buoyancy

Meaning: Pressure in a fluid increases with depth, so the fluid pushes up on the bottom of a submerged object harder than it pushes down on the top. That net upward force is the buoyant force, and it equals the weight of the displaced fluid. An object floats if its average density is less than the fluid's, sinks if greater, and hovers if equal.

Equation: Fb = ρfluid × Vsub × g, where Fb is the buoyant force (N), ρfluid is the fluid density (kg/m³), Vsub is the volume of fluid displaced, which is the submerged volume of the object (m³), and g is 9.8 m/s² (m/s²). For a floating object, Fb = weight, so the fraction submerged = ρobject/ρfluid.

On the MCAT: Expect an object hanging from a scale in water, where apparent weight = true weight − Fb. Another common setup is a floating block, ice or a boat moved into saltwater or another liquid, where you predict how much of it is submerged. A fully submerged object has the same Fb at any depth.

Common confusion: Students think Fb depends on the object's mass or density. It depends only on the fluid's density and the volume displaced. Density only decides whether Fb beats the weight. A steel ship floats because its hollow shape displaces a large volume of water.