bsellers37

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!

Cheers

Bill

--

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.

http://homepage.mac.com/~wis/

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

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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!

Cheers

Bill

--

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.

http://homepage.mac.com/~wis/

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

---------------------------------------------------------------

To unsubscribe send SIGNOFF BIOMCH-L to LISTSERV@nic.surfnet.nl

For information and archives: http://isb.ri.ccf.org/biomch-l

---------------------------------------------------------------