Eli5: How can hot air balloons fly by only using fire?

Question

Eli5: How can hot air balloons fly by only using fire?

In: Other

Answer 1

Heating a gas such as air causes the molecules to move faster. These faster moving molecules push against each other and cause the gas to expand and be less dense. There are fewer air molecules per cubic foot in hot air than in cold air.

Because the air in the balloon is hotter and thus less dense than the surrounding air, it weighs less. Lighter materials float on top of heavier ones, so the balloon rises.