A metal beam is balanced horizontally on a pivot. One end of the beam has a heavy load placed on it. Explain why the beam is no longer in equilibrium and what must be done to restore equilibrium.

Cambridge International IGCSE Physics (0625) — 1.5 Forces · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The heavy load produces a moment about the pivot, causing a turning effect.
The moments about the pivot are no longer balanced – the resultant moment is not zero.
To restore equilibrium a force (or load) must be applied on the opposite side of the pivot.
The opposing moment must equal the moment of the load so that the resultant moment is zero.

Examiner tips

  • Use the word ‘moment’ and mention the pivot explicitly.
  • Show that the resultant moment is non‑zero before the counter‑force is applied.
  • State that the counter‑force must create an equal and opposite moment.
  • Use the phrase ‘to restore equilibrium’ to match the mark scheme.

Common mistakes

  • Failing to mention the pivot or the turning effect.
  • Confusing torque with force or not recognising the need for an equal opposite moment.
  • Using vague language such as ‘balance’ without specifying moments.

Mark scheme (4 marks)

  1. The load creates a moment / turning effect about the pivot
  2. The resultant moment about the pivot is not zero / moments are unbalanced
  3. To restore equilibrium, a force (or load) must be applied on the opposite side of the pivot
  4. The opposing moment must equal the moment of the load so that the resultant moment is zero

Key terms in this question

equilibrium · pivot

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