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  1. #1
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    Default mass moment of inertia of an irregular area

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    Hi, sorry if question sounds stupid. I'm really pushed for time.. and I've tried reading the manual but it doesn't help. sorry guys I'm a biologist, so this is the first time I'm using AutoCAD.

    Basically:
    1) I have an irregular area
    2) I need to determine its mass moment of inertia about an axis (see picture)

    I'm not sure how, but I think this:
    1) trace the area into autoCAD (I'm using the polyline function)
    2) convert it into a solid
    3) use "massprop"

    Is this correct? Please help....

    Thanks! Your help is GREATLY appreciated.
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  2. #2
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    I am not sure, about this but I do not think AutoCad calculates moment of inertia, and I do not remember that far back to my calculas classes to do the equations, but I think you want MatLab, I do remember that program takes care of the calculations

    But if you draw that to scale you might be able to do the calculation, because if I recall a certain parallel axis theroem, where I see you have the line through the center, and you take the distance, from the 2 points, other than that, I am not sure.
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  3. #3
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    thanks. I'll have a look at matlab then

    but here:

    autocad gives you the mass moment of inertia:

    Mass properties for solids

    Mass property
    Description
    Mass
    The measure of inertia of a body. Because a density of one is used, mass and volume have the same value.
    Volume
    The amount of 3D space that a solid encloses.
    Bounding box
    The diagonally opposite corners of a 3D box that encloses the solid.
    Centroid
    A 3D point that is the center of mass for solids. A solid of uniform density is assumed.
    Moments of inertia
    The mass moments of inertia, which is used when computing the force required to rotate an object about a given axis, such as a wheel rotating about an axle. The formula for mass moments of inertia is
    mass_moments_of_inertia = object_mass * radiusaxis2
    Mass moments of inertia unit is mass (grams or slugs) times the distance squared.

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    Hmm maybe you can get it to work for you, but I would have no idea how, maybe someone else will know.
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    cheers again... if you know how to create a solid?

  6. #6
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    Well I think the first problem you would encounter, would be the object, since it is a 3 dimensional object, it would be hard to make the outline, I have tried massprop, but using a "box" which draws a 3d cube, and it seems to give you a detailed analysis, which is probably what you want.

    As for the 3d drafting, I would be unable to help you with that. Best bet would be to go in to the views dropdown, and select an isometric view, so you can try to draw the solid.

    I could be wrong, I hope someone else can help you here.
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  7. #7
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    If you are only concerned with a planar area then Regions will also give the Moment of Inertia via MASSPROP. If you align the Regions centroid with the WCS origin (as in attached) you should be able to glean some useful information.
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    thanks a lot g-prime and seant.

    Seant:
    here's the massprop data from your file:

    Moments of inertia: X: 1.03299381
    Y: 0.47709131
    Principal moments and X-Y directions about centroid:
    I: 0.47166819 along [0.09782044 -0.99520408]
    J: 1.03841677 along [0.99520408 0.09782044]

    1) it seems to give you the area moment of inertia, as opposed to the mass moment of inertia?
    2) I can't just multiply this area moment of inertia by a depth and density to give you the mass moment of inertia, or can I?
    3) this was the reason I wanted to make the object a solid instead of a region?


    4) because from the help file, for a region, it seems to give you "area_moments_of_inertia"
    for a solid, it seems to give you "mass_moments_of_inertia"
    Last edited by darkenergy; 20th Mar 2008 at 11:01 pm.

  9. #9
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    guys, I don't want to sound more stupid BUT:

    isn't this true:
    mass moment of inertia = area moment of inertia * density * depth?

  10. #10
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    Your original post had me assuming you were dealing with an area only. Any amount of volume will change the parameters. Is that area a straight extrusion?

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