CIE AS Physics (9702) exams from 2022

Revision Notes

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:

 

Upthrust, downloadable AS & A Level Physics revision notes

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

 

Worked example

Worked example - upthrust, downloadable AS & A Level Physics revision notes

 

Exam Tip

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

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