Explain why the water pressure at the bottom of a deep lake is greater than the water pressure near the surface.

Eduqas GCSE Physics — 3.2 Pressure and pressure differences in fluids · 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 at a point in a fluid is caused by the weight of the fluid above it.
1. The bottom of the lake is at a greater depth than the surface.
2. Therefore more water lies above the bottom than above the surface.
3. The weight of this extra water is greater.
4. Since pressure is weight per unit area, the greater weight per area at the bottom gives a higher pressure.

Examiner tips

  • Use the definition of pressure (weight/area) and link depth to weight of overlying fluid.
  • Show the logical chain: depth → more water → more weight → higher pressure.
  • Keep the answer concise and use the exact wording from the mark scheme.

Common mistakes

  • Confusing pressure with density or ignoring the weight per unit area concept.
  • Giving a qualitative answer without linking depth to weight and pressure.

Mark scheme (4 marks)

  1. There is a greater depth (of water) at the bottom of the lake
  2. There is a greater weight of water above a point at the bottom
  3. Pressure increases with depth (in a fluid)
  4. The weight of the fluid above acts on a given area, creating pressure / greater weight per unit area at greater depth

Key terms in this question

pressure

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