A wooden log floats on the surface of a river. Explain why the log floats rather than sinks.

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).

Model answer (4 marks)

Pressure in a fluid increases with depth, so the pressure acting upwards on the bottom of the log is greater than the pressure acting downwards on the top of the log.
This pressure difference produces an upward force on the log, called upthrust.
The log displaces water, and the weight of water displaced equals the upthrust acting on the log.
The upthrust is equal to (or greater than) the weight of the log, so there is no resultant downward force and the log floats.

Examiner tips

  • Use the term ‘upthrust’ and relate it to the pressure difference with depth.
  • Show the chain: pressure difference → upthrust → displaced water weight → balance with log weight.
  • Keep the answer concise – 4 points only, no extra explanation.

Common mistakes

  • Confusing buoyancy with weight of the log; forgetting to mention displaced water.
  • Using ‘buoyant force’ instead of ‘upthrust’ or vice‑versa without linking to pressure difference.
  • Writing a long paragraph instead of the required 4 concise points.

Mark scheme (4 marks)

  1. Pressure in a fluid increases with depth, so the pressure acting upwards on the bottom of the log is greater than the pressure acting downwards on the top of the log.
  2. This pressure difference produces an upward force on the log, called upthrust.
  3. The log displaces water, and the weight of water displaced equals the upthrust acting on the log.
  4. The upthrust is equal to (or greater than) the weight of the log, so there is no resultant downward force and the log floats.

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