Accidentally, Archimedes discovered that when a body is immersed partly or wholly in a fluid, in rest it is buoyed up with a force equal to the weight of the fluid displaced by the body. This principle is called Archimedes principle and is a necessary consequence of the laws of fluid statics.
The upthrust or buoyancy acts vertically upward (opposite to the weight of the body) through the center of gravity of displaced fluid (called center of buoyancy). Though we have derived this result for a body fully submerged in a fluid, it can be shown to hold good for partly submerged bodies or a body in more than one fluid also.
- Upthrust is independent of all factors of the body such as its mass, size, density, etc., except the volume of the body inside the fluid.
- Upthrust depends upon the nature of displaced fluid. This is why upthrust on a fully submerged body is more in sea water than in fresh water because its density is more than fresh water.