Upthrust
An object in water feels lighter. Water pushes it upwards with a force called upthrust. A spring balance shows this: an object weighs less when it hangs in water than in air.
- Archimedes' principle
- When an object is fully or partly in a fluid, the upthrust on it equals the weight of the fluid it displaces.
- density of the fluid (kg/m³)
- volume of fluid displaced (m³)
- 9.8 N/kg
The law of floatation
- Law of floatation
- A floating object displaces a weight of fluid equal to its own weight.
An object floats if its average density is less than the liquid's density. If its density is greater it sinks. A steel ship floats because its hollow shape gives it a low average density.
Try itFloat or sink?
Change the density of the block. Water has a density of 1 000 kg/m³. Find the density at which the block just floats.
Worked exampleUpthrust on a cube
A cube of volume is fully under water ( kg/m³, N/kg). Find the upthrust.
- Use U = ρ V g.
Answer: The upthrust is 20 N.
Key terms
- Upthrust
- The upward force of a fluid on an object in it.
- Displaced fluid
- The fluid pushed out of the way by an object.
- Density
- Mass per unit volume.
Summary
- Upthrust = weight of fluid displaced.
- A floating object has upthrust equal to its weight.
- It floats if its average density is less than the liquid's; otherwise it sinks.