A policeman uses a radar gun to detect the speed of drivers in a 65 mph (29.06 m

Question

A policeman uses a radar gun to detect the speed of drivers in a 65 mph (29.06 m/s) zone. The gun uses sound waves with a frequency of 875 Hz. As one driver passes, the return wave has a frequency of 1035 Hz. What is the drivers speed? I understand this to be a double Doppler effect problem but I'm getting mixed up. Any help is much appreciated! A policeman uses a radar gun to detect the speed of drivers in a 65 mph (29.06 m/s) zone. The gun uses sound waves with a frequency of 875 Hz. As one driver passes, the return wave has a frequency of 1035 Hz. What is the drivers speed? I understand this to be a double Doppler effect problem but I'm getting mixed up. Any help is much appreciated!

Explanation / Answer

f (observed) = { (vel of sound) / [ (velocity of sound) - (velocity of driver) ] } * f(source)

therefore,

(velocity of driver) = [ 1- (f (source) / f (observed)) ]* vel of sound

(velocity of driver) = [1- 875/1035]* 340.29 = 52.6 m/s

(velocity of driver) =

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