Question

A blood-flow meter emits a 1.5-MHz ultrasound pulse to measure the speed of blood moving directly away from the meter. The meter sensor detects the pulse reflected back from the blood at a frequency 28 Hz lower than the emitted frequency. Take the speed of sound in the tissues to be 1475 m/s

Answer 1

`v_(s)` = 0.0275 m / s

Step-by-step explanation:

The frequency change by the relative movement between the sound source and the observer is explained by the Doppler effect.

f ’= f v / (v- + vs)

Where f ’is the frequency received, v is the speed of sound and vs is the speed of the source, - + is for the source approaching and moving away, respectively.

Let us analyze our case carefully, the device emits the sound, reaches the blood and bounces, at this time it is the blood in motion that re-emits the sound, which corresponds to the sound source. They tell us that the blood is approaching so the correct sign is the negative

Let's calculate the speed of the source (blood)

f ’(v-
`v_(s)` ) = f v

f’
`v_(s)` = f’ v -vf

`v_(s)` = v (f ’-f) / f’

`v_(s)` = 1475 [(1.5 106-28) - 1. 5 106] / (1.5 106-28)

`v_(s)` = 1475 (28) / (1.5 106-28)

`v_(s)` = 0.0275 m / s