A scientist is studying pressure differences in the ocean. Explain why a sealed plastic bottle, which is partially filled with air, becomes crushed as it is pushed deeper below the water 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).

A sealed plastic bottle containing some air is pushed downwards from the ocean surface to a depth of several hundred metres.

Model answer (4 marks)

Pressure in a fluid increases with depth because the weight of the column of water above the bottle increases. The water therefore exerts a higher pressure on all surfaces of the bottle, pushing inwards. Inside the bottle the trapped air exerts an outward pressure, but this pressure does not increase with depth. At a certain depth the external water pressure exceeds the internal air pressure, so the bottle is crushed and compressed.

Examiner tips

  • Use the phrase ‘pressure increases with depth’ and explain it in terms of the weight of the water column.
  • Show that the water pressure acts on all surfaces and is directed inwards.
  • Explain that the trapped air pressure does not increase, so the external pressure dominates.
  • Mention that the bottle collapses when external pressure > internal pressure.

Common mistakes

  • Failing to state that the water pressure increases with depth.
  • Confusing the direction of the pressure forces (e.g., saying the water pushes outwards).
  • Ignoring that the internal air pressure remains constant and does not rise with depth.

Mark scheme (4 marks)

  1. Pressure in a fluid increases with depth
  2. This is because the weight / column of water above the bottle increases as depth increases
  3. The water exerts pressure on all surfaces of the bottle / the pressure acts inwards on the bottle
  4. The increased pressure force from the water is greater than the pressure of the trapped air inside, so the bottle is crushed / compressed

Key terms in this question

pressure

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