When a particular train whistle is sounded on a train which is at rest you hear

Question

When a particular train whistle is sounded on a train which is at rest you hear a sound wave with a frequency of 330 Hz. Knowing the speed of sound to be 330 m/s, you calculate the wavelength of the sound wave to be 1 m. If the train is quickly traveling away from you and the whistle is sounded, which of the following will be true about the sound wave which you hear.

Select one or more:

a. The wavelength is less than 1 m.

b. The frequency is less than 330 Hz

c. The frequency is more than 330 Hz.

d. The speed of the sound wave is more than 330 m/s.

e. The speed of the sound wave is less than 330 m/s.

f. The wavelength is more than 1 m.

Explanation / Answer

If the train is moving away from the observer, then each of the wave emitted by the train would be from a farther position form the observer than the previous one. This increases the arrival time between sucessive waves adn hance reduces the frequency. The distance between successive wave fronts is then increased, so the waves "spread out". This would increase the wavelength of the wave.

Thus, if the train quickly travels away from the observer and whistles, then the sould wave wouuld have frequency less than 330 Hz and the wavelength of the wave would be more than 1 m.

So, options (b) and (f) are the true.

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