Floating and sinking — Physical education, 7–10 years
An object can float when water pushes up enough to support it. Shape matters too, so a heavy steel ship can float while a small steel nail sinks.
Water pushes up
When you put something in water, the water pushes up on it. If that upward push can support the object’s weight, it floats; if not, it sinks. An object does not float simply because it is light: how much water its shape displaces matters too.
The problem boats solve
People needed to carry people and heavy goods across water instead of sinking. A solid lump of metal usually sinks, but a hollow shape can spread its weight and push away enough water to float. Boat builders therefore plan the shape, not just the material.
Testing clay
Take one ball of modelling clay and place it in water: it sinks. Remove it, reshape the same clay into a small boat with a hollow middle, and place it gently on the water. It may float because the boat shape displaces more water before the clay fills with water.
The common mistake
It is reasonable to think that anything made of metal must sink, because a metal key sinks in a cup. But floating is not decided by material alone. A ship has a hollow shape full of air, so its whole shape can displace water and receive enough upward support.
Where we use it
Boats, ferries and cargo ships are designed to displace enough water to stay up. Life jackets add a light, air-filled shape around a person, helping the water support them. Divers change their buoyancy by adding or releasing air from special equipment.
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