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Homework Help: Kinetic energy of rigid body

  1. Feb 6, 2010 #1
    when will i use 0.5mv^2 and when will i use 0.5Iw^2
    when will i use both?
    as far as i can see one represents the movement of the center of mass and the other
    the angular movement.
    so as far as i understand for a moving cylinder i would use the sum of those.
    but i can't think of any example where i use only one.
    for instance what happens in the following cases:
    1. circular motion of some ring
    2. a rod which is fixed to the wall on one side is being hit on the other and starts rising
    3. a rod that lies on a table, gets hit by clay and starts moving. this one is not fixed.

    thanks a lot
    this forum is really helpful in preparing for my exam
  2. jcsd
  3. Feb 6, 2010 #2

    Doc Al

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    Staff: Mentor

    If you take I about the center of mass, then you can always use both. One represents the KE due to the translation of the center of mass; the other is the KE due to rotation about the center of mass.

    But sometimes you can treat a body as being purely in rotation about some axis and just use KE = 0.5Iw^2, where I is about the axis of rotation.

    I don't understand this case.
    Since the rod is hinged at one end, you can treat its motion as being pure rotation about that end.
    Use both, with I about the center of mass.
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