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Balloons and blimps fly because of the physical differences between
different gases and how gases change with
temperature. When the gas inside
a balloon has a lower density than the
surrounding gases, the balloon gains
buoyancy. Some gases, like
helium, have a lower
molecular weight and
rise up through the heavier air.
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Gases are the most energetic of the three
phases of matter. Gas
molecules have little
attraction to each other and are spread far apart. Increasing
the temperature of a gas, increases the
molecular energy, pushing the molecules farther apart. This
decreases the density of the gas.
Hot-air balloons hold hotter and therefore
lighter air inside their envelopes and fly because of the buoyant
force pushing the lighter hot air up.
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