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 a speed of 10 m/s. Given that the speed of sound in still air is 340 m/s ______.

1. the frequency of sound as heard by an observer standing on the platform is 400 Hz.
2. the speed of sound for the observer standing on the platform is 350 m/s.
3. the frequency of sound as heard by the observer standing on the platform will increase.
4. the frequency of sound as heard by the observer standing on the platform will decrease.

Answer 1

a and b

Explanation:

Given, v₀ = 400 Hz, v = 340 m/s

Speed of wind, v_w = 10 m/s

a. As both source and observer are stationary. hence frequency observed will be the same as natural frequency v₀ = 400 Hz

b. The speed of sound v = v + v_w = 340 + 10 = 350 m/s

c. and d. There will be no effect on frequency because there is no relative motion between source and observer hence c. and d. are incorrect.