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. Then,

Given, ${\mathrm{v}}_0=400\mathrm{Hz},v=340 \mathrm{m}/\mathrm{s}$

Speed of wind  $v_w=10 \mathrm{m}/\mathrm{s}$

(i) As both source and observer are stationary, hence frequency observed will be same as natural frequency $v_0=400 \mathrm{Hz}$

(ii) The speed of sound  $v=v+v_w=340+10=350\mathrm{m}/\mathrm{s}$

(iii) and (iv) There will be no effect on frequency,because there is no relative motion between source and observer hence (iii),(iv) are incorrect.