# 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

## 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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