This Week's Experiment - #281 Moving Waves
While we were playing in the surf, my niece mentioned that her teacher had
told her that the waves move but the water does not. Now how can that be?
Let's find out. You will need:
a bathtub
water
something that floats. I used a rubber duck that Lisa bought me for the
experiment. You could also use a bottle of shampoo, a rubber ball, or any
other small object that floats.
Fill the tub about half full of water. Place your rubber duck (or whatever
you are using) into the water. Make sure the water is deep enough that the
toy floats and does not sit on the bottom.
The next thing is to make some waves. Put your hand into the water and swish
it from one end to the other. You should see a wave move in front of your
hand. When the wave hits the end of the tub it should seem to bounce back,
moving back towards where it started.
Make another wave, but this time instead of just paying attention to the
wave, pay attention to the rubber duck. Does it race along with the wave?
No. Instead, it stays in one area and bobs up and down as the wave passes.
Why?
Even though it seems that the water is all moving with the wave, it is not.
If it was, it would carry the toy along with it. Most of the water is just
moving up and down. Then what is moving from one end of the tub to the
other? Energy. You push on the water with your hand. As the water moves,
it pushes on the water in front of it. That water pushes on the water in
front of it, and so on and so on. While each bit of water only moves a short
distance, the energy of your push is transferred from one end of the tub to
the other and back again.
The same idea is true for ocean waves. Toss a float into the water and it
bobs up and down as the waves pass under it. The water does not move much,
but the energy of its motion is passed along.
This just goes to show you that you never know when some bit of knowledge
will be useful. I am sure that at the time, Lauren wondered why she had to
learn about waves. It turned out that she used her knowledge to beat the
rest of us at body boarding.
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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