# Homework Help: What were the magnitude and the direction of the velocity of the cars after the colli

1. Sep 24, 2009

### cutemale77

A railroad car that weighs 20,000 lbs. is traveling eastward with a velocity whose magnitude is 5 ft./sec. A second car, on the same track, that weighs 40,000 lbs. is also traveling in an easterly direction with a velocity of 7.81 ft./sec. When the cars collided, they became coupled together and then both cars moved with the same velocity. If friction between the cars and the rails is neglected, what were the magnitude and the direction of the velocity of the cars after the collision?

2. Sep 24, 2009

### UnknownFury

Re: what were the magnitude and the direction of the velocity of the cars after the c

You can use conservation of momentum. Momentum in a closed system is always a constant. To calculate momentum use p = mv (p = momentum, m = mass, v = velocity). As they are coupled together after the collision, you are just taking the combined mass and using the momentum from before the collision (work out the momentum of each car and add them) to work out v.

3. Sep 24, 2009

### cutemale77

Re: what were the magnitude and the direction of the velocity of the cars after the c

wow thanks for your big help

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