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Homework Help: Beats Question

  1. Oct 16, 2008 #1
    1. The problem statement, all variables and given/known data
    Suppose you're operating a machine with two motors. If the two motors are not operating at the same rpm, you will hear a periodic variation in the intensity of the sound-that is, beats. Suppose the tachometer that indicates the rotation rate of one motor is malfunctioning. The operational one shows 3600 rpm. You hear a beat every 21 s. You are certain the other motor is running fast.

    What is its rotational speed in rpm?

    2. Relevant equations
    w1-w2 is the frequency of amplitude change (according to the textbook).

    3. Attempt at solution
    delta_w = 2pi/T = w1-w2
    w1 - 2pi/T = w2

    w2 is not valid (given that I have converted all units to standard and back into rpm). So I think my formula isn't valid. Can someone derive a correct one please? Thanks.
     
  2. jcsd
  3. Oct 16, 2008 #2

    LowlyPion

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    Homework Helper

    How many times has one motor turned in 21 seconds?

    If during the beat the "faster" one precessed such that you discrened a "beat" wouldn't that mean that the faster one - over that time - rotated 1 more complete time than the other?
     
  4. Oct 16, 2008 #3
    >> How many times has one motor turned in 21 seconds?
    The first one rotated 3600 rpm * m/60s * 21s times. I don't know about the second one since it's the one I need to find.

    >> If during the beat the "faster" one precessed such that you discerned a "beat" wouldn't that mean that the faster one - over that time - rotated 1 more complete time than the other?

    Why would it be "1" more complete time than the other? How do I know how many more times it rotated? How is this tied into the formula? Thanks.
     
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