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Center Gravity and Moment of Inertia

  1. Apr 20, 2009 #1
    1. The problem statement, all variables and given/known data
    Four objects are held in position at the corners of a rectangle by light rods as shown in the figure below. The mass values are given below.
    M1 (kg) M2 (kg) M3 (kg) M4 (kg)
    3.50 1.50 3.90 1.70
    p8-29alt.gif
    (a) Find the moment of inertia of the system about the x axis.

    (b) Find the moment of inertia of the system about the y axis.

    (c) Find the moment of inertia of the system about an axis through O and perpendicular to the page.

    2. Relevant equations

    Center of Gravity: sumM1*X1+M1*X2.../M1+M2...

    3. The attempt at a solution

    I found the center of gravity for the x axis to be .037736...how do I find the moment of inertia with that information?
     

    Attached Files:

    Last edited: Apr 20, 2009
  2. jcsd
  3. Apr 20, 2009 #2

    LowlyPion

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    Homework Helper

    You need to find the x and y CoM.

    Knowing that then the moment I about the CoM is Σmr2.

    The axes are simpler in that for

    I_x = Σm_i*y_i2

    I_y = Σm_i*x_i2
     
  4. Apr 20, 2009 #3
    Oh, I reversed the two and that's why I got it wrong. Thanks.
    What would I use for part c?
     
  5. Apr 21, 2009 #4

    LowlyPion

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    What are the distances to each corner from O to each mass?

    I_o = Σmr2

    It's made a little easier by Pythagoras, so be sure and thank him.
     
  6. Apr 21, 2009 #5
    perfect! Thanks!!
     
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