**Rotational Inertia about an offset axis**
Problem gives 3 point masses all in the xy-plane. If I were asked to calculate the rotational inertia of the system about the z-axis, I would naturally use the magnitude of the position vector from each point to the origin where the closest distance to the z-axis is located. I can do it with vectors or magnitudes and get the same result for I total-system rotational inertia. But, some problems give and offset .. like (0,1) .. to the perpendicular axis through that point. I assume I can use the magnitudes from each point to (0,1) and those are the position vectors, or distances, I need for each point. But I never see that. Instead, I see a bunch of + and - signs that give very odd distances, and are certainly not equal to the magnitude of the distance from the point to (0,1). I have to assume that I simply do not understand what is meant by the offset point given. So what is really meant by that point ?
Here's an example of my thinking: point Q is at ( 4, 4 ) so the mag of the position vector is Sqrt( 4^2 + 4^2 ) ^2 which is 32. OK, if I'm given an offset point (1,0) for this calculation, now Q is sqrt( 3^2 + 4^2 )^2 and that is totally logical. But every example I see will give a ridiculous number for that distance from point Q to the offset point (1,0) through which the perpendicular axis is located. That makes no sense at all, unless there is some kind of offset jargon that I just don't understand.
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Last edited by johns123; Jun 15th 2014 at 09:00 PM.
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