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Homework Help: What is conserved in the system?

  1. Mar 8, 2014 #1
    1. The problem statement, all variables and given/known data

    A ball is attached to a rod at one end. The rod rotates about a point at the other end.
    A second ball hits the first ball with a horizontal velocity u, and then the two balls travel together with the same horizontal velocity v. The collision is inelastic.

    What is conserved in the system after the collision?
    The mechanical energy? Horizontal momentum? Angular momentum about the CM? Or angular momentum about the rod's axis?

    3. The attempt at a solution

    I do not believe the mechanical energy is conserved. Since this is an inelastic collision the kinetic energy after the collision is less than before, thus the mechanical energy is not conserved.

    I'm not sure if the momentum is conserved. In my book it says the momentum is conserved if the external forces can be neglected but I'm not sure if the second ball is an external force or not?
  2. jcsd
  3. Mar 8, 2014 #2


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    Are there OTHER external forces of roughly equal magnitude acting upon the second ball+pendulum system that actively prevents some type of motion to occur? (Hint: Look at the ceiling for a clue!)
  4. Mar 8, 2014 #3
    The moment of inertia? It makes it harder for the rod to rotate around the point on the ceiling?
  5. Mar 8, 2014 #4
    Is it correct to assume that there is conservation of angular momentum about the CM? Since the added weight (when the two ball join together) thus balance out the uneven weight by moving the center of mass thus the rotation has angular momentum conservation?
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