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Moment of inertia Question

  1. May 17, 2009 #1
    1. The problem statement, all variables and given/known data
    hello everyone ,, can someone tell me if I got the moment of inertia right or wrong of this question :
    Two blocks of masses m1 = 5 kg and m2 = 15 kg are suspended from the ends of 1.5 m
    rigid rod of weight 75 N that can rotate about point P, as shown in the figure. The rod is
    held in a configuration such that it makes an angle of 37° with the vertical, and then
    released. The two blocks can be considered as point particles and the moment of inertia
    of the rod about its center of mass is I(com) = ML^2/12.


    2. Relevant equations



    3. The attempt at a solution
    I=I(com)+M(L/3)^2+m1(2L/3)^2+m2(L/3)^2 = (7/36)ML^2+(m2L^2/9)+(4m1L^2/9),, my teacher told me that adding m2 and m1 is wrong ,, but what i don't get ,, in some questions we add them ,, so is he right or wrong ??
     

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    Last edited: May 17, 2009
  2. jcsd
  3. May 17, 2009 #2

    LowlyPion

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    Homework Helper

    And the figure shows what exactly?
     
  4. May 17, 2009 #3

    Doc Al

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    Staff: Mentor

    Attach the figure.
     
  5. May 17, 2009 #4
    sorry ,, forgot to add it
     
  6. May 17, 2009 #5

    Doc Al

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    Staff: Mentor

    The masses are not rigidly attached to the stick, so don't include them when calculating the rotational inertia.
     
  7. May 17, 2009 #6
    how come we don't include them ?? in the previous question (https://www.physicsforums.com/showthread.php?t=314630) we added them to the moment of inertia ,, in Fundamentals of physics book there is a question in Rotation chapter (11) Q:57P ,, in the answer .. it I*Alfa = (mL^2(1)+ mL^2(2))*alfa and it's almost the same as the question but without angle (it's horizontal) but I equaled : (mL^2(1)+ mL^2(2)) (the rod is mass less in this question)
    By rigidly ,, you mean it fixed and cant be taken away ? ,, BTW some questions say (without the cord slipping on the pulley) does it mean it fixed ?
     
    Last edited: May 17, 2009
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