Archimedes' principle

A body that is submerged in a fluid is buoyed up by a force equal in magnitude to the weight of the fluid that is displaced, and directed upward along a line through the centre of gravity of the displaced fluid.

The fluid may be a gas or a liquid.

The buoyant force is given by:

where
F = buoyant force [N]
g = standard acceleration due to gravity [ms-2]
ρflu = density of the fluid [kg m-3]
ρobj = density of the object [kg m-3]
Vobj = volume of object submerged [m3]
Examples
Balloon - The weight of the air displaced is equal to the upward force and if greater than the weight of the balloon the balloon will accelerate upwards. If the weight of the displaced air is less than the weight of the balloon the balloon will accelerate towards the ground.
Boat - The weight of the displaced water equals the upward force and this equals the weight of the ship.
Hydrometer - A floating instrument used to measure the density of liquids.
Submarine - Able to travel on the surface of the water or dive at will. Diving is accomplished by special ballast tanks with water, increasing the weight of the submarine. To rise the water is forced out of the tanks using compressed air.

See also: Archimedes, Buoyant Force.

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Subjects: Physics