GCSE Physics · AQA 8463 · Topic 4.5.5
Pressure and pressure differences in fluids
Key points
- A fluid is a liquid or a gas. Pressure in a fluid acts at right angles (normal) to any surface.
- Pressure: , in pascals (1 Pa = 1 N/m²).
- (Higher tier) Pressure due to a column of liquid: . Pressure increases with depth and with density.
- Upthrust: an object in a fluid feels a greater pressure on its bottom surface than its top, giving a resultant upward force. It floats if the upthrust equals its weight before it is fully submerged; objects less dense than the fluid float.
- Atmospheric pressure comes from air molecules colliding with surfaces. The atmosphere gets less dense with height, so pressure decreases with altitude.
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Why do your ears hurt at the bottom of a swimming pool, why does a steel ship float, and why is it hard to breathe on a high mountain? All three come down to pressure in fluids.
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