Buoya

Meet Buoya, a buoy who floats because the water pushes up as hard as she weighs.

Pressure in a fluid increases with depth, so the push on the bottom of a submerged object is greater than the push on its top. The net result is an upward buoyant force. Archimedes' principle says that force equals the weight of the fluid the object displaces: F_b = ρ_f V g.

A floating object sinks until it displaces its own weight of fluid. A fully submerged litre of anything in fresh water feels about 1,000 kg/m³ × 0.001 m³ × 9.8 m/s² ≈ 9.8 newtons of lift. If the object weighs more, it sinks; less, it rises.

Ships float despite steel being denser than water because their hulls enclose a large volume of air, displacing a great deal of water. Submarines and fish adjust their buoyancy by changing how much water or gas they hold, and hot-air balloons work the same way in air.

The principle holds for fluids at rest. Moving fluids add lift and drag, and surface tension matters for very small objects.