Acommuter train passes a passenger platform at a constant speed of 40.4 m/s. the train horn is sounded at its characteristic frequency of 350 hz. (a) what overall change in frequency is detected by a person on the platform as the train moves from approaching to receding? (indicate whether the frequency increases of decreases with the sign of your answer.) the response you submitted has the wrong sign. hz (b) what wavelength is detected by a person on the platform as the train approaches? your response differs significantly from the correct answer. rework your solution from the beginning and check each step carefully. m

(a) -83.6 Hz

Due to the Doppler effect, the frequency of the sound of the train horn appears shifted to the observer at rest, according to the formula:

where

f' is the apparent frequency

v = 343 m/s is the speed of sound

is the velocity of the source of the sound (positive if the source is moving away from the observer, negative if it is moving towards the observer)

f is the original frequency of the sound

Here we have

f = 350 Hz

When the train is approaching, we have

So the frequency heard by the observer on the platform is

When the train has passed the platform, we have

So the frequency heard by the observer on the platform is

Therefore the overall shift in frequency is

And the negative sign means the frequency has decreased.

(b) 0.865 m

The wavelength and the frequency of a wave are related by the equation

where

v is the speed of the wave

is the wavelength

f is the frequency

When the train is approaching the platform, we have

v = 343 m/s (speed of sound)

f = f' = 396.7 Hz (apparent frequency)

Therefore the wavelength detected by a person on the platform is

answer: he got 23

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