Question

A stone is dropped from a 50m tall building into a pond. When is sound of splash heard at top?
g=10m/s and speed of sound is 340m/s.

Answer 1

s = u t + ½ g t².

Since the stone is just dropped, the initial velocity u = 0 m/s, g = 10 m/s², and s = 500m. Substituting we get

500 = ½ × 10× t²; or t² = 100 or t = ± 10 sec. The t=-10 sec is a meaningless answer so time taken for stone to hit the top of the water pond = 10 s.

Hitting stone will produce a splashing sound which will travel all around @ 340 m/s. Time taken by sound to travel 500 m upwards @ 340 m/s =[ 500 m]/[340 m/s]= 25/17 s = 1.47s

Total time after which the sound will be heard after dropping of the stone = 10 s + 1.47 s = 11.47 s.