Catching Waves
By John Cowens

With a (gentle) tap of a hammer, water in a trash-bag-lined cardboard box is forced up and causes a wave motion.)

Waves are pulses of energy that move through the ocean. Most of the time, they are set in motion by wind. However, earthquakes and the gravitational pull of the moon also produce waves. Ocean waves begin as wind-stirred ripples on the surface of the water. As more energy is transferred from wind to water, the waves formed appear as great forward surges of water. However, the water is not moving forward at all! Only energy moves forward through the water, producing one wave after another. The energy is passed from one particle of water to another; as depth of ocean water increases, the motion of the energy particles decreases. At a certain depth, energy motion stops. Thus, in deep water, there are no waves except for those caused by tides and earthquakes. The height of surface waves depends upon three important factors: speed of wind, length of time for which wind blows and distance wind blows over water. If these factors increase, so does the height of a wave.

Wave characteristics

• The highest point of a wave is called the "crest" and the lowest point is called the "trough."
• The horizontal distance between two consecutive crests or troughs is called the "wavelength."
• The vertical distance between a crest and a trough is called the "wave height."
• The amount of time it takes consecutive crests or troughs to pass a given point is called the "wave period."
• The number of crests or troughs passing a given point in a certain wave period is called the "wave frequency."
• Out in the open ocean, swells are produced when waves stay about the same distance apart for thousands of kilometers, causing the wavelength to be constant until they approach the shore. When these waves finally crash forward as breakers and surge onto the shore, they are called "surf."
• As the water flows back to the ocean, an undertow is produced which may extend for several kilometer offshore. Beware, swimmers!

What's a tsunami?
Tsunami is the Japanese word for "large wave in a harbor." Tsunamis are the highest waves produced on Earth and are caused by earthquakes, volcanic explosions and underwater landslides (or "submarine slides"). Their wavelengths are long, very deep and carry an enormous amount of energy. As these waves enter shallow water, they slow down (due to compression against shallow underwater land), change shape, grow taller and get closer together. They gradually steepen and finally break in the surf when depth becomes less than 1.3 times their height. Waves change shape in depths depending on their wave length, but break in shallows relating to their height. How high a wave will rise depends on its wave length (period) and the slope of the coast land.

Make a miniature tsunami
Materials:

• Cardboard box trimmed to 45 cm x 50 cm x 5 cm


• Large trash bag (or plastic leaf bag)


• Two meter sticks


• Water


• Hammer


• Two chairs (or desks)

Procedure:

1. Line the cardboard box with the trash bag and make it water tight.
2. Place the chairs (or desks) approximately 50 cm apart. Chairs should face each other.
3. Bridge the chairs with two meter sticks, approximately 30 cm apart.
4. Place the box on the middle of the two sticks. Fill it halfway with water.
5. Using the hammer, lightly tap the underside center of the box. Observe what happens to the surface of the water. Water is forced up from the impact of the hammer, which causes a wave motion on the water's surface.
6. Turn the hammer upside down and use the handle to quickly push the bottom center of the box upward approximately one or two cm. Watch the surface of the water rise, then start wave motion.

Extension:
Do light waves and sound waves have similar characteristics as water waves?

Recommended books
What Makes an Ocean Wave? Questions and Answers About Oceans and Ocean Life by Melvin Berger, Gilda Berger and John Rice (Scholastic, 2001, ISBN: 0-439-14882-0).
Tsunamis by Luke Thompson (Children's Press, 2000, ISBN: 0-516-23568-0).

Good sites to surf
Oceanography: Waves: www.seafriends.org.nz/oceano/waves.htm
The Nature of a Wave: www.glenbrook.k12.il.us/gbssci/phys/Class/waves/u10l1a.html
International Tsunami Information Center: www.prh.noaa.gov