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 · View as Markdown

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

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

Model answer (4 marks)

1. Upthrust (buoyant force) on a balloon equals the weight of the air displaced.
2. The weight of the displaced air depends on the density of the surrounding air.
3. As the balloon rises, the atmosphere becomes less dense.
4. Therefore the weight of the displaced air – and hence the upthrust – decreases with altitude.

Examiner tips

  • Use the buoyancy equation (F_b = ρ_air V g) to link density to upthrust.
  • Show the relationship between density and altitude – less dense air gives less buoyant force.
  • Mention that pressure difference across the balloon reduces as density falls.
  • Avoid stating the opposite – upthrust decreases, not increases.

Common mistakes

  • Confusing upthrust with weight of the balloon; students often say it increases instead of decreases.
  • Failing to mention that density, not pressure alone, determines buoyant force.
  • Using the wrong sign – writing that upthrust increases with 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 in this question

upthrust · atmosphere

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