A 5,000 kg train is traveling at a velocity of 100 m/s and hits another train. The two trains stick together, and the new velocity is 50 m/s. 5,000 kg is the mass of the second train.
Solution:
(5000 kg) * (100 m/s) = 500,000 kg·m/s;
((5000+M) kg) * (50 m/s) = 500,000 kg·m/s;
Divide by 50 m/s and subtract 5,000 kg:
(5000 +M) kg = 10,000 kg;
M kg = 5000 kg

5,488 J with no time has an infinite power in Watts.