Since prehistoric times, people have known that certain substances, such as wood, will float in water while others, such as rocks, will sink. But no one could explain why. The answer was discovered by Archimedes, a Greek philosopher who lived in the 200's BC.
According to legend, Archimedes was asked to resolve a question by his king. Was the king's new crown made of pure gold, as the goldsmith claimed, or was it a mixture of gold and other metals? One day while pondering this question, Archimedes lowered himself into his bath and watched the water level rise. He suddenly realized that the water was rising because his body displaced, or pushed aside, an equal volume of water when it was submerged. With this realization, Archimedes knew how to answer the king's question.
He would find the volume of the king's crown by lowering it into a container of water, marking the level of the water before and after the crown was submerged, and then measuring the volume of water needed to raise the water level by that amount. This volume of water would equal the volume of the crown. Then he would weigh the crown and divide that weight by the crown's volume. This would give him what is called the weight per unit volume of the crown, or its density. By comparing this density to the density of pure gold, Archimedes could tell whether or not the crown was made of pure gold.
Finding the answer to the king's question led Archimedes to some other important discoveries. He realized that there must be some force that exerts an upward push on a floating object. This force is called the buoyant force or, simply, buoyancy. The buoyant force is the result of pressure exerted by the water, which pushes equally in all directions. In addition, because water pressure increases with depth, the pressure at the bottom of an object immersed in it is greater than the pressure at the top of the object. This results in an upward force that opposes the force of gravity, which pulls an object toward the center of the Earth.
The buoyant force applies not just to water but to any fluid. A fluid is a substance that is able to take the shape of a container in which it is placed. For example, both liquids and gases are fluids. Liquids that are less dense than water, such as alcohol and cooking oil, exert less buoyancy per volume of liquid displaced than water. Liquids that are more dense than water, such as mercury and salt water, exert more buoyancy per volume of liquid displaced. All gases are less dense than water, so their buoyant force is less. Some gases are more dense than others, however, so you will learn that their buoyant forces also differ from one another.
Through his observations and discoveries, Archimedes was able to arrive at a scientific principle about floating and buoyancy. Archimedes' principle states that an object floating or submerged in a fluid is buoyed upward by a force equal to the weight of the fluid it displaces. This is the basic principle that explains why some things float and others sink, and it applies to all liquids and all gases, including air.
An object placed in a fluid is buoyed upward by a force equal to the weight of the fluid displaced. If the buoyant force is equal to or greater than the object's weight, the object will float or rise. If the buoyant force is less than the object's weight, the object will sink in the fluid.