Moving Waves

We have had a marvelous week! My brother was here, with his kids and we had wonderful adventures. In addition to lots of beach time, we went to Discovery Cove in Orlando and swam with dolphins. We spent a day at Sea World, visited the St. Augustine Alligator Farm, toured the wax museum, ate enough sea food to sink a battleship, played Monopoly, and many other fun things.

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
  • small object that floats
  • something that floats. I used a rubber duck for the experiment. You could also use a bottle of shampoo, a rubber ball, or any other

You can also do this experiment in the classroom using a large container of water, as long as you don't mind making a bit of a mess.

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.