Question

A train, standing In a station-yard, blows a whistle of frequency 400 Hz In still air. The wind starts blowing in the direction from the yard to the station with at a speed of 10 ms^-1. what are the frequency, wavelength, and speed of sound for an observer standing on the station’s platform? Is the situation exactly Identical to the case when the air Is still and the observer runs towards the yard at a speed of 10 m s^-1? The speed of sound in still air can be taken as 340 m s^-1 .

Answer 1

Given f = 400 Hz v = 340 ms^-1 v_m = 1.0 ms^-1 : speed of air Since there is no relative motion between the source and the observer, the frequency of sound for the observer = f =400 Hz. The wind is blowing in the direction from the yard to the station, the effective speed of sound for an observer at the platform = v + v_m = 350 ms^-1

Wavelength of sound : λ= (v+v_m)/f

= \(\frac{350}{400}\) = 0.875 m

The situation is not identical to the case when the air is still and the observer runs towards the yard at a speed of 10 ms^-1 (= v₀). Because here there is a relative motion between the source and the observer.