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Homework Help: Finding velocity of a vehicle using Doppler Shift for sound

  1. Jan 8, 2013 #1
    1. The problem statement, all variables and given/known data

    You are standing on the side of the Trans-Canada Highway as the Physicsmobile approaches you. As it approaches, you hear an engine noise of 1100 Hz. After it passes, you hear an engine noise of 950 Hz. How fast was the Physicsmobile travelling?

    2. Relevant equations

    f2 = apparent frequency
    f1 = actual frequency emitted by source
    v = speed of sound in air
    vs = speed of source

    3. The attempt at a solution

    I'm not sure how to re-arrange the formula. If I fill in all variables that I'm given, there are still two left blank (vs and f).
  2. jcsd
  3. Jan 8, 2013 #2


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    2017 Award

    Staff: Mentor

    You have two unknown values (vs and f1) and two equations - one before and one after it passes you. You can solve this equation system to get vs.
  4. Jan 9, 2013 #3
    Does this look right?
  5. Jan 15, 2013 #4


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    Staff: Mentor

  6. Jan 15, 2013 #5
    Your first equation is wrong. The source approaches the observer, so the frequency should increase. To increase the frequency, the denominator in the equation should decrease.
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