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Conservation of angular momentum problem

  1. Mar 16, 2015 #1
    I have been busy with rotating objects and I have a question which I don't understand. http://dev.physicslab.org/Document.aspx?doctype=3&filename=RotaryMotion_AngularMomentum.xml (last question of the page, about the clay on the rod)
    What I don't understand is that the clay has a kinetic energy of KE=0,5*0,1*10^2=5J, but then one of the answers say the initial rotational kinetic energy, and thus the total energy of the system, is 2J. This is a violation of the law of conservation of energy. Please help?
     
  2. jcsd
  3. Mar 16, 2015 #2

    Doc Al

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    The clay makes an inelastic collision with the rod (they stick together). Mechanical energy is not conserved.
     
  4. Mar 16, 2015 #3
    But where does the 3J go?
     
  5. Mar 16, 2015 #4

    Doc Al

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    Random internal energy, deformation... things like that. (Things get warm.)

    Drop a lump of clay onto the floor. It goes splat. What happened to its kinetic energy? Same idea.
     
  6. Mar 16, 2015 #5
    What about a solid ball that's sticky or something, will it suddenly get 3J of just because it hits a hanging rod with 5J?
     
  7. Mar 16, 2015 #6
    * get 3J of heat
     
  8. Mar 16, 2015 #7

    Doc Al

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    The collision takes some time. But yes, the system loses mechanical energy; that lost energy will show up as "heat" and other forms of "random" energy.
     
  9. Mar 16, 2015 #8
    Aha, there is also a familiar example with the conservation of moment, called the ballistic pendulum problem.
    In conclusion; (angular) momentum is always (!) conserved, but energy can be transformed into other forms of energy, but the sum is also conserved.?
     
  10. Mar 16, 2015 #9

    Doc Al

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    That's right. Angular momentum is conserved in such problems because there is no external torque acting. (The pivot is frictionless.)
     
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