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Aishwarya Lall
Asked: 2022-11-05T22:01:28+05:30 In:

A boy is walking away from a well at a speed of `1.0 m//s` in a direction at right angles to the wall. As he walks, he below a whistle steadily. An observer towards whom the boy is walking hears `4.0` beats per second. If the speed of sound is `340 m//s` , what is the frequency of the whistle?
A. 480 Hz
B. 680 Hz
C. 840 Hz
D. 1000 Hz

A boy is walking away from a well at a speed of `1.0 m//s` in a direction at right angles to the wall. As he walks, he below a whistle steadily. An observer towards whom the boy is walking hears `4.0` beats per second. If the speed of sound is `340 m//s` , what is the frequency of the whistle?
A. 480 Hz
B. 680 Hz
C. 840 Hz
D. 1000 Hz
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1 Answer

  1. Correct Answer – B
    The frequency of direct sound of whistle heard by observer is
    `n_1=(v)/(v-v_S)n=(340)/(340-1)xxn=(340)/(339)n` ..(i)
    Frequency of sound of whistle reflected by wall is
    `n_2=(v)/(v+v_S)n=(340)/(341)n` .(ii)
    Given, `n_1-n_2=4`
    Therefore, `(340)/(339)n-(340)/(341)n=4`
    `impliesn=680Hz`

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