In: Physics
At a factory, a noon whistle is sounding with a frequency of 480 Hz. As a car traveling at 61.0 km/h approaches the factory, the driver hears the whistle at frequency fi. After driving past the factory, the driver hears frequency ff. What is the change in frequency ff - fi heard by the driver? Assume the speed of sound in air is 343 m/s.
Given
frequency of whistle(source) f0 = 480 Hz
speed of car (listner)
V0 = 61 kmph = 61*5/18 = 16.94 m/s
speed of sound V = 343 m/s
as the car reaches the whistle driver observes the higher frequency than moving away from the factory.
Doppler effect apparent frequency
approaching fi = f0(V+Vo / V)
moving away from whistle ff = f0(V-v0 / V)
change in
frequency = (f0/V)*(2v0)
= (480/343)*(2*16.94)
= 33.88 Hz
change in frequency observed by the car driver is 33.88 Hz