Buoyant force

Definition

The buoyant force exerted by a fluid on an object partly or wholly immersed in it is an upward force exerted by the fluid on the object to (partly or wholly) balance the gravitational force exerted on the object. Here, upward means opposite to the direction of the gravitational field.

For an object floating or sinking in a fluid, the buoyant force plays a role similar to the normal force that would exist between a pair of dry solid surfaces.

By the Archimedes principle, the magnitude of the buoyant force is the weight of the fluid displaced by the portion of the object that is immersed in the fluid.