That Sinking Feeling...

Heavy things sink, right? Not always!

Which is easier to lift? A small stone or an entire ship? A small stone won't float (unless you cheat and use pumice) even if it isn't very heavy, yet a huge oil tanker that is too heavy to pick up floats easily.

Surely the heaviest one should sink

Try this...

  • Take a piece of aluminium foil and split it into 2 identically sized pieces (with approximately the same weight).
  • Take one of the pieces and roll it into a tight, very compact, ball. Take the other piece and fold it into a flat shallow boat shape.
  • PREDICT: Ask your students "What will happen when I place these two into this sink (or tub) of water?"
  • OBSERVE: Watch what happens, remembering they have the same weight, when carefully placed onto the surface of the water.
  • EXPLAIN: What does this show us? (seniors might like to consider the area of the ball and boat in contact with the water)


Which one floats and which one sinks? Can you explain what you are looking at? It all works on the idea of buoyancy. Buoyancy is the loss in weight an object seems to undergo when placed in a liquid, as compared to its weight in air.

Archimedes was a man who lived centuries ago who figured out that an object fully or partly immersed in a liquid is buoyed upward by a force equal to the weight of the liquid pushed aside (displaced) by that object. A floating object pushes aside (displaces) an amount of liquid equal to its own weight.

The upshot of all this is that the foil boat pushes aside more water, as a larger area is in contact with the water compared to the foil ball. Even a ship with a hull made of concrete will float if the area of the object that makes contact with the water is large enough.

The ship is exerting a downward force on the water and the buoyancy of the water is exerting a upward force on the object. As long as the forces remain equal the ship will float. Put too much cargo on board and the downward force is greater... the ship sinks!


  1. Try to find out more about Archimedes.
  2. How do Submarines manage to rise and sink at will? Find out in the next model you can build, your own mini-submarine...


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