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Tanuja Bakshi
Asked: 2022-11-08T17:56:01+05:30 In:

A man is standing not exactly at the exactly at the centre, in between two parallel walls separated by a distance 990 m. He fires a gun and hears the first echo after 2 s, the time taken to hear 3rd echo after firing is ______ s. (take velocity of sound in air is `330 ms^(-1)`)
A. 5
B. 7
C. 6
D. 8

A man is standing not exactly at the exactly at the centre, in between two parallel walls separated by a distance 990 m. He fires a gun and hears the first echo after 2 s, the time taken to hear 3rd echo after firing is ______ s. (take velocity of sound in air is `330 ms^(-1)`)
A. 5
B. 7
C. 6
D. 8
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  1. Correct Answer – B
    He hears the first echo after 2 s. The distance between the 1st wall and the man is
    `d = v xx t = 330 xx 1 = 330 m.`
    So, the distance between the man and 2nd wall is
    `990 – 330 = 660 m.`
    So, time taken for 2nd echo time `= (“distance”)/(“velocity”) = (2 xx 660)/(330) = 4 s`
    The 3rd and 4th echo reach at the same time.
    Time taken for 3rd echo = time taken to hear the first echo + time taken to hear the 3rd echo
    `= 2 + (2 xx 660)/(330) = 2 + 2 xx 2 = 6 s.`

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