Buoyancy
The upward push of a fluid on anything immersed in it, equal to the weight of the fluid it pushes aside.
Buoyancy is the upward force a liquid or gas exerts on anything placed in it. Archimedes' principle, from his work On Floating Bodies of around 246 BC, says that this force equals the weight of the fluid the object pushes aside, or displaces.
Whether something floats depends on the balance between that push and its own weight. A solid iron block sinks because iron is nearly eight times as dense as water: a one-tonne block displaces only about an eighth of a tonne of water. Hammer the same iron into a wide bowl and it can displace far more water before it is submerged, so it floats. That is how a 10,000-tonne steel ship stays up: it simply has to displace 10,000 tonnes of water. Air works the same way, which is why a balloon filled with hot air or a light gas rises, and why every extra kilogram of load, ice or water counts against it.
The famous story of Archimedes leaping from his bath shouting "Eureka!" is a legend. Even in the tale, he used displaced water only to measure the volume of a crown, not to apply the principle that bears his name.
A common misconception is that heavy things sink and light things float. What matters is average density, weight compared with volume: a huge ship floats while a small pebble sinks.
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