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The equation for the doppler shift of a sound wave of speed v reaching a moving detector is f ? = f v + vd v ? vs , where vd is the speed of the detector, vs is the speed of the source, f is the frequency of the source, and f ? is the frequency at the detector. if the detector moves toward the source, vd is positive; if the source moves toward the detector, vs is positive. a train moving toward a detector at 29 m/s blows a 310 hz horn. what frequency is detected by a stationary train? the velocity of sound is 343 m/s.

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nickabrick1
 Assuming the speed of sound v = 343 m/s (the 20 C dry-air value), fd = 295*(343+0)/(343+33) = 269.11 Hz. 
Sloppily stated question. The train moves away in which direction, the same "away from the detector" direction as the emitting train? Which train is the 1st one, the stationary one or the 33 m/s one? Is the 21 m/s absolute or relative to the moving train? You need to find its absolute velocity and substitute it for the 0 in the equation for A.