Question

An echo-sounder in a fishing boat
receives an echo from a shoal of fish
0.45 s after it was sent. If the speed of
sound in water is 1500 m/s, how deep is
the shoal?
[Ans: 337.5 m
​

Answer 1

Given:-

• Time after which the echo was

received = 0.45s

• Speed of sound in water = 1500m/s

To find:-

• Depth of the shoal

Solution:-

Let the required depth be 'd' metre. Then, the depth will be equal to the distance travelled by the wave in given time interval.

Now, we know that :-

If 'd' is the distance of the shoal of fish from the fishing boat, then the total distance travelled by the wave in time interval 't' is '2d'.

∵ Speed = Distance/Time

=> v = 2d/t

=> vt = 2d

∴ d = vt/2

Where :-

• d is the distance travelled by the wave

[depth of the shoal].

• v is speed of sound in water.

• t is given time interval.

Substituting values, we get :-

=> d = 1500×0.45/2

=> d = 750×0.45

=> d = 337.5 m

Thus, depth of the shoal is 337.5m .