buoyant force

Meaning: Pressure increases with depth, so the upward push on the bottom of a submerged object exceeds the downward push on its top; the net is the buoyant force. Archimedes' principle says it equals the weight of the displaced fluid. An object floats at rest when Fb balances its weight, and a fully submerged object sinks if its weight exceeds Fb.

Equation: Fb = ρfluid V g, where Fb is the buoyant force (N), ρfluid is the fluid density (kg/m³), V is the volume of fluid displaced, equal to the submerged volume of the object (m³), and g is 9.8 m/s².

On the MCAT: Questions ask whether an object floats or sinks, or how much of it is submerged. A floating object has Fb = mg, so the submerged fraction equals ρobject/ρfluid. Apparent weight in a fluid is true weight minus Fb. Expect fresh-versus-salt-water comparisons.

Common confusion: Students think a heavier object feels a bigger Fb. For a given submerged volume, Fb does not depend on the object's mass or material. A floating object, however, settles until Fb equals its weight, so a heavier floating object displaces more fluid and feels a larger Fb.