Buoyancy
Definition
The upward push a fluid gives to anything immersed in it. It is why boats float, beach balls pop back up, and you feel lighter in a swimming pool.
Symbol: · Unit: newtons (N)
Technical definition
The net upward force exerted by a fluid on an immersed object, equal to the weight of the fluid the object displaces (Archimedes' principle): F_b = ρV_displaced·g. An object floats when the buoyant force balances its weight.
Example
A 70 kg swimmer displaces about 70 liters of water. The displaced water weighs roughly 686 N, nearly balancing the swimmer's 686 N weight — so the swimmer floats with just a little effort.
Buoyancy depends on the weight of displaced fluid, not on the object's own weight or its depth. A common mistake is thinking objects float 'because they are light' — a steel ship floats because its hollow shape displaces enough water, while a small solid steel nail sinks.