A seesaw in a playground has a pivot at its centre. A child of weight 400 N sits 1.5 m from the pivot on one side. An adult wants to balance the seesaw by sitting on the other side, closer to the pivot than the child. Explain, using the principle of moments, why the adult must sit closer to the pivot than the child.

Eduqas GCSE Physics — 3.3 Moments, levers and gears · Explain · 4 marks · View as Markdown

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

A seesaw is balanced when the turning effects on each side of the pivot are equal.

Model answer (4 marks)

The moment produced by a weight is the product of the weight and its perpendicular distance from the pivot. For the seesaw to be in equilibrium the moments on both sides of the pivot must be equal (principle of moments). The adult has a larger weight than the child, so to give the same moment the adult must sit at a smaller distance from the pivot than the child.

Examiner tips

  • State that moment = force × distance; mention the pivot; explain equality of moments; note adult’s greater weight requires a shorter distance

Common mistakes

  • Mixing up force and distance; forgetting to mention the pivot; assuming the adult can sit at the same distance because weight is larger

Mark scheme (4 marks)

  1. The moment (turning effect) depends on both the force and the perpendicular distance from the pivot
  2. For the seesaw to balance, the moments on each side must be equal (principle of moments)
  3. The adult has a greater weight (force) than the child
  4. Because the adult's force is larger, the distance must be smaller to produce the same moment as the child

Key terms in this question

moment · pivot · principle of moments

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