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Question
A pebble is dropped freely in a well from its top. It takes 20 s for the pebble to reach the water surface in the well. Taking g = 10 m s-2 and speed of sound = 330 m s-1. Find : The time when echo is heard after the pebble is dropped.
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Solution
Speed of sound = 330 m/s
Depth of well = 2000 m
Time is taken to hear the echo after the pebble reaches the water surface = `"Depth"/"speed"`
= `(2000/330)` s
= 6.06 ≈ 6.1 s
Time is taken for pebble to reach the water surface = 20 s.
Therefore, the total time taken to hear the echo after the pebble is dropped = 20 + 6.1 = 26.1 s.
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