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

Question

Quizplus · Accepted Answer

Momentum is conserved, so the total momentum before the lunch is equal to the total momentum after the lunch. The momentum of the larger fish before the lunch is 5 kg * 1 m/s = 5 kg m/s. The momentum of the smaller fish before the lunch is 1 kg * 4 m/s = 4 kg m/s. The total momentum before the lunch is 5 kg m/s + 4 kg m/s = 9 kg m/s. The total mass after the lunch is 5 kg + 1 kg = 6 kg. So the speed of the larger fish after the lunch is 9 kg m/s / 6 kg = 1.5 m/s.

A 5-Kg Fish Swimming at a Speed of 1 M/s