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Grades 8–10 Intermediate About 20 min Version 1 Awaiting review Premium

Floating, sinking and upthrust

When an object is put into a liquid, the liquid pushes up on it. This upward force is called upthrust (or buoyant force). Archimedes' principle says the upthrust equals the weight of the liquid the object displaces. Weigh an object in air and then in water: the reading drops by the upthrust. An object floats when it is less dense than the liquid, because it can displace its own weight of liquid before it is completely under. Salt water is denser than fresh water, so it gives more upthrust.

What you will learn

  • Measure upthrust as the loss of weight in a liquid.
  • State Archimedes' principle.
  • Use density to predict whether an object floats or sinks.

Before you start

  • Calculate density = mass ÷ volume.
  • Read a spring balance in newtons.

Materials

  • A wooden block, a plastic toy, an apple, an egg, a stone and an iron nail cluster
  • Fresh water, salt water and cooking oil

Equipment

  • Spring balance (0 to 2 N)
  • Large beaker

Procedure

  1. Hang the stone on the spring balance and record its weight in air.
  2. Lower it into fresh water and record the new reading. The difference is the upthrust.
  3. Lower the wooden block into the water and record what happens.
  4. Try the egg in fresh water and in salt water.

How this simulation is simplified

  • g is taken as 10 N/kg.
  • Densities: fresh water 1.00 g/cm³, salt water 1.10 g/cm³, cooking oil 0.92 g/cm³.