Atrain moving at a constant speed is passing a stationary observer on a platform. on one of the train cars, a flute player is continually playing the note known as concert a (f = 440 hz). after the flute has passed, the observer hears the sound with a frequency of 415 hz. what is the speed of the train? the speed of sound in air is 343 m/s. a) 7.3 m/s b) 12 m/s c) 21 m/s d) 37 m/s e) 42 m/s

C) 21 m/s

Explanation:

The general formula of the Doppler effect is:

where

f' is the apparent frequency

f is the original frequency

v is the speed of the wave in the medium

is the velocity of the receiver, positive if the receiver is moving towards the source

is the velocity of the source, positive if the source is moving away from the receiver

Here we have

f = 440 Hz

f' = 415 Hz

v = 343 m/s

(the observer is stationary)

is positive since we are considering when the train has passed the observer, so it is moving away from him

So we can rewrite the formula as

And solving for , we find the speed of the train

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