A big red fish with a mass of 20 kg swims with a velocity of 2 m/s toward a small, stationary green fish which has a mass of 10 kg. The big red fish opens its mouth and swallows the small green fish. What will the big red fish's velocity be immediately after swallowing its meal?

1.33m/s

Explanation:

Given parameters:

M1  = 20kg

V1  = 2m/s

V2 = 0m/s

M2  = 10kg

Unknown:

V  = ?

Solution:

As with all systems, the momentum must be conserved and it implies that the starting and ending momentum be the same.

  M1 V1  +  M2 V2  = V (M1 + M2)

M1 is the mass of the big red fish

V1 is the velocity of the big red fish

V2 is the velocity of the green fish

M2 is the mass of the green fish

V is the velocity after swallowing

           (20 x 2) + (10 x 0)  = V (20 + 10)

                      40  = 30V

                      V  = 1.33m/s