# 4.2.4 Upthrust

### Upthrust

• Upthrust is a force which pushes upwards on an object submerged in a fluid i.e. liquids and gases
• Also known as buoyancy force, upthrust is due to the difference in hydrostatic pressure at the top and bottom of the immersed object
• The force of upthrust is significantly larger in liquids than in gases, this is because liquids are much denser than gases
• Recall that hydrostatic pressure depends on the height (h) or depth that an object is submerged in from P = ρgh
• Therefore, the water pressure at the bottom of an object is greater than the water pressure at the top, as shown in the diagram below:

This can will experience upthrust due to the hydrostatic pressure difference

• Upthrust is a force and is directly proportional to the pressure. The force on the bottom of the can will be greater than the force on top of the can
• This resultant pressure causes a resultant upward force on the can known as upthrust
• Upthrust is why objects appear to weigh less when immersed in a liquid. If the upthrust is greater than the weight of the object, the object will rise up
• For an object to float, it must have a density less than the density of the fluid its immersed in

#### Exam Tip

Since upthrust is force it is influenced by pressure, not by the density of the object as commonly misunderstood.

### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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