# Train at constant velocity

1. Jan 17, 2015

### Brob

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.

2. Jan 17, 2015

### Staff: Mentor

I agree with your answer. Air drag on the coal before it reaches the train should be negligible (as all other effects of air), otherwise the problem statement would be misleading.
That is an unusual setup. Maybe he didn't mean that. Usually the coal is stationary relative to the ground.

3. Jan 17, 2015

### Brob

I agree but would my reasoning have been correct if the coal was stationary reltive to the train?

4. Jan 17, 2015

### Staff: Mentor

5. Jan 17, 2015

### Brob

Thanks for the help.

6. Jan 18, 2015

### Staff: Mentor

Very likely he believed he was simply confirming that the coal would have the same velocity as the train once dumped.