A level Physics · Edexcel 9PH0 · Topic 4A
Density and upthrust
Key points
- Density is mass per unit volume: , unit kg m (water is about 1000 kg m).
- To find density, measure mass on a balance and volume from dimensions (regular shapes) or by displacement of water (irregular shapes).
- Upthrust is the upward force a fluid exerts on an object in it. It equals the weight of the fluid displaced: .
- An object floats when it sinks until the weight of fluid displaced equals its own weight; it sinks if its average density is greater than the fluid's.
- Upthrust arises because pressure increases with depth, so the push up on the bottom is greater than the push down on the top.
- For a sphere falling through a liquid, the forces are weight (down), upthrust (up) and viscous drag (up).
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Density tells you how tightly mass is packed into a material, and upthrust is the upward push that a fluid gives anything immersed in it. Together they explain why some objects float and others sink.
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