Question

A 30 000 kg train is travelling at 15 m/s towards another 45 000 kg train traveling at 5 m/s in the same direction. After the two collide, they couple together. What is the final velocity of both trains?

Answer 1

Momentum is conserved, so

(30,000 kg) (15 m/s) + (45,000 kg) (5 m/s) = (30,000 kg + 45,000 kg) v

where v is the speed of the coupled trains.

Solve for v :

450,000 kg•m/s + 225,000 kg•m/s = (75,000 kg) v

675,000 kg•m/s = (75,000 kg) v

75v = 675 m/s

v = 9 m/s