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Any physics/ME dorks on here? Moments of inertia?


Trouble Maker

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just want to make sure I'm thinking about this right. It's been a while since I've touched this stuff and I figured I could ask here before I pulled out any books.

 

If I know the CG, weight (mass) of something and the point at which I'm rotating it around (how far is the CG from this point) I can use the simple equation of I = mr^2 rather than modeling each piece of a complex object... right?

 

E.G. trying to figure out the moment of inertia of a (few different) trailers.

 

TIA

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Sorry was a in bit of bad mood earlier.

 

Using the simplified I = mr^2 could be a decent guess I suppose. However, I = mr^2 works best for uniform bodies, i.e. simple regular shapes and uniform density.

 

I also don't remember a relatively easier way to calculate it that is a little more precise, either.

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Sorry was a in bit of bad mood earlier.

 

Using the simplified I = mr^2 could be a decent guess I suppose. However, I = mr^2 works best for uniform bodies, i.e. simple regular shapes and uniform density.

 

I also don't remember a relatively easier way to calculate it that is a little more precise, either.

 

Thanks. The part about non-uniform bodies makes sense and is the intuitive conclusion I came to but I couldn't pin down why so I decided it must be ok/correct to do it the 'easy' way. A good estimate will be enough as I don't need their actual moments of inertia but just need to compare them to each other. Just trying to set up a worst case scenario.

 

Edit: just so you know I did pose this on another forum and I'm getting that my first thought is correct. Just out of curiosity do you have a working knowledge in this area?

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As he said, thats a pretty basic approach. Its been forever since I did all that but depending on what axis you need the CG on, you could try to use a jack and balance the trailer on it. If you were able to balance it you found the CG horizontally.
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As he said, thats a pretty basic approach. Its been forever since I did all that but depending on what axis you need the CG on, you could try to use a jack and balance the trailer on it. If you were able to balance it you found the CG horizontally.

 

Berto, I have access to the right scales to find the CG. Longitudinal CG should be all I should need.

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