OCR AS Physics

Revision Notes

3.5.3 Archimedes' Principle

Archimedes' Principle

Upthrust on an Object in a Fluid

  • Pressure increases with depth in a fluid because of the force exerted by the increased weight of the fluid above
  • This change in pressure can be calculated using the equation of hydrostatic pressure:

Hydrostatic pressure equation, downloadable AS & A Level Physics revision notes

  • This equation can be derived in the following way:

Derivation for hydrostatic pressure (1), downloadable AS & A Level Physics revision notes

Derivation for hydrostatic pressure (2), downloadable AS & A Level Physics revision notes

Archimedes’ Principle

  • Archimedes’ principle states:

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:

Upthrust on a boat, downloadable AS & A Level Physics revision notes

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

Worked Example

Atmospheric pressure at sea level has a value of 100 kPa. The density of sea water is 1020 kg m-3.

At what depth in the sea would the total pressure be 250 kPa?

A. 20 m               B. 9.5 m               C. 18 m          D. 15 m

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

Exam Tip

When asked about the total pressure remember to also add the atmospheric pressure

Total pressure = Hydrostatic pressure + Atmospheric pressure

Atmospheric pressure (also known as barometric pressure) is equal to 101 325 Pa

Values for pressure can vary widely and depend on metric prefixes such as kPa or MPa. When you’re doing calculations make sure all the pressures are in the same units (otherwise you may be out by a factor of 1000!). To be on the safe side, you can convert them all to Pascals.

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