In this equation, vd is the speed of the detector, and vs is the speed of the source. Also, fs is the frequency of the source, and fd is the frequency of 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 away from a detector at 27 m/s blows a 290-Hz horn.
(a) What frequency is detected by a stationary train?
(b) What frequency is detected by a train moving away from the first train at a speed of 21 m/s?