Explain why a sealed plastic bottle, partially filled with air, floats on the surface of water rather than sinking.

AQA GCSE Physics (8463) — 4.5.5 Pressure and pressure differences in fluids (Phys only) · Explain · 4 marks · View as Markdown

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

A sealed plastic bottle contains some air and some water. When placed in a tank of water, it floats.

Model answer (4 marks)

The bottle displaces a volume of water equal to its own volume, so water is pushed out of the way.
The displaced water exerts an upthrust (buoyancy force) on the bottle, directed upwards.
The weight of the water displaced is greater than the weight of the bottle.
Because the upthrust exceeds the bottle’s weight, the resultant force is upwards and the bottle floats.

Examiner tips

  • Use the word 'displaces' and 'upthrust' to show understanding of buoyancy.
  • Show the comparison of forces (upthrust > weight) to justify floating.
  • Keep the explanation concise – one sentence per point.
  • Use UK spelling: 'displaces', 'weight'.

Common mistakes

  • Confusing the weight of the bottle with the weight of the displaced water.
  • Failing to mention that the upthrust acts upwards.
  • Using vague terms like 'pushes' instead of 'displaces'.

Mark scheme (4 marks)

  1. The bottle displaces water / pushes water out of the way
  2. The upthrust (buoyancy force) acts upwards on the bottle
  3. The weight of water displaced is greater than the weight of the bottle
  4. The upthrust is greater than the weight of the bottle, so the resultant force is upwards / the bottle floats

Related

More Pressure and pressure differences in fluids (Phys only) questions

▶ Try answering this question with AI marking (free) →