Question

a 5 kg fish swimming at a speed of 1m/s swallows an absent minded 1 kg fish swimming toward it at 4 m/s. the speed of the larger fish after lunch is?

Answer 1

Instantly after the "collision", you can apply the law of conservation of momentum, but shortly afterwards the speed will change if large fish continues to swim.

(5 kg * 1 m/s) + (1 kg * -4 m/s) = 6 kg * Vfinal

Vfinal = 1/6 m/s