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Homework Help: Doppler effect for radar gun

  1. May 13, 2012 #1
    1. The problem statement, all variables and given/known data
    Radar is used to measure the speed of passing vehicles. A stationary radar gun emits EM radiation, a proportion of which is reflected from the target vehicle which is moving away from the gun at 'u' ms^-1.

    a)State the frequency at the target vehicle (f.tar) in terms of the frequency at the radar gun (f.sou).

    The target vehicle then acts as a moving source emitting at frequency (f.tar), moving away from the radar gun.

    b) State the frequency of the reflected radiation (f.ref) at the radar gun in terms of the frequency reflected by the target vehicle (f.tar).

    c) Combine these expressions to relate the initial frequency (f.sou) to the reflected frequency (f.ref), simplifying the expression by assuming u<<c.

    2. Relevant equations

    3. The attempt at a solution

    I say f.tar = ((c-u)/c)*f.sou
    and f.ref = (c/(c+u))*f.tar

    so, f.ref = (c/(c+u))*((c-u)/c)*f.sou

    where f.ref = f.sou when u<<c. This surely can't be correct? What am i doing wrong?? Apparently there should be a beat frequency =S
  2. jcsd
  3. May 13, 2012 #2

    Why is this incorrect? In the case that u=0, you would expect f.ref = f.sou. If there's no relative velocity there is no doppler shift.
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