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How does a radar gun determine velocity?

  1. Feb 16, 2012 #1
    1. The problem statement, all variables and given/known data

    Radar guns emit radio waves. The radar gun measures the beat frequency between the returning waves. How does the radar gun determine velocity?

    A. By measuring beat frequency between two consecutive reflected waves.
    B. By measuring the beat frequency between a wave reflected from a stationary object and one reflected from a moving object.
    C. By measuring the beat frequency between a wave reflected from two separate objects moving at the same speed.
    D. By measuring the beat frequency between a wave reflected from two separate objects moving at different speeds.

    Answer is B.

    2. Relevant equations

    Fbeat = |F2 - F1|
    the doppler equation: Fs / v+or-vs = Fl / v+or-vl

    3. The attempt at a solution

    My interpretation. Correct me if i'm wrong, please:
    A wave is emitted, reflects off object, and goes back to the radar gun. The frequency of the wave changes due to doppler effect. As new waves are emitted, a beat is produced which can measure the difference between the shifted wave and the new wave.

    But beyond this, i'm not sure. Been staring at this question for an hour and could use some help. Thanks very much.
     
  2. jcsd
  3. Feb 16, 2012 #2

    berkeman

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    Staff: Mentor

    You have it pretty much correct. The wording of answer B is a bit weird, since the beat frequency is between the reflected wave (which is Doppler shifted) and the original source (not a reflected wave off of a stationary object). But the wording of your answer seems fine.
     
  4. Feb 16, 2012 #3
    Thanks for the response.

    I'm having difficulty translating B into my answer, though. The emitter is the stationary object. It doesn't really reflect a wave it only emits and receives it, doesn't it?
     
  5. Feb 16, 2012 #4

    berkeman

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    Staff: Mentor

    The difficulty comes because of the wording of answer B. As I said, and you are also saying, there is no reflected wave off of a stationary object involved. You can point to this wikipedia article on Radar Guns, for example:

    http://en.wikipedia.org/wiki/Radar_gun

     
  6. Feb 16, 2012 #5
    Alright i'm satisfied with my answer then.

    Thanks again for your help. Is very appreciated.

    edit: Would you mind if I ask another sound related question, please Or ought I start a new thread.
     
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