Momentum: I have the answer, now an explanation?

[TG3]

So, I know the "correct" answer to this problem, but it makes no sense to me at all. Why is the following the case?

Homework Statement

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?

Homework Equations

Momentum = Mass x Velocity

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...?

 

[Delphi51]

No friction or other force in the horizontal direction has been mentioned, so the car will continue in motion at its constant speed of 4 m/s. F = ma, F is zero so "a" is zero.

 

[LowlyPion]

Why do you think it is the right answer?

Are you saying that as soon as each lump of coal was dropped from the car it left its momentum in the car?

 

[TG3]

!
I obviously wasn't considering that the coal continues moving as it is dropped, and doesn't leave it's momentum behind, which is the premise I was operating under. Many thanks!