Moment of Inertia spreadsheet calculator

[hamerish]

Homework Statement

This isn't homework but its part of my assignment. I am 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. I am 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 I am unsure if they are correct.

Homework Equations

Cog

Σmi*di/Σmi

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.

The Attempt at a Solution

CoG

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.

Inertia

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 I am 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.

 

[hamerish]

If anyone needs me to explain it clearer then I will, or have I gone about it completely wrong