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Conservation of Energy for a rotating Rod

  1. Feb 15, 2009 #1
    1. The problem statement, all variables and given/known data
    rod%20problem.jpg


    2. Relevant equations

    Rotational Kinetic Energy = 1/2*I*w^2

    3. The attempt at a solution

    I was just wondering if someone explain to me why the potential energy is MgL/2 ?

    Is this not the same for the torque from gravity?
     
  2. jcsd
  3. Feb 15, 2009 #2
    As the hint in a) states, you can neglect the mass m. And since the mass M is uniform, you can presume all its weight is in the center, hence MgL/2
     
  4. Feb 15, 2009 #3
    Because the rod's center of mass is placed at L/2 over the ground.

    Edit: oops, already answered :)
     
  5. Feb 15, 2009 #4

    Doc Al

    User Avatar

    Staff: Mentor

    The rod is uniform. Where is its center of mass?
    The torque from gravity about the pivot point depends upon the angle of the rod.

    Edit: Already answered twice!
     
  6. Feb 15, 2009 #5
    ok so then its just mgh with h = L/2 . that makes sense

    then as it tips the hypotoneuse would be L/2, and the height would be this times the cos of theta.

    thanks
     
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