A stationary motion detector emits sound waves with a frequency of0.150 MHz towa

Question

A stationary motion detector emits sound waves with a frequency of0.150 MHz toward a truck approaching at a speed of 45.0 m/s. a) What is the frequency of the sound waves received bythe truck?

b)The sound waves received by the truck bounce back toward thedetector, acting as sound waves    emitted by amoving source. They have the same frequency as the sound wavesreceived by the truck (in part ‘a’ above). What is thefrequency of the sound waves picked up by the motiondetector? a) What is the frequency of the sound waves received bythe truck?

b)The sound waves received by the truck bounce back toward thedetector, acting as sound waves    emitted by amoving source. They have the same frequency as the sound wavesreceived by the truck (in part ‘a’ above). What is thefrequency of the sound waves picked up by the motiondetector?

Explanation / Answer

f = f0 * (v + vo) / (v -vs)   the equation for the Doppler effect forsound waves v = speed of sound vo = speed of observer moving towards stationarysource  (v0 negative if observermoving away from source) vs = speed of source moving towards stationaryobserver (vs negative if source moving away fromobserver) a) vo = speed of truck - theobserver       vs =0     f = .150 * (331 + 45) / 331 = .170Mhz    frequency received by truck (using 331as v at standard conditions) b) the truck now acts as a moving source     f = .170 * 331 / (331 - 45) = .197Mhz This is the same as a moving source and a movingobserver    f = .15 * (331 + 45) / (331 - 45) = .197Mhz  

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