# 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

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

## 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

- [Revision notes for AQA GCSE Physics (8463)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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