While sitting at a railroad crossing one day you measure the

While sitting at a railroad crossing one day, you measure the frequency of a train whistle to be 79.2 Hz as the train is approaching you. By making observations of the distance covered by the train in 10 seconds, you are able to approximate that the train is moving at 18.9 m/s.

A) What frequency does the train whistle have? (what you would hear if the train wasn’t moving)

B) A passenger on another train traveling in the opposite direction (on a parallel track), hears a frequency of 82.7 Hz. How fast is the second train moving?

Solution

observed frequency= Fo= 79.2Hz

velocity of train= 18.9 m/s

velocity of sound in air at 20 degrees = 344 m/s

using formula for doppler effect

Fo=[ Vs/Vs- Vt] F

F= [(Vs- Vt)/Vs] Fo

F=[( 344-18.9)/344]79.2 =74.8 Hz

(b)let V2 is the velocity of second train

Fo=[ (Vs+V2)/(Vs- Vt)] F

82.7 = [(344+V2)/(344-18.9)] *74.8

V2= 16.9 m/s