Two train whistles, A and B, each have a frequency of 392 Hz. A is stationary and B is moving toward the right?

—(away from A) at a speed of 35.0m/s. A listener is between the two whistles and is moving toward the right with a speed of 15.0m/s. No wind is blowing. a) What is the frequency from A as heard by the listener? b) What is the frequency from B as heard by the listener? c) What is the beat frequency detected by the listener?


(there are no question mark”?” in the right (away from A) is the continuations ty…)

3 Answers

  • this is doppler effect.

    observed frequency=f=((v+vl)/(v+vs))*fo

    v=velocity of wave in medium=343m/s

    vl=velocity of listener(+ if listener moving towards the source and – if moving away from source)

    vs=velocity of source(+if source moving away from observer and – if source moving towards observer)


    vl= -15



    fa can be found.





    fb can be found.

    c)beat frequency=fa-fb

  • with respect to A the listener is opening the distance at 15 m/s

    F = Fo ( V / ( V + Vl )

    where V is speed of sound and Vl is relative speed of listener

    F = 392 ( 343 / (343+15) = 375.6

    with respect to B the source is moving away from the listener at 20 m/s

    F = Fo ( ( V – Vs ) / V

    where Vs is the speed of the source

    F = 392 ( (343-20) / 343 ) = 369.1

    the difference gives 6.5 beats/sec

  • B

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