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Homework Help: Stuck for 2hrs on this frequency question

  1. Apr 8, 2013 #1
    The two cars have identical horns, each emitting a frequency of fs = 410 Hz. One of the cars is moving with a speed of 8.0 m/s toward a bystander waiting at a corner and the other car is parked. The speed of sound is 343 m/s. What is the beat frequency heard by the bystander?

    my attempt at the solution:

    fo = 410(343)/(343 - 8.0) = 419.791 Hz

    fb = f2 - f1
    = 419.791 - 410
    = 9.791 Hz

    i keep getting 9.79 Hz as my answer. i keep getting it wrong (thats what CAPA is telling me). can some one lead me in the right direction or tell me what i'm doing wrong???

    Thank you.
  2. jcsd
  3. Apr 8, 2013 #2


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    Hello icecubz. Welcome to PF !

    Does "CAPA" care about significant digits?
  4. Apr 8, 2013 #3
    yes it does, but if i do use the incorrect amount of sig figs, it tells me to use more or less digits. doesn't penalize for incorrect sig figs.

    nevertheless, i've tried entering 9.79, 9.791, and 9.80 and its come up as incorrect.
  5. Apr 8, 2013 #4

    f'= ((c+v)/c)*f

    this gives a different frequency, and thus a different beat frequency
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