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Homework Help: Find the Wavelength

  1. Dec 6, 2009 #1
    A standing wave is set up in a 200 cm string fixed at both ends. The string vibrates in 5 distinct segments when driven by a 120 Hz source. What is the wavelength?

    * 10 cm
    * 20 cm
    * 40 cm
    * 80 cm

    I need some help with this problem. I can't seem to find anything with an equation or any helpful information for this. I don't even know where to start with this. Any help would be much appreciated. Thanks.
  2. jcsd
  3. Dec 6, 2009 #2
    Visualize the string vibrating in 5 segments. Go http://id.mind.net/~zona/mstm/physics/waves/standingWaves/standingWaves1/StandingWaves1.html" [Broken] and choose 5th harmonic. You know that an up and down segment is one wavelength. So how many wavelengths fit on the string in this mode. How long is the string?
    Last edited by a moderator: May 4, 2017
  4. Dec 6, 2009 #3
    5 wavelengths. And it is 200 cm, or 2 m.

    Or would it be 2.5 wavelengths?
  5. Dec 6, 2009 #4
    2.5!!! Remember a wavelength consists of the positive and negative going forms--run thru all the harmonics on that page, in the first case when there is only one deflection, that is 1/2 a wavelength.
  6. Dec 6, 2009 #5
    Okay, I see. So where does the 120 Hz come in?
  7. Dec 6, 2009 #6
    It doesn't unless you are asked to compute the velocity.
  8. Dec 6, 2009 #7
    So what do I have to do now?
  9. Dec 6, 2009 #8
    the string is 2 meters, you have 2.5 wavelengths, calculate the length of a single wavelength.
  10. Dec 7, 2009 #9
    Okay, let me make sure I did this right:

    (2 m)/(2.5 m) = .8 m

    Is that right? So 80 cm is my final answer then if that is correct.
  11. Dec 7, 2009 #10
    Thats my best guess as well.
  12. Dec 7, 2009 #11
    Okay, thank you. Another one down! 2 to go! :biggrin:
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