Fluids push up on immersed objects — Physical education, 11–13 years
Water and air can push upward on objects inside them. This buoyant force explains floating, sinking and how large ships can stay on the surface.
A fluid can hold you up
When an object is partly or fully in water, the water pushes on it from all sides, including upward. The upward push is called buoyancy. If it balances the object’s weight, the object floats at that position; if it is weaker, the object moves down.
The problem of staying afloat
People wanted to carry cargo across water without making every ship from a naturally light material. They had to explain why a heavy steel ship can float while a small steel nail sinks. The answer is that the ship’s hollow shape displaces a large amount of water, creating enough upward push to balance its weight.
A block in water
A block weighs 15 N and, when fully under water, displaces 2 litres of water. One litre of water has a weight of about 10 N, so 2 litres give about 20 N of upward buoyancy. Because 20 N is greater than 15 N, the block rises. It stops when only about 1.5 litres are displaced, giving 15 N upward.
Heavy does not always mean sinking
A reasonable mistake is to think that anything heavier than water must sink. What matters is not just the object’s mass, but how much water its whole shape displaces and how that mass is spread through its volume. A steel ball sinks, while the same steel shaped as a hollow boat can displace enough water to float.
Ships, life jackets and submarines
Ship hulls are shaped to displace enough water for their weight, while life jackets add a large, light volume around a person. Submarines change how much water their tanks hold: taking in water makes them sink, and forcing water out makes them rise. These devices use buoyancy deliberately rather than fighting it.
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