# Buoyancy

## 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.

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## 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.

Q:

## Does buoyancy work the same in water?

A:

In a full bathtub you can easily raise yourself up on your fingertips, thanks to the buoyant effect of the water. Try the same thing while sitting on the floor, supported only by air—it’s not so easy! Water is dense enough to help support your weight. Air exerts a buoyant force on you too, but it is so small compared to your weight that it has almost no effect on you.

# Watch a Video

## Lift: Lighter Than Air Aircraft

Learn about the history and dynamics of hot-air balloons.

## Did You Know?

A column of water 10 meters (33 feet) deep weighs the same and therefore exerts the same amount of pressure as a column of air extending all the way up through the atmosphere.

## Pop Quiz

How does the density of helium compare to air?

A) Helium is two times denser than air.
B) Helium is two times less dense than air.
C) Helium and air have the same density.
D) Helium is seven times less dense than air.

Helium is seven times less dense than air.  A birthday balloon and helium together weigh less than an equal volume of air, so the balloon rises.  If you add weight to it, the balloon sinks.  If you add only enough to balance the force of buoyancy, the balloon will float in mid-air.