A large wooden sleeper used in a garden is placed flat on soft soil. A gardener then stands the same sleeper upright, so that only one end is in contact with the soil. The sleeper sinks deeper into the soil when stood upright than when lying flat. Explain why the sleeper sinks deeper into the soil when stood upright, even though its weight has not changed.

Cambridge International IGCSE Physics (0625) — 1.8 Pressure · 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 pressure exerted on the soil is the weight of the sleeper divided by the area of contact. When the sleeper lies flat the contact area is large, so the pressure is low. When it is stood upright the same weight acts over a much smaller area, so the pressure is higher. The greater pressure causes the soil to deform more, making the sleeper sink deeper.

Examiner tips

  • State the definition of pressure (force/area) first.
  • Explain that the weight is unchanged but the area changes.
  • Show the logical chain: smaller area → higher pressure → deeper sink.
  • Use the word ‘pressure’ and ‘contact area’ as in the mark scheme.

Common mistakes

  • Confusing weight with force or pressure.
  • Failing to mention that the weight is unchanged.
  • Using vague terms like ‘more force’ instead of ‘higher pressure’.

Mark scheme (4 marks)

  1. Pressure depends on the area over which a force acts
  2. When stood upright, the contact area between the sleeper and the soil is smaller
  3. The weight (force) acting on the soil remains the same
  4. The smaller area means the pressure on the soil is greater, so the sleeper sinks deeper

Key terms in this question

weight

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