Floating and sinking — Physics, 4–6 years
Find out why some things stay on water while others go down. Physics, 4–6 years.
Idea
An object floats when water can hold it up, and it sinks when the object moves down through the water. Shape matters as well as what the object is made from. A wide, hollow shape can float even when the same material sinks as a solid lump.
Why
People needed to know whether a log, basket or boat would stay on water before trusting it with people or goods. They learned by watching, testing and changing the shape. This idea solves a practical problem: choosing materials and shapes that keep things safely afloat.
Try it
Put a fresh orange in a bowl of water: it floats because its peel and air spaces make the whole orange light for its size. Peel it and put it back: it may sink. The orange has not changed its weight to zero; its shape and trapped air have changed.
Common mix-up
It is reasonable to think that heavy things sink, because a small stone goes down quickly in water. But a large steel ship can float because its hollow shape includes lots of air and spreads its weight over a wide space. Size alone does not decide the result.
Where it helps
Boats, canoes and ferries are shaped to stay on water while carrying people or objects. Life jackets contain light material or air, so they help a person remain near the surface. Designers test these shapes before they are used on real water.
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