Ok so the question is a freight car moves along a frictionless level railroad track at constant speed. The car is open on top. A large load of coal is suddenly dumped into the car. What happens to the velocity of the car? And I asked my teacher if the coal dumped in was stationary relative to the ground and he said it was traveling at the same velocity as the train when dumped. So I was thinking that the train speed would remain the same because the coal wouldn't need to be accelerated to the trains velocity because it is traveling at the same velocity as the train. Meaning it would not require any extra force to get it to the same velocity as the train. The force of normal would increase but since the train track is frictionless the force of normal would be irrelevant. However the correct answer was the speed of the train would decrease. Thoughts? I do understand that if friction of air was counted the coal would slow down before it fell into the train so it would need to be accelerated and would slow down the train. But, I doubt that that is the reason.