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 each added force, but then the spring starts to stretch much more for each extra force added, and eventually the spring does not return to its original length when the force is removed. Explain what is happening at each stage of this investigation, 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. At first the spring behaves predictably, but beyond a certain point the behaviour changes, and after removing all the weights the spring is permanently longer than it was at the start.
Model answer (5 marks)
1. In the first stage the extension is directly proportional to the force applied – the spring obeys Hooke’s Law, F = kx.
2. The spring undergoes elastic deformation, so it returns to its original length when the force is removed.
3. The point where the extension is no longer directly proportional to the force is called the limit of proportionality.
4. Beyond the limit of proportionality the spring no longer obeys Hooke’s Law – the extension increases more rapidly for each additional force.
5. When the spring does not return to its original length after the force is removed, it has undergone plastic deformation – the deformation is permanent.
2. The spring undergoes elastic deformation, so it returns to its original length when the force is removed.
3. The point where the extension is no longer directly proportional to the force is called the limit of proportionality.
4. Beyond the limit of proportionality the spring no longer obeys Hooke’s Law – the extension increases more rapidly for each additional force.
5. When the spring does not return to its original length after the force is removed, it has undergone plastic deformation – the deformation is permanent.
Examiner tips
- Use the exact terms: elastic deformation, plastic deformation, limit of proportionality, Hooke’s Law. Explain the change in behaviour at each stage. Show the proportional relationship in the first stage and the loss of proportionality after the limit.
- Include the consequence of plastic deformation – permanent change in length.
Common mistakes
- Confusing elastic with plastic deformation. Failing to mention the limit of proportionality. Using vague phrases like "the spring stretches more" without linking to 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, meaning it returns to its original shape / length when the force is removed
- The point at which the extension is no longer directly proportional to the force is called the limit of proportionality
- Beyond the limit of proportionality, the spring no longer obeys Hooke's Law / the extension increases more rapidly for each additional force
- When the spring does not return to its original length after the force is removed, it has undergone plastic deformation / the deformation is permanent
Key terms in this question
Hooke's Law · elastic deformation · plastic deformation · limit of proportionality
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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