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Transverse Wave on a String

  1. Dec 11, 2016 #1
    < Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown >

    A transverse wave on a string has an amplitude of 16cm, a wavenumber of 5.7m-1, and a frequency of 39Hz. What is the propagation speed of that wave?
    (a) 6.84 m/s
    (b) 39.2 m/s
    (c) 43 m/s
    (d) 6.24 m/s
    (e) 6.88 m/s
    This is a question on my physics study guide packet and while we have done wave problems in the past it was never anything like this. First: can someone explain what a wavenumber is. Second: what is propagation speed. Third: which equation should I be using. I thought maybe xmcos(ωt+Φ) but I don't know what the phase angle would be. I'm very confused, I would just like to have a basic understanding of what this is asking.
     
    Last edited: Dec 11, 2016
  2. jcsd
  3. Dec 11, 2016 #2

    davenn

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    Hi there
    welcome to PF :smile:

    I have asked for this to be moved to homework section

    from what you have studied so far, show us some of your thoughts


    Dave
     
  4. Dec 11, 2016 #3
    Oh I didn't know there was one, sorry.
     
  5. Dec 11, 2016 #4

    BvU

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    In particular: do you have an expression for the speed of a wave on a string ?
     
  6. Dec 11, 2016 #5
    wave speed? I have v=ω/k=λ/T=λf I believe but what I don't know is whether or not that's what propagation speed is.
     
  7. Dec 11, 2016 #6

    BvU

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    It is the good one in your case :smile:
     
  8. Dec 11, 2016 #7
    What do you mean by "the good one." I don't understand.
     
  9. Dec 11, 2016 #8
    Oh I got it. The variable k is wavenumber, my professor never made that clear. So using:
    ω=2π/T, T being the inverse of 39Hz or T=.0256
    ω=245.04
    v=ω/k=43m/s
    so the answer is c.
    This would've been a whole lot easier if that had been clear in my notes. Oops.
     
  10. Dec 11, 2016 #9
    Regarding your question about wavenumber, wavenumber is the number of waves that fit in a unit space. In other words it is 1/wavelength. Usually for convenience instead of how many waves per unit length it is defined as how many radians per unit length, that is

    k = 2 π/λ

    This is a convenience in that it simplifies a lot of equations and it is also proportional to energy.
     
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