A railroad train is moving in still air at speed 12.0 m/s toward a stationary de

Question

A railroad train is moving in still air at speed 12.0 m/s toward a stationary detector D. When the train is at rest, the frequency of the note it emits is 260 Hz, heard by an observer also at rest. To the right of the detector D is a stationary siren that also emits a note of frequency 260 Hz. See the diagram below for a schematic of the situation. Assume the speed of sound is 343 m/s. (25) What is the beat frequency of the sound that the detector D receives from both the siren and the moving train? (7) What is the wavelength of the sound coming front the moving train?

Explanation / Answer

(a)

For train moving towards stationary source, using dopper effect,

Frequancy,

f' = (v+v0 /v) *f

= 343+20 / 343 * 260

= 275 Hz

Frequency from sire will remain same as 260 Hz.

(b)

Wavelength (for train)

lambda = v/f'

=343/ 275 =1.247 m

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