original shape / length when the deforming force is removed
Key terms in this question
elastic deformation · plastic deformation · limit of proportionality · Hooke's Law · extension
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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