View Full Version : Moments of Inertia

01-31-2002, 09:55 PM
Dear All,

I've spent the last couple of days calculating various inertial
properties of geometric shapes in Mathematica and I've realised that my
understanding of exactly what I'm calculating isn't as good as it should
be... Generally I calculate the Ixx value by integrating the areas of
slices of the shape along the Y and Z axes (and similarly for the Iyy
and Izz values) and when I don't make any mistakes I get answers that
match the values quoted in text books (assuming I can find the shape I'm
interested in!). However simulation software generally asks for other
values (often Ixy, Iyz and Izx) which are usually set to zero. My first
question is what are these values and when are they non-zero?

Secondly, when doing 2D analysis I seem to remember that you generally
calculate the MOI by integrating along the length of the segment rather
than performing the two perpendicular integrations. Is that actually
correct or is it just an approximation since most segments can be
considered long and thin?

Thirdly, can anyone recommend a good source of MOI and centroid
equations for a variety of geometrical shapes since some of my results
are pretty complex and although it's an interesting exercise to
calculate them I would like to check that they are actually correct!


Dr. Bill Sellers, Editor Primate Eye Email: wis@mac.com
Department of Human Sciences Tel: UK (0)1509 228228
Loughborough University Fax: UK (0)1509 223941
Loughborough LE11 3TU UK.

Primate Society of Great Britain WWW site http://www.psgb.org/

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