A submarine dives from the surface of the ocean to a much greater depth. Explain why the pressure acting on the outside of the submarine increases as it dives deeper.

OCR GCSE Physics A: Gateway Science (J249) — P1.3 Pressure · Explain · 4 marks · View as Markdown

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

Submarines are designed to withstand very large pressures when operating at depth. Engineers must account for how pressure changes with depth when designing the hull.

Model answer (4 marks)

Pressure in a liquid increases with depth. As the submarine dives deeper, a greater height of water lies above it. The weight of this larger column of water is greater, and this weight acts over the same unit area of the hull. Therefore the force per unit area – the pressure – on the outside of the submarine increases.

Examiner tips

  • Use the phrase ‘pressure increases with depth’ first. Explain the role of the water column’s height and weight. Show the relationship ‘force = weight, pressure = force/area’. Keep the answer concise to fit the 4 marks.

Common mistakes

  • Confusing pressure with force; not mentioning the area. Saying only ‘more water means more pressure’ without linking to weight and area. Using vague terms like ‘more depth’ without explaining the column of water.

Mark scheme (4 marks)

  1. Pressure in a liquid increases with depth / greater depth means greater pressure
  2. There is a greater height (column) of water above the submarine at greater depth
  3. This greater column of water has a greater weight
  4. This greater weight acts over the same unit area, so the pressure (force per unit area) is greater

Key terms in this question

pressure

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