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The siren of a fire engine that is driving northward at 30.0 m/s emits a sound of frequency 2000 Hz.A truck in front of this fire engine is moving northward at 20.0 m/s.What is the frequency of the siren's sound that the fire engine's driver hears reflected from the back of the truck?,

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Kalahira
So this question based on doppler effect Here apparent freq = freq of sound *(speed of sound-speed of observer)/(speed of sound-speed of source) given freq of sound=2000 speed of observer= 20 speed of source= 30 taking speed of sound as 343 m/s apparent freq = 2000 [343 - 20] / [343 - 30] = 2063.90 Hz
aencabo
 n ' = n [v-vo] / [v-vs]
If, v=343 m/s (+x), vs
= 30 m/s (+x), v0
= 20 m/s (+x) 
then,
n ' = 2000 [343 - 20] / [343 - 30]
= 2063.90 Hz

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