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Constrained motion question (using moments of inertia/energy)

  1. Apr 23, 2006 #1
    I've attached a screenshot of the question. This is what i've tried so far:

    I've calculated the initial energy of the system (which is all potential energy):
    Energy of disc:
    0.5*I*w^2 + 0.5 *m*v^2 + m*g*h
    = 0 + 0 + 0

    Energy of arm:
    0.5*m*v^2 + m*g*h
    = 0 + 4*9.81*1.5Cos(45)
    = 41.62 joules

    Energy of collar:
    0.5*m*v^2 + m*g*h
    = 0 + 1*9.81*3Cos(45)
    = 20.81 joules

    Then i moved time on until the supporting arm BC was horizontal and calculated the energy at that stage (which would be all kinetic). When i do that, i end up calculating that the speed of the collar is 1.04 m/s. However, the answer is supposed to be 7.315 m/s.

    I'm doing something wrong, but i can't figure out what it is. Any hints on how this is supposed to be solved would be great.


    Last edited: Apr 23, 2006
  2. jcsd
  3. Apr 24, 2006 #2


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    Science Advisor
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    Gold Member
    Dearly Missed

    In all probability, your flaw lies in determining the kinetic energy term of the arm. Specify what you did.
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