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Concept Question - Momentum

  1. Jun 14, 2008 #1

    LHC

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    Hello,

    I have a question about momentum that showed up in my physics textbook. It's a True/False question, and I have the answer, it's just that I'm not sure how it works. Could someone please explain this to me? Many thanks.

    |Glider 1|-----> <------|Glider 2|
    -------------------------------------------
    Air Track
    -------------------------------------------

    Statement: In an isolated system, two gliders (m1 = m2) on an air track move toward each other at equal speed, collide, and then move away from each other at equal speed.

    Question (T/F): For this collision, if the change in momentum of glider 1 is -1.4 kg×m/s [W], then the change in momentum of glider 2 is 1.4 kg×m/s [W].

    The answer is true.

    I'm not sure how the numbers even got there. I recognize that they just happen to be [tex]\sqrt{2}[/tex], so I'm assuming there's some type of calculation to be made with the formula for kinetic energy [tex]E_{k}=\frac{1}{2}mv^2[/tex]. However, I just don't see how it works. I thought that the change in momentum was simply [tex]\Delta p=2mv[/tex]
     
  2. jcsd
  3. Jun 14, 2008 #2

    malawi_glenn

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    dont ask questions here....
     
  4. Jun 14, 2008 #3

    LHC

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    Oh, ha, I'm so sorry. How foolish of me.
     
  5. Jun 14, 2008 #4

    malawi_glenn

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    a mentor will move this thread, so just relax and don't make a new one.
     
  6. Jun 14, 2008 #5

    tiny-tim

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    Hi LHC! :smile:

    They just made up the 1.4. :smile:

    Hint: is momentum conserved in this collision?

    what does that mean? :wink:
     
  7. Jun 14, 2008 #6

    LHC

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    Ohhhh! I get it. Thanks!
     
  8. Jun 14, 2008 #7

    dynamicsolo

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    The thing that's funny about this T/F question is that the stated conditions are completely irrelevant. From Newton's Third Law or conservation of linear momentum, the change in linear momentum (also called the impulse) of glider 1 would be equal in magnitude and opposite in sign (since this is a one-dimensional collision) to the change in linear momentum of glider 2. Thus, the total momentum change of the two-glider system is zero (or the total linear momentum remains constant). That will be true regardless of the relative masses and initial velocities of the two gliders.
     
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