Physics
posted by Emily .
A person standing close to a railroad crossing hears the whistle of an approaching train. He notes that the pitch of the whistle drops as the train passes by and moves away from the crossing. The frequency of the distant approaching whistle is 540 Hz; it drops to 470 Hz after the train is well past the crossing. What is the speed of the train? Use 340 m/s for the speed of sound in air.

F = ((VVr) / (V+Vs))*Fs = 470.
((3400)/(340+Vs))*540 = 470
((340)/(340+Vs))*540 = 470
183600/(340+Vs) = 470
470(340+Vs) = 183600
Divide both sides by 470:
340+Vs = 390.64
Vs = 390.64340 = 50.64 m/s. = Velocity
of the source or train. 
NOTE:
V = Velocity of sound in air.
Vr = Velocity of the receiver or person.
Fs=Frequency of the source or whistle.
F = Freq. as heard by the person.
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