Upthrust and Flotation — AQA GCSE Physics (Separate)
Objects in a fluid experience an upward force called upthrust, which determines whether they float or sink.
What is upthrust?
When an object is submerged in a fluid (liquid or gas), the pressure increases with depth. This means the upward pressure on the bottom of the object is greater than the downward pressure on the top. The result is a resultant upward force called upthrust.
Archimedes' idea
The size of the upthrust equals the weight of the fluid displaced by the object.
Floating and sinking
Whether an object floats or sinks depends on comparing its weight with the upthrust:
- If the upthrust equals the object's weight, the object floats (forces balanced).
- If the weight is greater than the maximum upthrust, the object sinks.
An object floats if its average density is less than the density of the fluid. This is why a steel ship floats: although steel is dense, the ship's shape means it displaces a large weight of water, giving a large upthrust, and its average density (including the air inside) is less than water.
Exam tips
- Upthrust is caused by greater pressure at the bottom than the top of a submerged object.
- Upthrust = weight of fluid displaced.
- An object floats if upthrust = weight (and sinks if weight > maximum upthrust).
- Floating depends on average density compared with the fluid.