A student is testing two different materials, Material A and Material B, by applying increasing forces to each one. Material A returns to its original shape when the force is removed, even after large forces are applied, until it suddenly snaps. Material B permanently changes shape after a certain force is exceeded and does not return to its original shape. Explain the difference between the deformation behaviour of Material A and Material B, using the correct scientific terms.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student is testing two different materials, Material A and Material B, by applying increasing forces to each one. Material A returns to its original shape when the force is removed, even after large forces are applied, until it suddenly snaps. Material B permanently changes shape after a certain force is exceeded and does not return to its original shape.
Model answer (5 marks)
Material A behaves elastically – the deformation is reversible and the material returns to its original shape when the force is removed. It is brittle, so it snaps instead of deforming plastically once the elastic limit is exceeded.
Material B undergoes plastic deformation – beyond a certain force the material no longer follows Hooke’s law. The force at which it stops returning to its original shape is the elastic limit (or limit of proportionality). When the force is removed beyond this point, the material retains a permanent distortion.
Material B undergoes plastic deformation – beyond a certain force the material no longer follows Hooke’s law. The force at which it stops returning to its original shape is the elastic limit (or limit of proportionality). When the force is removed beyond this point, the material retains a permanent distortion.
Examiner tips
- Use the terms ‘elastic deformation’, ‘brittle’, ‘plastic deformation’, and ‘elastic limit’ exactly as in the mark scheme.
- Show the sequence: elastic → brittle snap for A; elastic limit → plastic deformation for B.
- Mention that the deformation of B is permanent when the elastic limit is exceeded.
Common mistakes
- Confusing ‘elastic limit’ with ‘yield point’ – they are the same in this context.
- Using ‘ductile’ for Material A – it is actually brittle.
- Omitting the word ‘permanent’ when describing B’s deformation after the elastic limit.
Mark scheme (5 marks)
- Material A undergoes elastic deformation (up to the point it snaps)
- Material A is described as brittle because it snaps rather than undergoing plastic deformation after the elastic limit
- Material B undergoes plastic deformation beyond a certain force
- The force beyond which Material B no longer returns to its original shape is called the elastic limit (or limit of proportionality)
- For Material B, when the deforming force is removed beyond the elastic limit, the object does not return to its original shape (permanent distortion)
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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