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?

Options

A. 480 Hz
B. 680 Hz
C. 840 Hz
D. 1000 Hz

Answer

B

Explanation

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`