Buoyancy is simply defined as the ability of an object to float in a fluid (liquids and gases). When an object with a mass is submerged in any fluid, an upward force exerted by the fluid is experienced by the object that tends to push the object upwards. This upwards acting force that pushes the immersed body towards the surface of the fluid is termed buoyant force. This buoyant force is due to the pressure differences on the top and bottom surfaces of the immersed object. The object will experience an upwards buoyant force if the object is immersed completely or even partially. An object, when immersed in a fluid, will always feel lighter than it actually is because of the buoyant force, also otherwise known as upthrust. The following are the factors that influence the force of buoyancy.
The buoyant force is given by F_{b} = ρVg
where
Buoyancy can be commonly experienced by humans when they are swimming. As the swimmer tries to go deeper into the water, there seems to be a force that is trying to push them back to the surface. However, there seems to be no issue when swimming on the surface of the water body. This is because the pressure acting upwards on the swimmer is greater than the pressure acting on top of the swimmer (atmospheric pressure), keeping him afloat. As the swimmer dives and goes deeper, the force from pressure exerted downward on the top of the swimmer will be less than the force from pressure exerted upward on the bottom of the swimmer. Pressure increases as the swimmer goes deeper because there are many layers of water on top of them. This pressure can be very dangerous and even fatal at times if the swimmer is not careful. This is why it is suggested that swimmers do not ever dive deeper than 60 feet in any water body.
First discovered by Archimedes of Syracuse in 212 BC, Archimedes’ principle has been the foundation for further research and developments in the study of hydrostatics. For fluids, in terms of force, Archimedes’ principle is stated as “an object, when immersed partially or completely in a fluid, is buoyed up by force equal to the weight of the fluid displaced by the object”. Even though Archimedes’ principle does not consider the surface tension of the fluid, the idea that buoyancy = weight of displaced fluid holds true because the surface tension will modify only the amount of water displaced. If the surrounding fluid has uniform density, the weight of the displaced fluid is directly proportional to the volume of the displaced fluid. The principle further establishes that the force of buoyancy acting upwards on an object is equal to the weight of the fluid displaced by the object. Therefore, when objects with the same mass are immersed in a fluid, the object with more volume displaces relatively more water.
Buoyancy is an important part of hydrostatics, and the appropriate calculation of it allows us to make objects float on fluids. Some real-life examples where buoyancy can be observed are discussed below.