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Homework Help: Momentum problem (should be pretty easy I think)

  1. Nov 12, 2007 #1
    1. The problem statement, all variables and given/known data

    A 20 metric ton train moves toward the south at 50 m/s. What speed must it travel to have two times its original momentum?

    2. Relevant equations

    Change in momentum = mass(FinalVelocity - InitialVelocity)

    3. The attempt at a solution

    so I thought you would simply times 50 by 2 and get 100 to be the answer, but thats not right. First question is do you have to convert 20 metric tons to kilograms?
     
  2. jcsd
  3. Nov 12, 2007 #2

    PhanthomJay

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    did you put in the units for its speed?
     
  4. Nov 12, 2007 #3
    you mean 50 m/s yeah?...if thats not what you mean then I'm confused
     
  5. Nov 12, 2007 #4

    PhanthomJay

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    No, I meant 100m/s. In order for the momentum vector to be twice its original momentum, the new momentum must have a magnitude of twice as much and a direction in the same direction as the original momentum vector. Since its momentum doubles, and the mass is constant, then its velocity must double. So the velocity is 100m/s south. The speed is 100m/s. Unless we both missed something.
     
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