Let's make waves!
Discover the forces that put the motion in the ocean
Summer is a great time of year to go swimming, especially if you can get to the beach. The waves make water so much more interesting. If you can't go to the beach, you can still put on your swimsuit and do some "wave exploring" at home.
Waves make fascinating study for scientists, who are still trying to understand more about their nature. The basics, however, can be easily studied in your backyard.
This first wave activity helps you understand a fundamental fact about waves: When they move, they don't take the water with them. To make your own wave tank, you'll need an aquarium (no fish, please), water to fill the aquarium, a cork, masking tape, and a small block of wood.
Fill the aquarium about two-thirds full with water. Float the cork in the center. Mark the outside of the tank with masking tape where the two sides of the cork appear to be.
Make waves by moving the block of wood gently up and down at one end of the tank. Watch how the cork goes in a circle but doesn't get pushed to the side. That's because waves transmit energy through the water - they don't move the water sideways.
You can see the same type of wave if you tie a piece of rope to a doorknob and shake the loose end up and down. Waves will move along the rope, but the rope itself doesn't move toward the doorknob.
The top of each wave is called the crest, and the lowest point is the trough. The distance from the top of one wave to the top of the next is the wavelength. The distance from the trough to the crest is the wave height.
The waves in your tank can be measured in inches, but in the ocean, the wind can blow them dozens of feet high. The highest wind wave ever recorded was 112 feet.
To see how the wind forms waves, blow across the water and watch how the water ripples. One of the reasons wind can create waves is that on the water's surface, water molecules are pulled toward each other - not the air. This pull is called surface tension, and it creates a kind of film on the water. In the ocean, wind blows against the water and tries to "stretch" its surface. Surface tension makes the water pull back together. As the wind keeps blowing, the water gets more agitated and waves form. The more the wind pushes, the bigger the waves get.
Here's one way to see how surface tension works. You'll need a drinking glass, a pitcher of water, a tablespoon, some salt, and a few pennies.
Put the glass on a level surface and carefully fill it to the top. Now drop a penny gently into the glass, keeping an eye on the water at the edge. The water will rise, but won't overflow.
The water is able to rise above the rim of the glass because surface tension holds the water together. See how many pennies you can add before the water spills over.
Repeat the activity, but add a tablespoon of salt to the water. You won't be able to add as many pennies because salt reduces the surface tension.
Some waves can be pretty big by the time they reach the shore. Like the cork in the wave tank, water in the waves is moving in circles. When it gets close to the shore, the bottom part of the circle gets cut off by the ground beneath. The top of the wave is pushed onto the shore. Then it flows back downhill into the ocean.
When the waves are very strong for a long time, they can carry a lot of the beach's sand back with them. During the winter many California beaches become rocky and barren as sand is pulled into sandbars offshore by the powerful winter waves. New sand is deposited by gentle summer waves, however. Some of this sand comes from the sandbars, and new sand is added as rivers carry silt into the sea.
You can prove how sand is put back onto the beach. Get a bucket, some water, a board at least two feet wide and four feet long, a rock or other object to lift one end of the board about six inches, and a few pebbles.
Prop one end of the board on the rock so it slants downward. Put a few inches of water in the bucket. Stand at the bottom of the board and gently toss the water at the top, keeping the bucket low to imitate a wave coming onto a beach. When the water hits, it splashes all over, but then runs straight back down the board. It runs down more slowly than you threw it because it rubs against the board on the way back. This is called friction.
Since the water has less energy on the way back, a wave can't carry as much of the sand and debris that's floating in the water - so it leaves them on shore.
Next, add some pebbles to the water before you toss it at the top of the board. Some of the pebbles will be left on the board. This is how gentle waves can put sand onto a beach, while more active waves have enough energy to carry sand off the beach.
Now stand off to one side of the board. Toss water at the top of the board at an angle. The water quickly stops flowing sideways and runs back down the board.
When wind blows waves from an angle onto the shore, the waves flow back to the sea at right angles to the shore. Pebbles or sand carried off the shore in one wave will be shoved sideways farther down the beach by the next wave. This is called longshore drift. It moves sand along the beach so it becomes built up some areas and worn down others.
After you have tried these experiments, the next time your parents take you to the beach or to a wave pool, you can explain how waves work.
Sources: The aquarium experiment is from the Water on the Move website, www.mos.org/oceans/motion/wind. html; the pennies in the glass experiment can be found in 'Bathtub Science,' Shar Levine and Leslie Johnstone, Sterling Publishing Co.
While most ocean waves are caused by wind, another type of wave is caused by earthquakes, volcanoes, or avalanches in the ocean. It is called tsunami (sue-NAH-me), a Japanese word for "large waves in the harbor." While most of these waves are small and harmless, a few big ones have hit the shore with much force.
Tsunamis are often only a few feet high in the open sea. People in ships may not even notice if one passes beneath them. But these waves move quickly and have a lot of energy. The average ocean wave caused by wind moves at a speed of about 35 miles per hour. In very deep water a tsunami may travel more than 500 miles an hour.
On rare occasions, the wave travels to a land mass, and the water gets shallow. The ocean floor acts like a brake. The bottom of the wave slows down and the top rises. A two-foot wave in the open sea can grow as high as 30 feet before it reaches shore. To protect people and property, 26 countries track events that might cause these waves and watch for evidence of them coming toward coastal areas.
About five tsunamis are detected each year, and most are not a threat. Coastal cities are prepared, however, to move people away from the shore quickly if one of these huge waves threatens. This is one wave that even surfers don't want to meet.