The principle that a body wholly or partly immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces.
- The upward push (buoyant force) equals the weight of the displaced fluid; this is why objects feel lighter in water.
- An object floats if its average density is less than that of the fluid and sinks if it is greater; a steel ship floats because its hollow shape displaces a large volume of water.
- The law of flotation: a floating body displaces a weight of fluid equal to its own weight.
- Applications include ships, submarines (which adjust buoyancy by filling and emptying ballast tanks), hydrometers (measuring liquid density), hot-air balloons, and lactometers (testing milk).
- The principle was discovered by the Greek thinker Archimedes; the famous "Eureka" story relates to detecting impurity in a gold crown by comparing densities.
Buoyancy, flotation, density, and the working of ships, submarines, and hydrometers are recurring everyday-physics facts and a frequent source of MCQs.
A heavy object can still float if its average density (mass divided by total volume, including hollow space) is lower than the fluid; flotation depends on density, not weight alone. Buoyant force depends on the displaced fluid's weight, not on the object's own weight.
Upthrust equals the weight of fluid displaced; an object floats when its density is less than the fluid's.