If a light object, such as a cork stopper, is held under water and released, it quickly returns to the surface. By contrast, a heavy object, such as an iron ball, sinks in water. This phenomenon was understood by the classical Greek mathematician, Archimedes, who described it in his famous principle. Archimedes' principle states that an object completely or partially submerged in a fluid experiences an upward force equal to the weight of the displaced fluid.
Density is defined as the amount of mass contained in a unit volume of a substance, usually expressed in grams (g) per cubic centimeter or in pounds per cubic foot. A light object floats on water because its density is less than that of water and an iron ball sinks because its density is greater than that of water. Specific examples using Archimedes' principle will clarify why this is so.
Consider an aluminum block of volume 100 centimeters-cubed (cm-cubed) that is immersed in a container of water. The density of aluminum is 2.7 g/cm-cubed; therefore, its weight is 100 cm-cubed x 2.7 g/cm-cubed = 270 g. The density of water is 1 g/cm-cubed; therefore, the weight of 100 cm-cubed of water is 100 g. According to Archimedes' principle, the "buoyant force" acting upward on the aluminum block is 100 g, which, in effect, reduces the weight of the block to 270 -- 100 = 170 g. From this it is clear that the block will sink because a downward force due to its effective weight remains.
A wooden block of the same volume, 100 cm-cubed, when immersed in water will float. The density of wood is typically about 0.6 g/cm-cubed. A wooden block of this density weighs 100 cm-cubed x 0.6 g/cm-cubed = 60 g. By invoking Archimedes' principle, it is seen that the buoyant force, 100 g, is greater than the weight of the block. Therefore, the buoyant force pushes the block to the surface, where it remains partially submerged. The wooden block immerses itself to a depth where the weight of the displaced water balances the weight of the block. Since the block weighs 60 g, it displaces 60 cm-cubed of water. Therefore, 40 cm-cubed, or 40 percent, of the block remains above water.