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Homework Help: Moment of Inertia spreadsheet calculator

  1. Nov 16, 2012 #1
    1. The problem statement, all variables and given/known data

    This isn't homework but its part of my assignment. Im creating a center of gravity calculator for a car using excel which I can enter the locations of different components and their masses, and it will give me the center of gravity for the car. Im then using this to give me moments of inertia for different vectors of the car. The spreadsheet I made has columns for mass, (component cog location, x, y and z from the nearside corner of the car. I then calculate the inertia about Ixx, Iyy and Izz. I have created some equations for doing this but im unsure if they are correct.

    2. Relevant equations



    mi are the individual masses and di is the distance from datum point

    for moment of inertia

    I = m*r^2

    when m is mass and r is distance from reference

    so because the is a right angle

    I = M*SQroot(x^2+y2)^2

    boot the SQroot and ^2 cancel each other out so
    I = M * (x^2 + y^2)

    And for clarification the x coordinate is the length, y is the width and z is the height all from the front nearside bumper.

    3. The attempt at a solution


    For the x coordinate

    (Xdistance/(total sum of masses)* individual mass) = Mx

    I then added each individual component answer(Mx) to get the x coordinate of the car cog.


    For Ixx for the individual component inertia contribution

    (Component Mass*(((Car Height^2+Car Width^2)/ Car Length)+(Y-Mz)^2+(Z-Mz)^2))

    the I total all the individual inertia's to get the cars moment of inertia

    I did these spreadsheets about 2 weeks ago and im lost to how I came about the equations and am unsure if they are correct so any help would be appreciated.

    A screen shot of the spreadsheet is below to give you an idea of what I an trying to accomplish.

    Attached Files:

    • cogm.png
      File size:
      27.9 KB
  2. jcsd
  3. Nov 22, 2012 #2
    If anyone needs me to explain it clearer then I will, or have I gone about it completely wrong
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