A man is lying on the floor of a large, empty hemispherical hall, in such a way that his head is at the centre of the hall. He shouts ''Hello'' and hears the echo of his voice after0.2s. What is the radius of the hall ?

A man is lying on the floor of a large, empty hemispherical hall, in s

1.	A man is lying on the floor of a large, empty hemispherical hall, in such a way that his head is at the centre of the hall. He shouts ''Hello'' and hears the echo of his voice after0.2s. What is the radius of the hall ?
Answer» Solution :Let the distance travalled by the sound WAVE = 2d m Time to HEAR the echo `(t) = 0.2s` , Speed of sound (v) = 340 m/s We KNOW that `v = (2d)/(t)` `implies 340 = (2 xx d)/(0.2) implies 2d = 340 xx 0.2` `implies d = (340 xx0.2)/(2)` = 34 m. As the head of the man is at the centre of hemispherical room, then 'd' is the RADIUS of the hall. Radius of the hall = 34 m. .