Trouble Maker Posted July 17, 2011 Report Share Posted July 17, 2011 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 Quote Link to comment Share on other sites More sharing options...
Furloaf Posted July 17, 2011 Report Share Posted July 17, 2011 No. Quote Link to comment Share on other sites More sharing options...
Trouble Maker Posted July 17, 2011 Author Report Share Posted July 17, 2011 Would you like to actually help? Quote Link to comment Share on other sites More sharing options...
Patterson Posted July 17, 2011 Report Share Posted July 17, 2011 (edited) ... Edited July 18, 2011 by Patterson Quote Link to comment Share on other sites More sharing options...
Furloaf Posted July 17, 2011 Report Share Posted July 17, 2011 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. Quote Link to comment Share on other sites More sharing options...
Trouble Maker Posted July 18, 2011 Author Report Share Posted July 18, 2011 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? Quote Link to comment Share on other sites More sharing options...
Furloaf Posted July 18, 2011 Report Share Posted July 18, 2011 Yeah that should be fine since you're just doing a comparison between two similar objects. I studied ME for a while, that's all my knowledge consists of. Quote Link to comment Share on other sites More sharing options...
Berto Posted July 18, 2011 Report Share Posted July 18, 2011 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. Quote Link to comment Share on other sites More sharing options...
Trouble Maker Posted July 18, 2011 Author Report Share Posted July 18, 2011 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. Quote Link to comment Share on other sites More sharing options...
Consynx Posted July 18, 2011 Report Share Posted July 18, 2011 you have to account for rotating around a point that isn't the COG. iirc, it's not just mr^2 in this case. Quote Link to comment Share on other sites More sharing options...
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