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Boats

Air and fluids · level 2, lesson 4 of 6

A question to start

Steel is much heavier than water for its size. A steel nail sinks at once. So how can a giant steel ship float?

The idea

It is all about shape.

Roll a lump of modelling clay into a ball and drop it in water: it sinks. Now press the same clay out flat and shape it into a little boat with high sides. It floats!

The boat shape pushes a lot of water out of the way, and the water pushes back up hard. A boat is also mostly air inside. So the whole boat, the clay and the air in it together, is lighter than the same space full of water.

a ball: sinksa boat: floats

When you put things in a boat, it sits lower in the water. Load it too much and the water comes over the sides, it fills up, and it sinks.

Worked examples

1. Clay boats. Make the sides higher and the boat can carry more coins before it sinks.

2. A big ship is a huge steel shell full of air. It pushes away a vast amount of water, so the water pushes up hard enough to hold it.

3. A loaded ship sits lower in the water than an empty one. Ships have a line painted on the side to show how low is safe.

Try it

  1. A ball of clay sinks. How can you make the same clay float?
  2. Why does a steel ship float?
  3. What happens to a boat when you load it up?
  4. Why do boats with higher sides carry more?
Answers
  1. Shape it into a boat.
  2. It is a shell full of air, so the whole ship is lighter than the same space full of water.
  3. It sits lower in the water.
  4. They can sit lower before water comes over the sides.

Common mistake

"Steel floats in a ship." The steel on its own would sink. It is the ship's shape, with all the air inside, that floats.

Where it leads