Question

Blood flow rates in the umbilical cord can be found by measuring the Doppler shift of the ultrasound signal reflected by the red blood cells. If the source emits a frequency f, what is the measured reflected frequency fR? Assume that all of the red blood cells move directly toward the source. Let c be the speed of sound in blood and v be the speed of the red blood cells.


A) f (c+v)/c
B) f (c+v)/c-v
C) f (c+v)/v-c
D) f (c-v)/v+c

Answer 1

fb = frequency heard then emmitted by blood particle
fb = (c+v) f /c

fl = frequency heard back at the source
f receiver = fr = fb c/(c-v)

so the result is:

fr = f * (c+v)/(c-v)
which is B (sort of, parenthesis missing)