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Frequency Shift Question

  1. Oct 16, 2008 #1
    1. The problem statement, all variables and given/known data
    What is the frequency shift of a 72 GHz police radar signal when it reflects off a car moving at 111 km/h? (Radar waves travel at the speed of light = 340 m/s.)

    --------
    f' = f(1 +- u/v) (moving observer, + appr, -receding)
    f' = f/(1 +- u/v) (moving source, - appr, +receding)
    --------

    f = 72 GHz = 72 * 10^9 Hz.
    u = 111 * 10^3 / 3600 m/s
    v = 340 m/s

    f' = f(1+u/v)
    f'' = f(1+u/v)/(1-u/v)
    f'' = (72 * 10^9 Hz)(1+(111 * 10^3 / 3600 m/s)/340)(1-(111 * 10^3 / 3600 m/s)/340)

    Calculating this formula does not give the correct result. What am I doing wrong?
     
  2. jcsd
  3. Oct 16, 2008 #2

    Redbelly98

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    The speed of light is actually a lot larger than 340 m/s. It seems you're using the speed of sound instead.
     
  4. Oct 16, 2008 #3
    Wow, that is probably the reason. The whole chapter was on sound so I thought the problem was, too, and didn't read it correctly. Did I make any other mistakes? Thanks.
     
  5. Oct 16, 2008 #4

    Redbelly98

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    I don't remember the exact formulas and don't have them handy, I just spotted the velocity error so I posted.
     
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