Question

Question 1. A deep well has water at a depth of 80m below the top of the well. A stone is dropped from the top. If the speed of sound is 320m/ s and g =10m/s sq then the splash of the sound hitting the water is heard after how many second later?

Answer 1

Speed = distance / time
As this is a case similar to that of echoes, we need to take twice the distance(i.e., forward distance + backward distance = 2(distance) = 2(height)).    
So,
Speed = 2(height) / time
Time = 2 (80) / 320 
        = 1/2
        = 0.5 seconds

Hence, the splash of the sound hitting the water is heard after 0.5 seconds.

Answer 2

Time taken by the stone to reach water, $t_{1}=\sqrt{\frac{2h}{g}}=4s$.
Time taken by sound wave to reach top, $t_{2}=\frac{h}{s}=0.25s$.
Total time, $T=t_{1}+t_{2}=4.25s$.