Buoyancy

Our atmosphere and ocean share much in common. Both consist of fluids (matter that flows), both create pressure that changes with depth, and for both the same forces to movement apply. We are indeed bottom dwellers in a “sea of air.”

The key to a boat sailing the seas and a hot-air balloon soaring through the skies is the same force—buoyancy.

Buoyancy is the upward force exerted by a fluid, like water or air, on an object that works against the downward force of an object’s weight.

All liquids and gases in the presence of gravity exert an upward force on any object immersed in them. If the object weighs less than the liquid or gas it displaces, buoyancy will make it float.

To make something like a hot-air balloon or blimp float, it must weigh less than the air it displaces. Since the balloon and basket are heavier than air, how can you make them light enough to float?

The answer is to fill the hot-air balloon or blimp with a large volume of something much lighter, either hot air or a very light gas, such as helium. Because hot air and helium are less dense, the combined weight of the balloon and the gas is less than the weight of an equal volume of surrounding air, so the balloon rises.