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Homework Help: Doppler Effect: Moving Observer riding on moving source! With reflection off wall!

  1. Mar 11, 2010 #1
    1. The problem statement, all variables and given/known data

    A bus is moving at 37.00m/s towards a wall. The sound from the bus has an original wavelength of 0.1500m. The sound from the bus reflects off the wall. What frequency sound does an observer on the moving bus hear from the reflection??

    2. Relevant equations

    Moving Observer: fo = fs (1 + vo/v)
    v = LaTeX Code: \\lambda f

    3. The attempt at a solution

    Is this doppler effect??

    vo = 37.00m/s

    Since v = LaTeX Code: \\lambda f

    vi = LaTeX Code: \\lambda fi
    f = v / LaTeX Code: \\lambda
    = (343m/s)(0.1500) = 2286.667hz

    Subbing into equation:

    fo = (2286.667hz)(1 + 37m/s / 343m/s) = 2533hz



    Ok, I ALSO tried another method...

    Vs = 37 m/s

    Therefore the speed of the wavefront is: vs + v
    where v = 343m/s (speed of sound in air)

    fs = 343m/s / 0.15m = 2286.667hz

    Frequency observed is thus:

    (v+vs/v-vs)fs = (343+37 / 343-37) (2286.667) = 2839.65hz

    Both of these answers are choices in the multiple choice part, so this is becoming a frustrating question for me.

    Help appreciated! Thanks!
     
    Last edited: Mar 12, 2010
  2. jcsd
  3. Mar 11, 2010 #2
    Re: Doppler Effect: Moving Observer riding on moving source! With reflection off wall

    The frequency of source does not change on reflection. Could you explain the thought behind your assumption in the first part?
     
  4. Mar 12, 2010 #3
    Re: Doppler Effect: Moving Observer riding on moving source! With reflection off wall

    I was confused about that too, but am now thinking I misread some of my notes... which would explain A LOT of why that didn't make sense to me.

    Could you check my two possible solutions and let me know if either is correct?
     
  5. Mar 12, 2010 #4
    Re: Doppler Effect: Moving Observer riding on moving source! With reflection off wall

    Well the second one is right.
    I was thinking if frequency and velocity of propogation of sound do not change in reflection wavelength shouldn't change too.
     
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