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Homework Help: Initial velocity (and momentum)

  1. Nov 28, 2007 #1
    A 2020kg car moving at 20.0m/s collides and locks together with a 1400kg car at rest at a stop sign. Show that momentum is conserved in a reference frame moving at 10.0m/s in the direction of the moving car.

    initial momentum

    final momentum


    I don't understand why that the answer for initial momentum is not 20200 kg*m/s due to a reference frame moving at 10.0m/s in the direction of the moving car. Can anyone help me out here, that will be awesome
     
    Last edited by a moderator: Jan 7, 2014
  2. jcsd
  3. Nov 28, 2007 #2

    Doc Al

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    Staff: Mentor

    In that new reference frame, what's the initial velocity (and momentum) of the 1400 kg car?
     
    Last edited by a moderator: Jan 7, 2014
  4. Nov 28, 2007 #3
    was it 10.0m/s and 14000kg*m/s in the opposite direction?
     
  5. Nov 28, 2007 #4

    Doc Al

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    Exactly.
     
  6. Nov 28, 2007 #5
    then do I have to sum it together (20200+ (-14000))kg*m/s? and the answer should be 6200kg*m/s for the initial momentum?
     
  7. Nov 28, 2007 #6

    Doc Al

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    That's correct.
     
  8. Nov 28, 2007 #7
    is that mean the final momentum is also 6200kg*m/s?
     
  9. Nov 28, 2007 #8

    Doc Al

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    Sure. Momentum is conserved.
     
  10. Nov 28, 2007 #9
    thanks Doc Al
     
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