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 after 0.2 s. What is the radius of the hall? (Speed of sound in air = 340 m/s)
Since the man receives the echo after 0.2 seconds. And echo is returned after reflection from the walls of hall.
∴ Time taken by sound wave to travel to wall of hall is 0.1 s
⇒ Time (T) = 0.1 s
Speed (v) of sound in air = 340 m/s)
We know that 
⇒ 340 = 
⇒ D = 
Hence the distance travelled by sound wave is 34 m.
∵ the hall is hemispherical therefore the distance of the man from the walls of the hall is same and is equal to the radius of the hall.
Hence radius of hall is 34 m.
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