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Archimedes’ Principle

Buoyancy of Liquids. — It is a matter of common experience that bodies are lighter in water than they are in air. A fresh egg will sink in pure water but will float in water to which a considerable quantity of salt has been added. A piece of iron sinks in water but floats in mercury. This is because the density of the mercury is greater than that of the iron. When a diver lifts a stone under water and brings it to the surface, he finds that the stone is heavier above the surface. In the case of lighter bodies, such as wood or cork, this lifting effect may be sufficient to keep parts of the body above water.

This resultant upward pressure of a liquid on a wholly or partly immersed body is called buoyancy. It is a force acting vertically upward and counterbalancing in whole or in part the weight of the body. A body may float (Figure 27) by being buoyed by more than one liquid at the same time.

Sphere

\ Less dense liquid

Denser liquid

Figure 27 - A sphere floating in two

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