So, I know the "correct" answer to this problem, but it makes no sense to me at all. Why is the following the case? 1. The problem statement, all variables and given/known data A railroad hopper car has mass 40,000 kg when empty and contains 50,000 kg of coal. As it coasts along the track at 4 m/s the hopper opens and steadily releases all the coal onto a platform below the rails over a period of 4 s. How fast does the car travel after all the coal is dumped? 2. Relevant equations Momentum = Mass x Velocity 3. The attempt at a solution 40,000+ 50,000 = 90,000 90,000 x 4 = 360,000 360,000 / 40,000 = 9 m/s. This seems simple and obvious, yet the computer insists that the correct answer is 4 m/s. Why would this ever be....?