Question

The sound of an explosion on the surface of the lake is heard by a boatman 100 m away and a driver 100 m below the point of explosion.
(i) Explain who will hear the sound of explosion first.
(ii) If the sound takes 't' s to reach the man, how much time will it take to reach the driver?

Answer 1

For the boatman above the surface of the water.

Speed of sound in air = 344 m/s
Distance covered = 100 m
Time taken by sound = 't' sec

Velocity = $\frac{Distance}{Time}$
344 = $\frac{100}{t}$
t = $\frac{100}{344}$
t = 0.29 sec

Now, for the man below the surface of water.

Speed of sound in water = 1533 m/s
Distance covered = 100 m
Time taken = t

Velocity = $\frac{Distance}{Time}$
1533 = $\frac{100}{t}$
t = $\frac{100}{1533}$
t = 0.06 sec

So, the man under the water hears the sound of the explosion first.