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Energy travels in waves. There are two types of waves, transverse and longitudinal also called compression waves. A transverse wave moves perpendicular to the line of direction or up and down. Think about when you stick your arm out the window of the car, making your hand dive up and down while you ride; you are making a transverse wave. See the diagram below and click on the link to watch a video clip of a transverse wave.

The diagram above is a transverse wave. You may also view a QuickTime video showing a transverse wave.
 
A longitudinal wave moves back and forth in the line of direction, not up and down like the transverse wave. If you take a slinky, lay it on the table, pull back about 10 links and let loose, you’ll observe a longitudinal wave. Notice that the slinky has areas that are darker where the rings have compressed (compression), and areas where it is stretched out (rarefaction). Longitudinal waves are sometimes referred to as compression waves for this reason. Click on the link below to watch a short video clip of a longitudinal wave in action. The concept of measuring how far molecules move is difficult to measure, so amplitude is usually only discussed in terms of transverse waves. You may view a QuickTime video of a longitudinal wave.
 
Amplitude is a measure of the amount of energy in a wave. In a transverse wave, amplitude is the measure from the resting position to either the crest (high point of the wave) or to the trough (low point of the wave.)
 

In the following examples, which has the greatest amplitude?

A

B

C

 
Highlight the box below to check your answer.
B
 

Think of amplitude in terms of a wave on water, the bigger (taller) the wave the more the boat moves up and down because of greater amplitude!  It takes energy to move a boat and the water. But you'll notice the boat stays in the same place, it only goes up and down.  This is because the energy that made the wave is transferring through the water, it is not transferring the water. The water is the medium that the energy is being transferred though. The definition of a wave is: "transfer energy through matter or space." Smaller waves cause less movement because they have less energy or a smaller amplitude. Larger waves like a tsunami hold a huge amount of energy. That is why they are so damaging.

Another factor of a wave is the wavelength. Wavelength is a measure of the distance between two crests or two troughs of a wave. The greater the wavelength, the more energy a wave has. "AM" radio stations have a big wavelength. You might have tuned the radio dial at night and heard stations that were far away and unable to be heard during the daytime. Similarly, an FRS radio is only able to transmit for a short distance because it uses a short wavelength and low power (amplitude.)

 
Notice in the QuickTime movie at the left how a drop of falling water has energy when it hits the pond below it. The drop's energy is transferred to the pond when it hits. As the energy is transferred to the pond, the waves move away from the drop in yet another transfer of energy. (Replay the video and watch the waves move away from the drop again.)
 

Activity:

In this activity you and a partner are going to simulate and draw some transverse waves.

 

Materials:

  • Butcher Paper or Poster Board
  • Four Markers- Black, Blue, Red, Green
  • One Meter of flexible rope
  • Meter stick
 
Safety concerns: As with all science lab activities, the most important safety rule is to follow all teacher directions.
 

Procedure:

  1. Copy this table onto a piece of paper to record your data.
    Wave
    Wavelength (cm)
    Low Energy  
    Medium Energy  
    High Energy  
  2. Lay your paper lengthwise and draw a horizontal line down the center in black.
  3. Label this line, in black as the “resting position.” Make a color coded legend in the lower right hand corner stating which color will represent your low (red), medium (blue) and high (green) energy waves.
  4. Each partner will hold one end of the rope and one partner will add a small amount of energy creating a transverse wave. See picture below.
  5. Once you feel like you have a smooth looking wave, you have to stop quickly to “freeze” the shape of the wave. This may take several attempts.
  6. Now that you have a frozen a wave, keep in mind that you must be able to see at least two crests or two troughs in order to measure the wavelength. Trace the wave with the assigned color for the low energy wave. (Begin tracing from the left of the page and go all the way to the right.)
  7. Label only the first wave with a crest, trough, amplitude and measure the wavelength using your meter stick. Have your teacher check your labels for accuracy and record the measurement for wavelength on your data table.
  8. Begin again with the rope at the resting position and make a new wave by adding more energy to the rope.
  9. Freeze this wave, trace it in blue and then measure and record the wavelength on your data table.
  10. Repeat steps 8 and 9 only add more energy and trace your final wave in green. Record data.
 

Analysis:

  1. Which wave “color” has the most energy? How do you know?
  2. What happens to the wavelength as energy increases?
  3. Let’s say that you have three waves with the wavelength measurements of 20 cm, 50 cm and 85 cm. Which wave has the lowest energy?
  4. What kind of waves do you use on a daily basis?

Review Science safetey rules here.

Get the plug-ins: Get Adobe Acrobat Reader , and Get Quicktime Player. (The QuickTime plug-in is needed to play sounds and movies correctly.)

Want to share photos of you or your friends doing this activity? Send it in an e-mail with the following information:

  1. The title of the activity
  2. The URL (Internet address)
  3. Your name.

Remember that no pictures can be used that show student faces or student names on it. 

Teachers should view the Teacher Site Map to relate Sci-ber text and the USOE 8th grade science core.


Updated October 24, 2008 by: Glen Westbroek

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