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Buoyancy

Physics: Cambridge International A Level, topic 4.1.4

Buoyancy

There is a net upward or buoyant force on any object in any fluid due to the increase in pressure with depth.

Buoyancy

Buoyancy

  • The buoyant force is the net upward force on any object in any fluid.
  • The pressure due to the weight of a fluid increases with depth since P=hρgP = h\rho g.
  • This pressure and associated upward force on the bottom of the cylinder are greater than the downward force on the top of the cylinder.
  • Their difference is the buoyant force FBF_B.
Buoyancy

Buoyancy

Archimedes' principle

Archimedes' principle

  • According to this principle, the buoyant force on an object equals the weight of the fluid it displaces.
  • In equation form, Archimedes' principle is:
  • FB=WfF_B = W_f
Equation

Equation

  • The mass of the displaced fluid can be calculated from its density and volume, and so the buoyant force is:
  • FB=ρVgF_B = \rho V g
  • Where ρ\rho and V are the density and volume of the displaced fluid and g is the gravitational field strength.
Float or sink?

Float or sink?

  • If FB>WF_B>W, where W is the weight of the object, the object will rise to the surface and float.
  • If FB<WF_B<W, the object will sink.
  • If FB=WF_B=W, the object will remain suspended at that depth.

Test yourself on Buoyancy

  • In which direction does the buoyant force act?

  • What will happen to a rubber duck with weight 5 N that is dropped into a bath, experiencing a buoyant force of 7 N?

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