MENU
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 m s–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 -

For the stationary observer: 400 Hz; 0.875 m; 350 m/s

For the running observer: Not exactly identical

For the stationary observer:

Frequency of the sound produced by the whistle, ν = 400 Hz

Speed of sound = 340 m/s

Velocity of the wind, v =10 m/s

As there is no relative motion between the source and theobserver, the frequency of the sound heard by the observer will be the same asthat produced by the source, i.e., 400 Hz.

The wind is blowing toward the observer. Hence, theeffective speed of the sound increases by 10 units, i.e.,

Effective speed of the sound, ve =340 + 10 = 350 m/s

The wavelength (λ) of the sound heardby the observer is given by the relation:

For the running observer:

Velocity of the observer, vo =10 m/s

The observer is moving toward the source. As a result ofthe relative motions of the source and the observer, there is a change infrequency ().

This is given by the relation:

Since the air is still, the effective speed of sound = 340+ 0 = 340 m/s

The source is at rest. Hence, the wavelengthof the sound will not change, i.e., λ remains 0.875 m.

Hence, the given two situations are not exactly identical.

Comment(S)

Show all Coment

Leave a Comment

Free - Previous Years Question Papers
Any questions? Ask us!
NodeJS Interview Questions Answers
ASP.Net Interview Questions Answers
Interview Questions Answers
×