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Homework Help: Rotational Motion

  1. Dec 17, 2013 #1
    1. The problem statement, all variables and given/known data

    Consider a uniform rod of mass 12 kg and length 1.0 m. At its end, the rod is attached to a fixed, fricition free pivot. initially the rod is balanced vertically abbove the pivot and begins to fall (from rest) as shown. Determine

    a) the angual acceleration of the rod as it passes through the horizontal at B
    b) the angual speed of the rod as it passes throught the vertical at C

    2. Relevant equations

    i want to say.....

    w=wo + at
    θ = wot + 0.5at^2

    3. The attempt at a solution

    I have absolutely no clue how to start this off... what i wanted to do was find some information at point A such as the angle it makes there or the velocity

    then from this information.. find angular acceleration at B for partt a.

    then use the info from a to find angular speed at C

    but i have no clue how to start this off.. if any1 can help me start it off it will be a big help!!!!

    Attached Files:

  2. jcsd
  3. Dec 17, 2013 #2
  4. Dec 17, 2013 #3
    I=mr^2 and Torque=Ia(angular)… you saying to use these. If i find I(inertia) at point a then i use that and find angular acceleration at B? is that what your hinting towards?
  5. Dec 17, 2013 #4
    Hello, yes, that is what I mean. Consider that rod is rotating about it's end.
  6. Dec 17, 2013 #5
    i just realized how can i find inertia (I) at a because i don't know what Torque is ….

    i worked something out and both torque and I were 12 :S that looks wrong
  7. Dec 17, 2013 #6


    User Avatar
    Gold Member

    You don't need to know the torque to find the moment of inertia of the rod; it's solely a function of the geometry and the mass distribution. The moment of inertia of a uniform thin rod about an axis through its end is well known. (Hint: you can either look this up or use elementary calculus.)

    For determining the torque, you might wish to consider the rod at a displacement ##\theta## from it's initial equilibrium position, and then consider the forces acting upon it.
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