Buoyancy is the net upward force exerted by a fluid on a body that is partially or completely immersed in the fluid. It results from the variation of fluid pressure with position, particularly the fact that pressure increases with depth in a gravitational field. The lower portions of an immersed object therefore generally experience greater pressure than the upper portions, producing a resultant force with an upward component. For a body in a stationary fluid, the magnitude of the buoyant force is equal to the weight of the displaced fluid under the conditions described by Archimedes' Principle. Buoyancy applies to liquids and gases and explains floating, sinking and neutral suspension.
The buoyant force depends on displaced fluid volume and fluid density rather than directly on the material density of the submerged object. For an object fully submerged in a uniform fluid, the buoyant force is rho fluid g V displaced. If the object's weight is greater than this force, it accelerates downward when other forces are absent. If the buoyant force exceeds its weight, it accelerates upward until another equilibrium condition is reached. An object with equal weight and buoyant force can be neutrally buoyant under suitable conditions. These comparisons must include all relevant forces when the body is moving or subject to additional constraints.
Floating bodies are only partly submerged when their average density is lower than that of the surrounding fluid and a stable equilibrium position can be established. At static equilibrium, the buoyant force equals the object's weight. The submerged volume therefore adjusts until the weight of the displaced fluid equals the object's weight. This explains why a large hollow steel ship can float even though steel itself is denser than water: the ship's overall average density, including the volume of enclosed air and structure, can be less than that of the displaced water. Changing cargo, ballast or trapped air changes the average density and consequently the equilibrium immersion depth.
Buoyancy also operates in gases. Hot air balloons rise because the buoyant force from surrounding air can exceed the combined weight of the balloon envelope, equipment and heated air. Helium balloons rise for a similar reason because helium has lower density than surrounding atmospheric air under comparable conditions. Atmospheric buoyancy is important in meteorology because parcels of air can accelerate vertically when their density differs from that of the surrounding atmosphere. The resulting motion is influenced by temperature, pressure, moisture and atmospheric stratification, so buoyancy in gases can contribute to convection and large scale atmospheric circulation.
The stability of a floating body depends on how the buoyant force changes when the body is displaced or tilted. The centre of buoyancy is the geometric centre of the displaced fluid volume when the fluid is homogeneous and the relevant assumptions apply. The centre of gravity is the point through which the body's weight acts. For floating bodies, the relative positions of the centre of gravity, centre of buoyancy and metacentre influence rotational stability. Naval architecture uses metacentric height and related concepts to assess whether small angular disturbances tend to produce restoring or destabilising moments. Thus simply knowing that an object floats does not establish that its orientation is stable.
Buoyancy can be analysed as the integrated result of a pressure field rather than as a separate mysterious force. Pressure on every portion of an immersed surface acts normal to that surface, and the vector sum of these pressure forces produces the net buoyant force. This viewpoint explains why buoyancy remains present even when the object's shape is irregular. It also connects buoyancy directly to hydrostatic pressure and provides a general route for calculating forces on submerged surfaces. In nonuniform fluids, density variation can modify the pressure distribution and therefore the buoyant force, while in moving fluids additional hydrodynamic forces can complicate the simple hydrostatic result.
Buoyancy has extensive engineering and scientific applications. Ships, submarines, hydrometers, floats, life saving devices, underwater instruments and density measurement systems all depend on buoyant forces. A hydrometer floats at a depth determined by its weight and the density of the surrounding liquid, allowing liquid density to be inferred from calibrated markings. Submarines alter buoyancy by changing ballast tank contents and therefore their displacement and average density. Underwater vehicles can use buoyancy and ballast systems to achieve approximately neutral buoyancy, reducing the propulsion force required for vertical position control.
Common misconceptions about Buoyancy include the belief that it depends primarily on an object's surface area or that only objects less dense than water can experience buoyant force. Any object immersed in a fluid experiences pressure forces and therefore a buoyant effect, even if it ultimately sinks. The magnitude depends on displaced volume and fluid density under the standard hydrostatic relationship.
For a completely submerged object in a uniform fluid, changing its depth does not change the buoyant force if fluid density remains constant, although pressure itself increases with depth. Buoyancy is therefore fundamentally a consequence of pressure variation and displaced fluid volume, while floating or sinking depends on the balance between buoyancy, weight and any other relevant forces.
