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Grade 8 Beginner About 15 min Version 1 Awaiting review Premium

Shape and floating: the plasticine boat challenge

Plasticine is denser than water, so a ball of it sinks. But if you press the same plasticine into a hollow boat shape, it can push aside much more water. The upthrust equals the weight of water pushed aside, so the boat floats as long as it can push aside water weighing as much as the boat and its load. A wider, deeper hull can push aside more water and so carry a heavier load. This is how steel ships float.

What you will learn

  • Explain why the same material can sink as a lump and float as a boat.
  • Relate the load a boat can carry to the volume of water its hull can displace.
  • Explain how steel ships float.

Before you start

  • Know that upthrust equals the weight of liquid displaced.

Materials

  • 50 g of plasticine (modelling clay)
  • Marbles of 5 g each

Equipment

  • Tank or large bowl of water

Procedure

  1. Drop the ball of plasticine into the water and record what happens.
  2. Shape it into the narrow boat and record it.
  3. Add marbles one at a time to the narrow boat until it sinks, and record each time.
  4. Shape the plasticine into the wide boat and see how many marbles it can carry.

How this simulation is simplified

  • The plasticine is 50 g with a density of 1.5 g/cm³; each marble is 5 g.
  • The narrow hull can displace 80 cm³ of water and the wide hull 140 cm³ before water pours in.