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Homework Help: Physics of Waves & Sounds

  1. Feb 9, 2010 #1
    1. The problem statement, all variables and given/known data
    A train moving toward a detector at 29 m/s blows a 295-Hz whistle. What frequency is detected on each of the following?
    (a) a stationary train
    (b) a train moving toward the first train at 21 m/s

    2. Relevant equations
    V = f * Wavelength
    (f = frequency, V = velocity in m/s)

    T = 1/f
    (T= period in sec.)

    3. The attempt at a solution
    1. i know the doppler's effect is involved here somehow, and that it has to do with frequency increasing as the space between the two objects (one emitting the waves) grows smaller. but my teacher never gave us any sort of formula for this scenario, and i just absolutely have no idea how to approach this problem or determine by how much the frequency increases given just velocity & frequency. please help..

    Any or all help appreciated...I apologize for my lack of knowledge. I really was never good at physics or anything math related...

    Last edited: Feb 9, 2010
  2. jcsd
  3. Feb 9, 2010 #2
    The formula needed is F = F' * (V +/- Vd)/(V +/- Vs) where F is the frequency observed, F' is the frequency emitted, V is the speed of sound, Vd is the speed of the detector and Vs is the speed of the source. You have to determine the signs based on your knowledge of how sound works.
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