Question

A train approaches a railroad crossing, blowing its whistle the entire time. When does an observer at the crossing hear the lowest frequency of the train’s whistle?

Answer 1

after the train has passed

Step-by-step explanation:

Answer 2

The observer will hear the lowest frequency of the whistle as the train passes. As the train approaches, the whistle will appear to increase in pitch as it approaches. When it passes, the sound will decrease in pitch as it moves further away. The effect observed here is the Doppler Effect.  It occurs when there is motion between the source of the sound and an observer or receiver.