To solve this problem we will apply the concepts related to the conservation of momentum. Momentum is defined as the product between mass and velocity of each body. And its conservation as the equality between the initial and final momentum. Mathematically described as
Here
= Mass of big fish
= Mass of small fish
= Velocity of big fish
= Velocity of small fish
= Final Velocity
The big fish eats small fish and the final velocity is zero. Rearrange the equation for the initial velocity of small fish we have
Replacing we have,
The negative sign indicates that the small fish is swimming in the direction opposite to that of the big fish.
Therefore the speed of the small fish is 10m/s