National Physical Laboratory

What is buoyancy and how does it affect weighing? (FAQ - Mass & Density)

Buoyancy is the upward force exerted on an object when it is immersed, partially or fully, in a fluid - and strictly where the fluid is subjected to a gravitational force but is not in free fall - and its value is equal to the weight of the fluid displaced by the object. All objects that are surrounded by air or water on the surface of the Earth experience buoyancy to some degree. (But, for what it is worth, similar objects inside a pressurised, orbiting spacecraft do not experience buoyancy because they are in free fall, constantly accelerating towards the centre of the Earth. In orbit objects have no weight and hence can have no buoyancy.)

Buoyancy cannot affect the mass of an object; by definition, mass is a measure of the amount of material in a body and immersing it in a liquid does not change the amount. It cannot affect weight either as this (in one of its three meanings) is the gravitational force acting on a mass. But buoyancy can and does alter our perception of both mass and weight by affecting the measurement processes, making it more difficult.

Last Updated: 25 Mar 2010
Created: 9 Aug 2007