How do hot air balloons float?
(Lansing State Journal, August 24, 1994)

Hot air balloons rely on buoyancy to float up into the air.  Something is buoyant if it is less dense than the surrounding gas or liquid.  When air is heated, it expands, becoming less dense than the air that is not heated.  If the hot air is contained as it is in a balloon, it can force additional weight up into the air.

Why is the hot air less dense?  When one heats matter, its individual molecules vibrate more and more. These vibrators force the molecules apart, causing the substance to expand.  In solids and liquids, this expansion isn't very noticeable to the naked eye, but can be measured.  With gases it is aquite different.  Because gas molecules aren't strongly attracted to one another, the molecules are free to move about.  Heating causes the molecules to move faster and make collisions harder.  The result is a more massive expansion.  In fact, gases will increase their volume by roughly 1 percent for every 6 degrees Fahrenheit.  So, hot air actually has fewer molecules in a given volume than cool air.

When the balloons are on the ground, the air inside is just as dense as that outside.  Using gas burners nd fans, the balloon operators force hot air into the balloon.  Because the heated air is somewhat trapped inside, the balloon starts to fill up and float.  Constant heating of the air is required to keep the balloon aloft.  As the air in the balloon cools, it contracts, allowing outside cool air to enter, and the density increases.  When this is carefully controlled by the pilot, the balloon can land as gently as it rose.

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