A railroad flatcar of weight 7840 N can roll without friction along a straight h

Question

A railroad flatcar of weight 7840 N can roll without friction along a straight horizontal track. Initially, a man of weight 612 N is standing on the car, which is moving to the right with speed 25 m/s; see the figure. What is the change in velocity of the car if the man runs to the left so that his speed relative to the car is 4.5 m/s?

An MSU linebacker of mass 114.0 kg sacks a UM quarterback of mass 80.0 kg. Just after they collide, they are momentarily stuck together, and both are moving at a speed of 2.90 m/s. If the quarterback was at rest just before he was sacked, how fast was the linebacker moving just before the collision?

Explanation / Answer

1)
The car will go faster... until our man comes to the car's rear end and stop.However, where you find frictionless rails, I find infinitely long railroad cars. The only consideration is the change the man's momentum - and add that to the flatcar's momentum:
612N x 4.5 m/s is the added momentum. = 2754N-m/s
The system was (7840N + 612N) = 8452N
8452N x 25m/s = 211,300 N-m/s
The system is now 211,300 + 2754 = 214,054 N-m/s
214,054N-m/s / 8452N = 25.3258 m/s
Whew! The change in velocity will be 0.3258 m/s to the right.

2)
(114+80)x2.9=562.6
562.6/114=4.93508m/s

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