
#1
Mar1910, 10:53 AM

P: 1

1. The problem statement, all variables and given/known data
A Doppler blood flow unit emits ultrasound at 5.0 MHz. What is the frequency shift of the ultrasound reflected from blood moving in an artery at a speed of 0.20 m/s? Express your answer using two significant figures. 2. Relevant equations f' = f0(v/vVs) frequency shift = f'  f0 f0 = 5*10^6 Hz v = 1500 m/s (assumed) Vs = 0.20 m/s 3. The attempt at a solution Plugging in the numbers we get the following equation for frequency shift: (5*10^6)*[1500/(15000.2)]5*10^6 Which comes out to approx 666 Hz. or 6.7 *10^2 Hz using 2 significant figures. However, this problem comes from an online hw set and according to that 6.7 *10^2 is not the correct solution. Does anyone have an idea of what I am doing wrong? Thanks in advance for any and all help! 



#2
Mar1910, 12:02 PM

HW Helper
P: 4,442

Hi grigri9, welcome to PF.
We have here a case of the "listener" (the blood corpuscles) moving away from a stationary source, and thus the frequency, f corpuscle that will be reflected by the corpuscles will be f corpuscle = f(transmitted)[(Vs  Vb)/Vs For the reflected waves, reflection, the corpuscles are the source and the "listener" is the receiver. The frequency at the receiver will be f(receiver) = f(corpuscles)*Vs/(Vs + Vb) from which we get f(receiver) = f(transmitted)*(VsVb)/(Vs+Vb) 


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