Hot-air balloons float because the air inside the balloon is less dense than the outside air.
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Energy Density
A tornado is the most destructive force in nature; that doesn't mean it
has the most energy. Thunderstorms which produce tornadoes can have
40,000 times as much energy as a tornado! Tornadoes are so destructive
because they have a higher
energy density than
thunderstorms; the energy is concentrated in a small area while
the energy in a thunderstorm is spread-out over a much greater area.
Energy and energy density can be compared to weight and force. Energy
density is energy per unit volume and force is weight per unit area. If
a 100 pound person in flat shoes stepped on your foot, you'd feel it. If
the same person stepped on your foot while wearing very narrow
high-heels, you might end up with a broken toe. That's because the
person's weight is concentrated in a small area. The same holds with
tornadoes. They may not have as much total energy as thunderstorms, but
the energy they do have is concentrated in a very small area. So small,
in fact, that tornadoes have destroyed houses while not damaging the
neighbors.
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