The helicopter’s blades encounter air as it falls. The blades are at an angle,
so that each little bit of air that hits them bounces off and pushes them to
the side. If you have them bent correctly, each one will be pushed around
in the same direction, causing the helicopter to spin. The air also pushes
up the blades, causing the helicopter to fall more slowly.
Real helicopters do this in emergencies. If a helicopter’s engine fails,
the recovery procedure is to begin “autorotation.” This simply means that
instead of the engine turning the blades, the air flowing past the falling
craft will turn the blades. In most cases, this provides enough upward
force to slow the descent, allowing the pilot to find a suitable space below
for an emergency landing.
This toy requires air to function. If you tried it on the moon, here
is what would happen: It would rise to six times the height it rises to on
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Earth, because the moon’s gravity is six times less than the Earth’s. Then
it would fall back to the Earth without any spinning at all, because there
is no air on the moon.
Let’s follow the energy used in this project. You provide the original
energy to stretch the rubber band. Your body got this energy by metabolizing
the food you ate earlier. When the rubber band is stretched, it holds
potential energy. When released, it gives this energy to the helicopter.
The helicopter begins accelerating upward. As the helicopter leaves the
rubber band, it is traveling as fast as it will go during its flight. The potential
energy of the rubber band has been converted into kinetic energy.
The helicopter slows down as it continues upward because gravity is
pulling steadily downward on it. It loses kinetic energy as it gains potential
energy of height. At its highest point, it stops moving up and has lost
all kinetic energy. It holds the maximum potential energy of height it will
have. It then starts down.
If the blades spring out properly, they encounter air, which pushes
them around. As the helicopter begins rotating, it gains rotational kinetic
energy, which uses up some of the potential energy. Some of the helicopter’s
potential energy is also given to the air, heating it up a bit. Because of
these losses, the speed (and kinetic energy) of the helicopter is a bit less on
the way down than it was on the way up. Upon impact with the ground,
all remaining energy is converted to heat. You won’t feel this heat because
there is not much energy involved.