A stationary train emits a whistle at a frequency f. The whistle sounds higher or lowers in

Question:

A stationary train emits a whistle at a frequency f. The whistle sounds higher or lowers in pitch depending on whether the moving train is approaching or receding. Derive a formula for the difference in frequency Δf, between the approaching and receding train whistle in terms of u, the speed of the train, and v, the speed of sound.