# A hot air balloon rises through the atmosphere. Explain why the upthrust acting on the balloon increases as it rises to greater altitudes.

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

> A hot air balloon is inflated and released. As it rises, the atmosphere gets less dense with increasing altitude.

## Mark scheme (4 marks)

1. Upthrust arises because the balloon experiences greater pressure below it than above it
2. The atmosphere becomes less dense with increasing altitude
3. Pressure in a fluid depends on the density of the fluid, so lower density means lower pressure at any given depth/height
4. Therefore, as the balloon rises into less dense air, the pressure difference between the bottom and top of the balloon decreases, so the upthrust decreases (not increases) — CORRECTION: upthrust decreases as altitude increases because the surrounding air is less dense

## Key terms

- [upthrust](https://www.gradenine.co.uk/glossary/upthrust)
- [atmosphere](https://www.gradenine.co.uk/glossary/atmosphere)

## 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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-hot-air-balloon-rises-through-1a80cb64) · Published by Druglandscape Ltd.