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Homework Help: KE of a bar rotating at an angle

  1. May 31, 2007 #1
    KE of a bar rotating at an angle- please help!

    1. The problem statement, all variables and given/known data

    A slender prismatic bar DE Of length l(=1 m) and weight W(= 89 N) is rigidly attached at it's midpoint C to a shaft AB and makes an angle (alpha(=45degrees)) as shown in the figure. Find the kinetic energy T of the bar if it rotates with constant angular velocity (omega(=200 rpm)) about AB.

    2. Relevant equations

    Work done= change in KE


    Work done= integration of moments about the axis of rotation

    3. The attempt at a solution
    I'm supposed to use the work- energy relation to solve this question but am clueless as to how to go about it..... please help!!

    Attached Files:

    • AAA.JPG
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    Last edited: May 31, 2007
  2. jcsd
  3. May 31, 2007 #2


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    You need to find the moment of inertia of the slender rod about the axis. There is a formula that connects the kinetic energy to the MoI and the angular velocity ( omega). Have a look at this, down the page to rotational KE.

  4. May 31, 2007 #3
    thanks for replying!!!:smile:
    the problem is i don't know how to get the moment of inertia... should i assume the bar to be two rotating cones or something like that?
    and after i get this, should i also include translation to find the energy?:confused:
  5. May 31, 2007 #4


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    In general, the MoI of a mass m rotating a distance r from its axis is m*r^2.

    Work out the center of gravity for each arm, then use the perpendicular distance from the center of mass to the axis to get the MoI of each side.

    There's no translational energy, I think.
  6. May 31, 2007 #5
    are you supposed to derive the monent of inertia or are you allowed to look up the formula??

    if ou allowed to look it up then go here
  7. May 31, 2007 #6
    i'll try doing that now...
  8. May 31, 2007 #7
    i think i don't need to derive the formula for moment of inertia...
    this page gives moment of inertia for a rod rotating perpendicular to its axis of rotation, but in this case its inclined... won't that change the situation?
  9. May 31, 2007 #8
    is its axis of rotation perpendicular to the rod ?
  10. May 31, 2007 #9
    no it's not perpendicular...
    the axis passes thro' the rod's midpoint and the rod rotates such that it forms a double cone....
  11. Jun 1, 2007 #10


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    See post #4
  12. Jun 1, 2007 #11
    the answer to the question is 84 joules...
    that is given in the book but the working out isn't...
  13. Jun 1, 2007 #12
    i tried doing it using what you said...
    Please see the attachment along with this post...

    Any clues?

    Attached Files:

    • AAA.JPG
      File size:
      20.2 KB
  14. Jun 2, 2007 #13
    some help please!!:smile:
  15. Jun 3, 2007 #14
    you can't go wrong with a suggestion...:wink:
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