Question

A train moving at 25 m/s emits a whistle of frequency 200 Hz. If the speed of sound in air is 340 m/s, find the frequency observed by a stationary observer.

1. if the observer is in front of the source.
2. if the observer is behind the train.

विकल्प

• 186 Hz, 216 Hz

• 216 Hz, 186 Hz

• 172 Hz, 220 Hz

• 220 Hz, 172 Hz

Answer 1

216 Hz, 186 Hz

Explanation:

(i) The Doppler effect is followed for the sound of the approaching source and the receding source.

Here, f = observed frequency

f₀ = source frequency

v = velocity of sound

v_s = velocity of the source

f = `"v"/("v"-"v"_"s")xx"f"_0`
= `340/(340-25)xx200` Hz 

= 216 Hz

(ii) f = `"v"/("v"+"v"_"s")xx"f"_0`

= `340/(340+25)xx200` Hz 

= 186 Hz