Ncert Exercises & Solutions

A train standing in a yard blows a whistle of frequency 400Hz 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. Read the following statements.

(a) the frequency of sound as heard by an observer standing on the platform is 400Hz.

(b) the speed of sound for the observer standing on the platform is 350 m/s.

(d) the frequency of sound as heard by the observer standing on the platform will decrease.

Choose the correct alternatives:

1. (a, b)

2. (a, c)

3. (b, c)

4. (c, d)

(3) Hint: Use the concept of Doppler's effect.

Step 1: Find the apparent frequency of the sound.

Given,

ν_{0} = 400 Hz, v = 340 m / s

speed of wind, v_{w} = 10 m / s

As both source and observer are stationary, hence the frequency observed will be the same as the natural frequency,

ν_{0} = 400 Hz

Step 2: Find the speed of the sound.

The speed of sound,

v' = v + v_{w} = 340 + 10 = 350 m / s

There will be no effect on frequency because there is no relative motion between source and observer hence 3, 4 are incorrect.

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