Upthrust, floating and sinking · Grade 9

When something is put in a liquid it pushes some liquid aside, and the liquid pushes back up with a force called upthrust. If the upthrust can balance the weight, the object floats. Whether something floats depends on its density compared with the liquid, not just on how heavy it is.

Key facts

  • Upthrust is an upward force from a fluid on any object in it.
  • Archimedes’ principle: upthrust = weight of fluid displaced = ρ(fluid) × V(displaced) × g.
  • A floating object: upthrust = weight.
  • Objects less dense than the liquid float; denser ones sink.
  • Apparent weight in water = real weight − upthrust.
  • Fraction of a floating object under the surface = ρ(object) ÷ ρ(liquid).

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