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Homework Help: Frequency of Horn

  1. Jan 28, 2010 #1
    1. The problem statement, all variables and given/known data

    While blowing its horn of frequency 451 Hz, a car accelerates at 0.64 m/s2.

    The car starts from rest by your side and moves away.

    How long will it take for the frequency you hear to decrease by 5%?

    Hint: Do not forget to include the time it will take for sound to reach you.

    Give your answer in s, and to three significant figures.

    Only answer in numerical values, no units.

    2. Relevant equations

    speed of sound in air is 340 m/s

    3. The attempt at a solution

    fsource = fo (1/(1+vsource/c))

    0.95 = (1/(1+vsource/c))

    vsourve = 17.89m/s

    V= v0 +at
    t= 17.89/0.64
    t = 28.0 seconds

    This is wrong
  2. jcsd
  3. Jan 28, 2010 #2
    You probably just need to take into account the time it takes for sound to reach the observer. If the time you got is right, then at 28.0s the car would have traveled some distance that sound would have to cover in some time to get to the observer.
  4. Jan 28, 2010 #3
    OK. I am not sure how to account for this. What equation would i use?

    Thanks alot
  5. Jan 28, 2010 #4
    The car traveled a distance x = .5at2 from the observer, in which t = 28.0s. This would mean that for the sound carrying the relative frequency to get to the observer, it would have to travel through this distance to get to the observer. Your total time would take this into account.
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