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Homework Help: Easy collision problem I keep getting wrong!

  1. Dec 29, 2009 #1
    1. "Consider a minor parking-lot accident. Car A backs out at 30 cm/s toward the west, and Car B looks for a place to park, driving north at 40 cm/s. Both cars mass 1,000 kg. What is the total system momentum before the collision? Remember that momentum is a vector quantity. Also, be careful with your units."

    2. Momentum= Mass * Velocity

    3. First, I convert 30 cm/s and 40 cm/s to m/s, which gets me .3 m/s and .4 m/s. Then, I multiply mass and velocity together and I get 400 kg*m/s for Car B and 300 kg*m/s for Car A. I combine the momentums to get 700 kg*m/s in a NW direction. The book, however, says 500 kg*m/s is the correct answer (without explanation). Can anyone help? I'm just starting out, trying to learn basic physics on my own.
  2. jcsd
  3. Dec 30, 2009 #2


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    Homework Helper

    Hi SoulInNeed, welcome to PF.
    In the problem they have instructed to remember that the momentum is a vector.
    The two cars are moving at right angle to each other. So their momentum are perpendicular to each other. So what is the resultant of two vectors which are perpendicular to each other?
  4. Dec 30, 2009 #3
    The important trick here is in the question itself. Momentum is a vector -- draw the momenta out as vectors if you like and the answer should pop out.
  5. Dec 30, 2009 #4
    Thanks guys, I got it. Use the Pythagorean to find it.

    I've learned a lesson, I need to use a better book to teach me this. This one made no mention of using the Pythagorean theorem when dealing with perpendicular vectors, and I had no idea you could do that in momentum vectors. I gotta get a textbook, lol.
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