Water Displacement Explained Simply
When you dip an object into water, it pushes water out of the way - it displaces it. That displaced water needs somewhere to go, so the water level rises.
Whether something floats or sinks isn't just about weight - it's about density: a massive steel ship floats because its shape displaces a huge amount of water - the buoyant force from that displaced water equals the ship's weight. A small solid stone, on the other hand, displaces far too little water compared to its weight, so it sinks.
đź’¬ Mia asks Grandpa Theo
But a steel ship is super heavy - steel always sinks otherwise, doesn't it?
A solid lump of steel really does sink. But a ship is hollow inside - it's mostly air, wrapped in a thin steel shell. That gives it a huge volume compared to its weight.
Ah, so it's not just weight that matters, but how much space the thing takes up?
Exactly - that's called density, weight per volume. The trick behind it is over 2000 years old: Archimedes discovered that an object gets exactly as much buoyant force as the weight of the water it displaces. If it displaces more water-weight than it itself weighs, it floats.
Fill a bowl right to the brim with water and place a glass in it. The water that spills out weighs exactly as much as the buoyant force the glass gets - you can measure that directly.
Submarines use exactly this principle too: they fill tanks with water to become heavier and sink, then pump them back out with air to rise again. For more on the link between weight and volume, see Mass and Density.
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