A student stretches a spring by hanging masses from it. At first, the spring obeys Hooke's Law, but after adding too many masses, it no longer does. Explain what Hooke's Law states and describe what happens to the spring's behaviour once the elastic limit has been exceeded.

Edexcel GCSE Physics (1PH0) — 15.1 Forces and matter (Phys only) · Explain · 4 marks · View as Markdown

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

A student is investigating how the extension of a spring changes as more force is applied to it by adding masses one at a time.

Model answer (4 marks)

Hooke's Law states that the extension of a spring is directly proportional to the force applied to it, i.e.
F = kx, where k is the spring constant.
The law only holds up to the limit of proportionality (the elastic limit).
Once the elastic limit is exceeded the spring undergoes plastic distortion or permanent deformation.
After this point the extension is no longer directly proportional to the applied force.

Examiner tips

  • Use the equation F = kx to show proportionality. Mention the elastic limit explicitly. Explain that beyond this the spring deforms plastically. Keep the answer concise and use UK spelling.

Common mistakes

  • Saying Hooke's Law applies to all forces, ignoring the elastic limit. Confusing the elastic limit with the maximum load. Using vague terms like "breaks" instead of "plastic deformation".

Mark scheme (4 marks)

  1. Hooke's Law states that the extension of a spring is directly proportional to the force applied to it
  2. Hooke's Law only applies up to the limit of proportionality
  3. Beyond the elastic limit the spring undergoes plastic distortion / deformation
  4. The extension is no longer directly proportional to the force applied (after the elastic limit)

Key terms in this question

Hooke's Law · elastic limit

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