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?,

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

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