A pebble is dropped freely in a well from it’s 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: (i) the depth of water surface, and (ii) the time when echo is heard after the pebble is dropped.

(i) g = 10 m s^-2

time (t) = 20 s

Initial velocity (u) = 0

h = ut + 1/2 gt²

= 0 × 20 + 1/2 × 10 × 20²

= 5 × 400 = 2000 m

Hence, the height of the well is 2000 m

(ii) time (t) = distance/speed

t = 2000/330 = 6.1 s

As the pebble reaches the ground after 20 s, hence echo will be heard after (20 + 6.1) s = 26.1 s