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Doppler's Effect

  1. Nov 7, 2007 #1
    1. The problem statement, all variables and given/known data
    A physics professor demonstrates the Doppler effect by tying a 600 Hz sound generator to a 1.0-m-long rope and whirling it around her head in a horizontal circle at 100 rpm.


    2. Relevant equations
    f =fo(1/ (V +/- Vsource/V))


    3. The attempt at a solution
    T = 1/(100r/min)*((1min/60s)) = 0.6seconds
    d = 2pi(1.0m)
    vsource = d/t = 2pi /0.6 = 10.4719 m/s

    f+ = 1.749Hz
    f- = 1.749Hz

    which is obviously wrong...because they should be hearing different frequencies when source is moving towards or away from the students. can anyone tell me what i am doing wrong?
     
  2. jcsd
  3. Nov 7, 2007 #2

    Dick

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    Sure, one shift is added to 600Hz and the other subtracted from 600Hz. Corresponding to the +/- in the formula. But your frequency shifts look like they are off as well, by something like a factor of 10.
     
  4. Nov 7, 2007 #3
    omgosh ... i got the equation wrong...lol it's fo /(1 +/- vs/v)
    my bad. thanks
     
  5. Nov 7, 2007 #4

    Dick

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    It's correct if you put another pair of parentheses in f =fo(1/ ((V +/- Vsource)/V)), which then turns into what you just said. Which I thought is what you meant. No problem.
     
  6. Dec 1, 2007 #5
    Oh! but how do we find the velocity (V) ? It's 100 rpm, so do we convert that to revolution per second? Thanks!
     
  7. Dec 1, 2007 #6

    Dick

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    One minute has sixty seconds.
     
  8. Dec 1, 2007 #7
    Ok, so that means 1.67 revolution per second. If I substitute that into the equation
    f =fo(1/ ((V +/- Vsource)/V))

    f+ = 600(1/ ((1.67 + 10.4719)/1.67))
    = 82.39
    f- = 600(1/ ((1.67 - 10.4719)/1.67))
    = -113.60

    Is that right?
     
  9. Dec 1, 2007 #8
    no, first of all your v is the speed of sound. Your v_s is the speed 10.4719. You only use the period to find the velocity.

    so its

    f+ = 600/(1- (10.4719/343) ( you got the signs messed up in your equation, - for approaching,+ for receding)

    try that see if it works
     
    Last edited: Dec 1, 2007
  10. Dec 1, 2007 #9
    Draco!! Yes!! Thanks a lot!!!!!! :D:D:D

    f+ = 618.9
    f- = 582.2
     
  11. Dec 1, 2007 #10
    Check your messages in your inbox on this site!!
     
  12. Dec 1, 2007 #11
    hey Draco, do u go to UTSC?
     
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