Question

The siren of a fire engine has a frequency of 500 Hz. (a) The fire engine approaches a stationary car at 20 m/s. What frequency does a person in the car hear? (b) The fire engine stops and the car drives away from it at 20 m/s. What frequency does the person in the car hear?

Answer 1

Given
frequency of fire engine is F = 500 Hz, approaching the satationary car

the frequency of fire engine,a person can hear is

doppler effect
       f' = f(v-v0/v-vs)

here v0 = 0 m/s, vs = 20 m/s, v= 343 m/s speed of sound in air

       f' = 500( 343 -0/343-20) = 530.96 Hz

when the fire engine with frequency 500 Hz at rest vs = 0 m/s,and the car moving away from the fire engine with speed v0 = 20 m/s

   the frequency the person in car can hear is

   f' = f(v-V0/V-vs)

   f' = 500(343-20/343-0) Hz

   f' = 470.8454 Hz