A steel spring is stretched by hanging weights from it. At first, the extension increases in proportion to the load. When a very heavy load is applied, the spring does not return to its original length after the load is removed. Explain what happens to the spring at each stage, using the terms limit of proportionality, elastic deformation, and plastic deformation in your answer.

Pearson Edexcel International GCSE Physics (4PH1) — 1.3 Forces, movement, shape and momentum · Explain · 4 marks · View as Markdown

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

A steel spring is hung vertically and weights are added one at a time. For small loads, each equal increase in load produces the same increase in extension. Beyond a certain point, equal increases in load produce larger and larger increases in extension, and the spring is permanently stretched.

Model answer (4 marks)

For small loads the spring is in the elastic region – it obeys Hooke’s law and the extension is directly proportional to the load. The proportionality continues up to the limit of proportionality. When the load exceeds this limit the spring enters the plastic region – the extension is no longer proportional to the load and the material deforms irreversibly. After the very heavy load is removed the spring has undergone plastic deformation and therefore does not return to its original length.

Examiner tips

  • Use the terms in the correct order: elastic deformation, limit of proportionality, plastic deformation.
  • Show the change from proportional to non‑proportional behaviour.
  • Mention that the spring no longer returns to its original length after the heavy load is removed.
  • Keep the answer concise – 4 marks only.

Common mistakes

  • Confusing elastic with plastic deformation; writing that the spring still returns to its original length after the heavy load.
  • Using the wrong order of terms or omitting the limit of proportionality.
  • Giving extra detail about the material’s micro‑structure that is not required for 4 marks.

Mark scheme (4 marks)

  1. For small loads, the spring undergoes elastic deformation
  2. Up to the limit of proportionality, extension is directly proportional to the load / Hooke's law is obeyed
  3. Beyond the limit of proportionality, extension is no longer directly proportional to the load
  4. When the very heavy load is applied and removed, the spring has undergone plastic deformation, so it does not return to its original shape / length

Key terms in this question

limit of proportionality · elastic deformation · plastic deformation · extension

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