This Week's Experiment - #216 Pendulum Perils
This week's experiment requires a lot of faith in the laws of science. When
it is done at science centers, it usually involves a bowling ball rushing
towards your face, but we will use a less dramatic version. You will need:
string
an apple that still has the stem on it
a high place to tie the string. I used a shower curtain rod.
Tie the string to the stem of the apple. You can use something besides an
apple, as long as it is round and easy to tie the string to. If you use
something that is not round, it may smack you in the nose, so I suggest
looking for an apple.
Tie the other end of the string to the rod that holds your shower curtain.
You want to measure the string so that if you sit in the bathtub, facing
towards the curtain rod (sideways in the tub), you can pull the apple over so
that it touches your forehead.
That is exactly what you are going to do. Lean your head back against the
wall, so that it does not move forward or backwards. Pull the apple towards
you until it touches your forehead. Now comes the part where you have to
trust me. Release the apple, letting it swing away from you. That wasn't so
bad, now was it. But wait! Now it is swinging back towards you! Will it
smack you in the face?
No, as long as you do not move your head, the apple will not hit you. Why
not? As the apple moves away from you, it is picking up speed. It is
converting potential energy, from its height, into kinetic energy, the energy
of motion. Once it passes the lowest point, the opposite begins to happen.
It is now moving against gravity, and some of its kinetic energy is converted
into potential energy. Once all of the kinetic energy has been converted, it
stops and starts to move downwards again.
In a perfect system, this would keep happening over and over, with no energy
lost. In reality, there is resistance from the air, friction with the
string, all sorts of things to drain away some of the energy. This means
that each swing will not go quite as high as the one before it. Because we
know that our apple on a string system is not perfectly frictionless, we know
that the apple will not make it back up high enough to hit your face. Just
be sure not to move, and use a small apple, just in case.
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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