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Activity: Orbital Motion of Waves

NGSS Science and Engineering Practices:

NGSS Crosscutting Concepts:

NGSS Disciplinary Core Ideas:

Materials

  • Fig. 4.21
  • Table 4.10
  • Long wave tank
  • One paddle that fits snugly in the width of the wave tank
  • Water
  • Three rulers
  • Masking tape
  • Metronome or sound recording
  • Dry-erase marker, felt-tip marker or grease pencil
  • Towels
  • Materials that are more dense than water, less dense than water, and neutrally buoyant, for example (see Step 4),
    • Corks or other small buoyant objects about 1–3 cm in diameter
    • Monofilament fishing line
    • Small lead sinkers or other weights
    • Sand
    • Food coloring and plastic pipette
    • Small bits of paper
    • Boats (made of paper, foil, or other material that floats)

Procedure

Safety Note: If your wave tank is sliding on the table, place towels underneath it. Immediately mop up all water spills to prevent slipping.

 

A. Generate standing waves.

  1. Set up the long wave tank as shown in Fig. 4.21.
    1. Fill the wave tank about halfway with water.
    2. Tape one ruler to the end of the wave tank as a backstop. The backstop should be positioned to prevent the paddle from going past vertical.
    3. Tape a second ruler along the top edge of the wave tank (the yellow ruler in Fig. 4.21)
    4. Tape a third ruler 5 cm in front of the backstop as the paddle-stop. The paddle-stop will help you control the amount of water pushed by limiting the distance the paddle can move.

<p><strong>Fig. 4.21.</strong> Wave tank set up for observing orbital wave motion. (This image is not to scale; the paddle, paddlestop, and ruler have been enlarged relative to the size of the tank.)</p><br />


 

  1. Set your paddle in the paddle groove and practice generating standing waves. Recall that standing waves do not advance; they appear to move up and down in place.
    1. Take turns creating waves. Observe as others create waves and let the wave maker know when a standing wave has been created.
    2. Determine the best method for creating a standing wave.

 

B. Observe the orbital motion of waves

  1. Make predictions about how you expect water to move at the bottom of the tank, mid-water, and at the surface of the water when you start generating waves.
     
  2. Place objects in the water so you can observe the motion of water on the bottom, at the surface, and near mid-water levels (Table 4.10).
    1. Choose one or more methods for each location in the tank.
    2. Note that some of these methods (e.g., food coloring) will cloud your tank, so you will have to work quickly. You made want to save these messier methods for after you have tried other options.
    3. You can also develop your own methods based on the materials available to you.
       
  3. Use the metronome to produce standing waves at a consistent frequency.
     
  4. Observe water motion at each level of the tank and record your observations.
     
  5. With a marker or a grease pencil, trace onto the side of the tank the movements of materials at the surface, at mid-water, and at the bottom of the tank.

 

Activity Questions: 
  1. As wave energy moves through water, the water moves in an orbital (circular) motion (Fig. 4.18), with orbital motion and wave energy decreasing with depth. Are your results consistent with Fig. 4.18? If you did not observe orbital motion, explain why.

<p><strong>Fig. 4.18.</strong> (<strong>A</strong>) If a small buoy (black circle) was on the surface of the water, it would move in a circular motion, returning to its original location due to the orbital motion of waves in deep water. (<strong>B</strong>) As deep-water waves approach shore and become shallow-water waves, circular motion is distorted as interaction with the bottom occurs.</p><br />


  1. Did you observe more motion at the at the surface than at mid-water or at the bottom? Explain what you observed.
     
  2. Did all the methods you used show orbital motion in the same way? Evaluate which method was best for your goals.
     
  3. If you could repeat the experiment, how would you change your procedure?
Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.