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First teaching 2020

Last exams 2024

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Archimedes Principle (CIE AS Physics)

Revision Note

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Katie M

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Katie M

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Archimedes' Principle

  • Archimedes’ principle states that

An object submerged in a fluid at rest has an upward buoyancy force (upthrust) equal to the weight of the fluid displaced by the object

  • The object sinks until the weight of the fluid displaced is equal to its own weight
    • Therefore the object floats when the magnitude of the upthrust equals the weight of the object
  • The magnitude of upthrust can be calculated by:

Upthrust equation, downloadable AS & A Level Physics revision notes

  • Since m = ρV, upthrust is equal to F = mg which is the weight of the fluid displaced by the object
  • Archimedes’ Principle explains how ships float:

4-2-6-upthrust-on-a-boat-new

Boats float because they displace an amount of water that is equal to their weight

Worked example

Icebergs typically float with a large volume of ice beneath the water. Ice has a density of 917 kg m-3 and a volume of Vi.

The density of seawater is 1020 kg m-3.

What fraction of the iceberg is above the water?

A. 0.10 Vi          B. 0.90 Vi          C. 0.97 Vi          D. 0.20 Vi

Worked example - Archimedes' principle iceberg (2), downloadable AS & A Level Physics revision notes

Examiner Tip

Don't get confused by the two step process to find upthrust.

  • Step 1: You need the volume of the submerged object, but only because you want to know how much fluid was displaced
  • Step 2: What you really want to know is the weight of the displaced fluid.

A couple of familiar equations will help;

  • m = ρV to get mass (and that's the V from step 1 out of the way),

   then

  • W = mg to get weight

If you are feeling particularly mathematical, you can combine your equations, so that W = ρVg

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.