A submarine is submerged in the ocean. As the submarine descends deeper into the ocean, the crew must ensure the hull can withstand increasing pressure. Explain why the pressure of the water acting on the hull increases as the submarine descends, and state TWO factors, other than depth, that determine the pressure at any point in a fluid.

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)

As the submarine descends, a greater weight of water lies above it, forming a taller column of fluid.
This increased weight exerts a larger force on a given area of the hull, so the pressure rises.
Other than depth, the pressure at a point in a fluid also depends on the fluid’s density.
It also depends on the local gravitational field strength, g.

Examiner tips

  • Use the depth–weight–force chain to justify pressure rise. Mention density and g explicitly to gain the two factor marks. Keep the answer concise and use the exact terminology.
  • common_mistakes
  • :
  • Writing only depth as a factor. Forgetting to mention gravitational field strength or density. Using vague terms like ‘more water’ without linking to weight and force.

Mark scheme (4 marks)

  1. As the submarine descends, there is a greater weight / column of water above it
  2. This greater weight of water results in a greater force acting on a given area of the hull, so pressure increases
  3. One correct factor: the density of the fluid (liquid)
  4. Second correct factor: gravitational field strength (g)

Key terms in this question

pressure · fluid · depth

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