# Homework Help: Conservation of linear momentum

1. Oct 30, 2012

### daivinhtran

1. The problem statement, all variables and given/known data
A runaway 14,000 kg railroad car is rolling horizontally at 4.00 m/s toward a switchyard. As it passes by a grain elevator, 2000 of grain suddenly drops into the car. Suppose the there is a small vertical chute in the bottom of the car so that the grain leaks out at 10 kg/s. How long does it take the car to cover 500m

2. Relevant equations
Pi = Pf

3. The attempt at a solution
Picture attached

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2. Oct 30, 2012

### Andrew Mason

What is the initial horizontal momentum of the car and how does that relate to the horizontal momentum of the car + grain after the grain drops? Can you determine the speed of the car + grain after the drop? What effect does the vertical leak of grain have on the horizontal speed of the car?

AM

3. Nov 2, 2012

### daivinhtran

I already included those in my paper (attached).
However I can't determine the speed of the car + grain after drop unless I know the time.

4. Nov 3, 2012

### Andrew Mason

Sure you can. How does the horizontal momentum before the collision compare to the horizontal momentum after the collision? (hint: are there any forces acting in the horizontal direction that are external to the car - grain system?).

AM

5. Nov 3, 2012

### haruspex

Andrew is asking what the speed is immediately after the drop, i.e. t = 0.
Separately, he asks what affect the grain leak has on the car thereafter.