Waves in a Spring Lab

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Purpose

To familiarize you with the properties of waves using a spring. The general procedure will be to stretch a spring between two people on the floor, create a pulse by a quick jerk of your hand, and observe the results.

Procedure

1. Create a clump of coils at one end, and let it go. What kind of wave have you created? Sketch the wave pulse below:
2. Move your hand quickly to one side and back to create a transverse wave.
   1. How is this wave different from the wave you created in #1? Sketch the transverse wave below.
   2. Does the size of the pulse change as it travels along the spring? Why or why not?
   3. Does the pulse reflected from the far end return to you on the same side or the opposite side of the original pulse? What is happening to the wave at the "fixed" (hand-held) end?
   4. How would you make the wave go faster, that is, increase its speed through the spring? Is this the same as increasing the frequency of the wave?
3. Have your partner send a wave pulse toward you on the same side that you send a pulse toward your partner, so that the pulses meet in the middle of the spring. The interaction of the two springs is called interference.
   1. When the two pulses meet, does the displacement of the spring at that instant get larger or smaller? Make a sketch below.
   2. Did the two pulses go through each other or bounce off each other? Explain.
   3. What is a wave made of?
4. Have your partner send a wave pulse toward you on the opposite side that you send a pulse toward your partner, so that the pulses meet in the middle of the spring.
   1. When the two pulses meet, does the displacement of the spring at that instant get larger or smaller? Make a sketch below.
   2. Formulate a general statement about the diplacement caused by the addition (or subtraction) of two pulses at the same point. (You might want to call this the Principle of Superposition).
5. By vibrating your hand steadily back and forth, you can produce a train of pulses called a periodic wave. The distance between any two neighboring crests on such a periodic wave is called the wavelength. The rate at which you vibrate your hand will determine the frequency of the periodic wave. Produce short bursts of periodic waves so that you can observe them without interference by reflections from the far end.
   1. How does the wavelength depend on the frequency?
6. Have your partner generate a continuous periodic wave while you try to match you partner's speed and amplitude. You have produced a standing wave.
   1. Are there places that the spring does not appear to move? What are these places called?
   2. What are the loops called?
   3. Make a sketch of your standing wave, labeling the answers to (a) and (b).
   4. What is the general relationship between the number of loops and the frequency of the wave?