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Why Isn't Linear Momentum Conserved?

  1. Mar 28, 2014 #1
    1. The problem statement, all variables and given/known data

    A thin metal bar, 2.00 m and a mass of 9.18 kg hangs vertically from a ceiling by a frictionless pivot. Suddenly it is struck 1.50 m below the ceiling by a small 3.00 kg ball, initially travelling horizontally at 10.0 m/s. The ball rebounds in the opposite direction with a speed of 6.00 m/s.

    (a) Find the angular speed of the bar just after the collision. ***The answer in the textbook is 5.88 rad/s, and that makes sense to me.***

    (b) During the collision, why is the angular momentum conserved but not the linear momentum?

    2. Relevant equations

    m*v(initial)*l = Iω + m*v(final)*l

    3. The attempt at a solution

    I have absolutely no idea how this is possible. I was always taught that momentum is always conserved.
  2. jcsd
  3. Mar 28, 2014 #2


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    Staff: Mentor

    What are forces at the pivot point?
  4. Mar 28, 2014 #3
    There are tension and gravity forces, but I don't understand how they would affect the linear momentum of the ball in the horizontal dimension.
  5. Mar 28, 2014 #4

    Doc Al

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    Staff: Mentor

    Hint: Does the pivot move? Why not?
  6. Mar 28, 2014 #5
    It would be useful here to keep in mind what criteria need to be met for linear momentum to be conserved.
  7. Mar 28, 2014 #6


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    Linear momentum is conserved (if you consider the whole system).

    Linear momentum is not necessarily conserved (if you only consider part of the system).

    E.g. bouncing ball: momentum conserved if you consider the Earth's momentum; momentum clearly not conserved if you consider the ball only.
  8. Mar 28, 2014 #7
    Yeah, I should have knew that, thanks everyone!
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