A commuter train passes a passenger platform at a constant speed of 40.0 m/s. The train horn is sounded at its characteristic frequency of 320 Hz. What overall change in frequency is detected by a person on the platform as the train moves from approaching to receding?

Answer 1

2 (V/a)*fo = (80/340)*320 Hz

assuming a = 340 m/s is the speed of sound.

There is a more accurate formula for moving sources and stationary observers, but this should be close enough

Answer 2

A commuter train passes a passenger platform at a constant speed of 40.8 m/s. The train horn is sounded at its characteristic frequency of 320 Hz.