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Water Waves: Surface Disturbances Due to Energy Transfer (page 2)

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Author: Janice VanCleave

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As a wave travels through water, each water molecule moves up and down in a circular motion, ending up nearly in the same position as where it started. Repeat the experiment to prepare a model of the movement of a water molecule as a wave passes through water. Modify the plastic piece by laying it over the circle in Figure 21.3 so that the top left corner of the plastic piece matches the corner formed by the dashed lines. Trace the circle, dots, and arrows. Tape the plastic back onto the viewer. Then, holding the viewer with one hand, pull the paper sideways through the viewer with the other hand so that the top dot on the circle is centered on the crest of a wave. Slowly pull the paper in the direction of the wave, toward the right, until the bottom dot on the circle is centered on the trough of the wave. The dots on the circle represent the location of a water molecule at the crest and at the trough. The arrows indicate the direction in which the molecule moves.

 Water Waves: Surface Disturbances Due to Energy Transfer

Design Your Own Experiment

  1. A way to demonstrate that surface water does not move with waves is to fill a long Pyrex baking dish about three-fourths full with water. Place a small cork in the center of the water in the dish. Wait about 30 seconds to allow the water to become calm. Tap the surface of the water at one end of the dish with your finger. Observe the movement of the cork.
  2. When waves come from different directions, they overlap each other. The placement of one wave atop another when they meet is called superposition. The waves can combine in two different ways. When crests and troughs of colliding waves match, constructive interference occurs. When the crests and troughs don't match, destructive interference occurs. Use the water-filled baking dish from the previous experiment. Simultaneously touch the surface of the water at both ends and observe the movement of the waves.

Get the Facts

While wind causes most ocean waves, underwater disturbances such as volcanoes, earthquakes, or landslides can cause freakishly long, high-speed waves called tsunamis. What is tsunamis' wave height in deep water? at the shore? What is their wavelength? For information about tsunamis, see Don Groves, The Oceans (New York: Wiley, 1989), pp. 120-122.

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