Archimedes and the “buoyant force”

September 6, 2010 | In: Science facts

While stepping into a public bath in the third century B.C., the Greek scientist Archimedes noticed how submerging his foot caused water to run over the side. He realized that the volume of water displaced was equal to the volume of the object submerged, his foot. He is said to have been so excited about this insight that he ran naked into the street, shouting about his discovery.

The displacement Archimedes noticed occurred because when a solid object like his foot is submerged in a liquid or gas it pushes aside, or “displaces,” some of the liquid or gas molecules to make space for itself. We know today that this displacement gives rise to a force called the “buoyant force”. The buoyant force pushes up on any submerged object. The strength of the force is equal to the weight of the displaced fluid. That is the reason you feel “lighter” when walking through water in a swimming pool. It is also the reason that hot air rises.

If a balloon is filled with hot air, helium, or any gas lighter than air, then the buoyant force, equal to the weight of an equal volume of air, may be powerful enough to make the balloon rise. If the balloon is big enough, the force may even be powerful enough to carry people with it. This principle enabled the first humans to fly, in the Montgolfier balloons, in eighteenth century France.