The forces at work in buoyancy. The object floats at rest because the upward force of buoyancy is equal to the downward force of gravity.

In physics, buoyancy (n-/)[1][2] or upthrust, is an upward force exerted by a fluid that opposes the weight of an immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pressure at the bottom of a column of fluid is greater than at the top of the column. Similarly, the pressure at the bottom of an object submerged in a fluid is greater than at the top of the object. The pressure difference results in a net upward force on the object. The magnitude of the force is proportional to the pressure difference, and (as explained by Archimedes' principle) is equivalent to the weight of the fluid that would otherwise occupy the volume of the object, i.e. the displaced fluid.

For this reason, an object whose average density is greater than that of the fluid in which it is submerged tends to sink. If the object is less dense than the liquid, the force can keep the object afloat. This can occur only in a non-inertial reference frame, which either has a gravitational field or is accelerating due to a force other than gravity defining a "downward" direction.[3]

The center of buoyancy of an object is the centroid of the displaced volume of fluid.

Archimedes' principle

A metallic coin (an old British pound coin) floats in mercury due to the buoyancy force upon it and appears to float higher because of the surface tension of the mercury.
The Galileo's Ball experiment, showing the different buoyancy of the same object, depending on its surrounding medium. The ball has certain buoyancy in water, but once ethanol is added (which is less dense than water), it reduces the density of the medium, thus making the ball sink further down (reducing its buoyancy).

Archimedes' principle is named after Archimedes of Syracuse, who first discovered this law in 212 B.C.[4] For objects, floating and sunken, and in gases as well as liquids (i.e. a fluid), Archimedes' principle may be stated thus in terms of forces:

Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object

— with the clarifications that for a sunken object the volume of displaced fluid is the volume of the object, and for a floating object on a liquid, the weight of the displaced liquid is the weight of the object.

More tersely: Buoyancy = weight of displaced fluid.

Archimedes' principle does not consider the surface tension (capillarity) acting on the body,[5] but this additional force modifies only the amount of fluid displaced and the spatial distribution of the displacement, so the principle that Buoyancy = weight of displaced fluid remains valid.

The weight of the displaced fluid is directly proportional to the volume of the displaced fluid (if the surrounding fluid is of uniform density). In simple terms, the principle states that the buoyancy force on an object is equal to the weight of the fluid displaced by the object, or the density of the fluid multiplied by the submerged volume times the gravitational acceleration, g. Thus, among completely submerged objects with equal masses, objects with greater volume have greater buoyancy. This is also known as upthrust.

Suppose a rock's weight is measured as 10 newtons when suspended by a string in a vacuum with gravity acting upon it. Suppose that when the rock is lowered into water, it displaces water of weight 3 newtons. The force it then exerts on the string from which it hangs would be 10 newtons minus the 3 newtons of buoyancy force: 10 − 3 = 7 newtons. Buoyancy reduces the apparent weight of objects that have sunk completely to the sea floor. It is generally easier to lift an object up through the water than it is to pull it out of the water.

Assuming Archimedes' principle to be reformulated as follows,

then inserted into the quotient of weights, which has been expanded by the mutual volume

yields the formula below. The density of the immersed object relative to the density of the fluid can easily be calculated without measuring any volumes.:

(This formula is used for example in describing the measuring principle of a dasymeter and of hydrostatic weighing.)

Example: If you drop wood into water, buoyancy will keep it afloat.

Example: A helium balloon in a moving car. During a period of increasing speed, the air mass inside the car moves in the direction opposite to the car's acceleration (i.e., towards the rear). The balloon is also pulled this way. However, because the balloon is buoyant relative to the air, it ends up being pushed "out of the way", and will actually drift in the same direction as the car's acceleration (i.e., forward). If the car slows down, the same balloon will begin to drift backward. For the same reason, as the car goes round a curve, the balloon will drift towards the inside of the curve.

Other Languages
Afrikaans: Dryfvermoë
العربية: طفو
asturianu: Flotabilidá
Bân-lâm-gú: Phû-le̍k
беларуская: Плывучасць
български: Плаваемост
bosanski: Potisak
català: Flotabilitat
čeština: Vztlak
chiShona: Simudzo
Ελληνικά: Άνωση
español: Flotabilidad
فارسی: شناوری
français: Flottabilité
Gaeilge: Buacacht
한국어: 부력
hrvatski: Uzgon
Bahasa Indonesia: Gaya apung
Kreyòl ayisyen: Flotabilite
Bahasa Melayu: Keapungan
မြန်မာဘာသာ: ဖော့ဂုဏ်
日本語: 浮力
norsk: Oppdrift
norsk nynorsk: Oppdrift
ਪੰਜਾਬੀ: ਉਤਪਲਾਵਨ ਬਲ
polski: Siła wyporu
português: Impulsão
română: Flotabilitate
русский: Плавучесть
Simple English: Buoyancy
slovenščina: Vzgon
srpskohrvatski / српскохрватски: Uzgon
suomi: Noste
svenska: Flytkraft
Türkçe: Batmazlık
українська: Плавучість
中文: 浮力