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Homework Help: Momentum of two balls

  1. Apr 10, 2008 #1
    Two balls X and Y collide elastically. Their speeds before and after collision are as shown.
    Before

    After
    Which of the following equations is correct?

    A Ux +Uy = Vx +Vy
    B Ux +Uy = Vx - Vy
    C Ux - Uy = Vx +Vy
    D Ux - Uy = Vx - Vy
     

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  2. jcsd
  3. Apr 10, 2008 #2
    the picture is attached
     
  4. Apr 10, 2008 #3

    Doc Al

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    Staff: Mentor

    What do you think? What can you say about elastic collision?
     
  5. Apr 11, 2008 #4
    Ux- Uy = -Vx+Vy

    Ux- Uy = -Vx+Vy
    I think this is the right equation.
     
  6. Apr 11, 2008 #5

    Doc Al

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    Staff: Mentor

    Why? What's your reasoning? What physics principle or result are you appealing to? (Hint: The directions of the velocities are given; Treat the U's and V's as positive numbers.)
     
  7. Apr 12, 2008 #6
    The conservation of momentum.
    It is elastic collision. Overall speeds are equivalent before and after the collision.
     
  8. Apr 12, 2008 #7
    Let the direction of Ux be positive and thus that of Vx is negative.
     
  9. Apr 12, 2008 #8

    Doc Al

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    Staff: Mentor

    Good.
    I don't know what this means. Hint: Conservation of momentum and energy leads to a very specific conclusion about velocities before and after the collision.

    OK.
     
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