A student presses down on a foam cushion and then removes the force. The foam returns to its original shape. The student then presses much harder and removes the force. This time the foam does not return to its original shape. Explain why the foam behaves differently in each case.

AQA GCSE Physics (8463) — 4.5.3 Forces and elasticity · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

In the first case the foam undergoes elastic deformation, so it returns to its original shape when the force is removed. In the second case the applied force exceeds the elastic limit, causing plastic deformation, so the foam does not return to its original shape.

Examiner tips

  • Use the terms "elastic deformation" and "plastic deformation"
  • Explain that elastic deformation is reversible and plastic is irreversible
  • Mention the elastic limit as the threshold

Common mistakes

  • Confusing elastic with plastic deformation
  • Forgetting to state that the foam returns in the first case
  • Using vague language such as "stiff" instead of "elastic limit"

Mark scheme (4 marks)

  1. In the first case the foam undergoes elastic deformation
  2. Elastic deformation means the foam returns to its original shape when the force is removed
  3. In the second case the force applied exceeds the elastic limit
  4. This causes plastic deformation, so the foam does not return to its original shape

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