# Physics

Ultrasound reflected from an oncoming bloodstream that is moving at 30 cm/s is mixed with the original frequency of 1.3 MHz to produce beats. What is the beat frequency? (Velocity of sound in blood = 1540 m/s.)

Beat frequency = Hz

HELP: This problem involves two Doppler shifts.

HELP: In the first shift, the source is fixed and the observer is the moving blood. Why?

HELP: In the second shift, the source is the moving blood and the observer is fixed. Why?

HELP: It might be helpful to think of a tiny ultrasound recorder mounted on a blood cell. The recording happens when the ultrasound arrives at the cell, and the echo leaves when the recorded sound is "played back".

HELP: Once you know the frequency of the Doppler-shifted echo, how do you find the beat frequency between the echo and the original frequency?

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