Kinetic Energy and Friction

Pendulum

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 on a chain, 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 the apple 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.

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