When an object is immersed in water, it feels lighter. In a cylinder filled with water, the action of inserting a mass in the liquid causes it to displace upward. In 212 B.C., the Greek scientist Archimedes discovered the following principle: an object is immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object. This became known as Archimede's principle. The weight of the displaced fluid can be found mathematically. The fluid displaced has a weight W = mg. The mass can now be expressed in terms of the density and its volume, m = pV. Hence, W = pVg.

It is important to note that the buoyant force does not depend on the weight or shape of the submerged object, only on the weight of the displaced fluid. Archimede's principle applies to object of all densities. If the density of the object is greater than that of the fluid, the object will sink. If the density of the object is equal to that of the fluid, the object will neither sink or float. If the density of the object is less than that of the fluid, the object will float.