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Homework Help: Physics wave question

  1. May 29, 2009 #1
    1. The problem statement, all variables and given/known data
    a cyclist and a car are approaching each other at the same speed. the car makes a 500 Hz sound at 343 m/s. the cyclist hears the sound as 535 Hz. How fast are they moving?

    2. Relevant equations

    f2 = f1(v/v[tex]\pm[/tex]vs)

    3. The attempt at a solution
    535 hz = 500 hz(343 m/s[tex]\div[/tex]343 m/s-vs)
    vs = 22.4 m/s

    but the answer should be 11 m/s
    HELP ??
  2. jcsd
  3. May 30, 2009 #2
    In this case, both the source and the receiver move, so u must use the general Dopple formula, not the specific formula for each case the source moves or the receiver moves.

    f' = f.(V+Vr)/(V-Vs)

    f': frequency received

    f: frequency created.

    V: speed of the wave (sound or light).

    Vr: velocity of the receiver: positive if moving toward the source, otherwise it's negative.

    Vs: velocity of the source: positive if moving toward the receiver, otherwise it's negative.

    In this case, Vs=Vr, and they're both positive. U will get the result 11,6 m/s. Be happy :D
  4. May 30, 2009 #3
    great thanks!
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