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## Homework Statement

A man (weighing 915 N) stands on a long railroad flatcar (weighing 2805 N) as it rolls at 18.0 m/s in the positive direction of an x axis, with negligible friction. Then the man runs along the flatcar in the negative x direction at 40.00 m/s relative to the flatcar. What is the resulting increase in the speed of the flatcar?

## Homework Equations

Pi=Pf

m1v1=m2v2

## The Attempt at a Solution

(915+2805)(18)=(915)(-40+18)+(2805)(v+18)

v=13.05 m/s

Well this is the wrong answer and I have no clue why. I know that the fact that the weights are given and not mass is not an issue since W=ma and a would cancel out on both sides so I cannot figure out why this answer is wrong. Anything else that I can try???