What Makes a Hot-Air Balloon or a Blimp Float?
Hot-air balloons and blimps can float in the air thanks to buoyancy, an upward force that the air exerts on them. The balloon must weigh less than the air it displaces (the air that would occupy the same space if the balloon wasn’t there).
Since the balloon itself is heavier than air, it must be filled with a large volume of something much lighter—either hot air or a very light gas, such as helium. Because the combined weight of the balloon and the gas is less than the weight of an equal volume of surrounding air, the balloon rises.
What Is Buoyancy?
All liquids and gases in the presence of gravity exert an upward force—called buoyancy—on any object immersed in them. If the object is less dense than the liquid or gas, buoyancy will make it float. A cork floats in water because it is less dense than a cork-size volume of water. But it won’t float in air because it is denser than the same volume of air.