Find the final velocity of the railroad car.

ScullyX51

1. The problem statement, all variables and given/known data
A 1.00×104 railroad car is rolling at 4.00 when a 2000 load of gravel is suddenly dropped in. What is the car's speed just after the gravel is loaded?


2. Relevant equations
f=ma
pi=pf


3. The attempt at a solution
I am very confused with this problem, but this is what I have tried so far:
I drew a "before" picture of mass1: 2000kg, and mass2: 1.00*10^4kg. both in the y direction.
Then I drew an "after" picture of "m1+m2" moving in the x direction (m1+m2)Vf. and vf is given to be 4 m/s.
I don't know what to do from here, and I am also very confused because it seems as though it is not conservation of momentum in the collision in the y direction because of the force of gravity, and the normal force from the train. I would appreciate if someone could give me some hints on how to approach this problem.
Thank you. :)

Doc Al

What do you mean "in the y direction"?
This is good.
No, 4 m/s is the initial velocity of the empty railroad car.

Hint: Just worry about the x-direction.

asleight

Conservation of horizontal momentum is what you're after.