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Homework Help: Doppler effect measure speed in blood cells

  1. Jan 19, 2009 #1
    1. The problem statement, all variables and given/known data

    A Doppler blood flow unit emits ultrasound at 5.0 {\rm MHz}.

    What is the frequency shift of the ultrasound reflected from blood moving in an artery at a speed of 0.20 m/s?

    What equations would I use to calculate the doppler shift? It is not in my textbook...
  2. jcsd
  3. Jan 19, 2009 #2
    I believe off hand that the ratio of wavelenghts is equal to the ratio of speeds of the object;

    L = wavelenght
    L0 = wavelenght before reflection
    V = velocity of wave in medium
    V0 = velocity of wave.

    V0=fL0 where f is the frequency of the ultrasound emitter.

    L / L0 = V / V0

    I think that's all the infomation required for the solution. See if you can do it now. If not highlight the below;

    L / L0 = 0.2 m/s / speed of sound ( 340 m/s) = Y

    Where I'm just using Y to represent the ratio. The wave travels the same speed V in the medium of flesh (roughtly) hence;

    Y = ( V/f ) / (V/f0)

    Rearrage for f;

    1 / [(Y*( V / f0 )) / V ] = f

    However that might not be the question being asked or not, as you would require extra infomation to solve that numerically.

  4. Jan 19, 2009 #3
    How do I solve for the wavelength if the wavelength is not given?
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