Floating and sinking — Science, 4–6 years
An object floats when water can support it at the surface, and sinks when it cannot stay up.
Water can hold some things up
Some objects stay at the water’s surface, while others go down to the bottom. A cork usually floats and a stone usually sinks. What matters is not only what an object is made of, but also its shape and how much space it takes for its weight.
Why test this before building
People need to know what will stay on water when they make boats, rafts or floating toys. A heavy piece of metal may sink, but a hollow metal boat can float because its shape includes lots of air and spreads its weight. Testing in water helps reveal what will happen.
Test three objects
Fill a bowl with water and choose a plastic block, a coin and a cork. Place the block in gently: it floats, so mark F. Place the coin in: it sinks, so mark S. Place the cork in: it floats, so mark F. The result is F, S, F.
Heavy does not always mean sinking
It is reasonable to think every heavy thing sinks, because a stone drops in water. But a large ship is heavy and still floats. Its wide, hollow shape pushes aside enough water to support the ship and its load. So test the whole shape, not just one piece.
Boats and life jackets
Boats are shaped to float while carrying people and things. Life jackets contain light material or air spaces, so they help a person stay at the surface. These objects work because their shape and contents let the water support them.
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