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RE: Off topic, Moment of Inertia

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Apparently this is a difference between r14 and 2000.  A little playing
shows that the moments of inertia returned by AutoCad r14 are indeed about
the origin.  (Make a 1x1 box with its centroid at 0,0,0 into a region.  The
moments of inertia are 1/12.  Move it so that the centroid is at 10,10,0.
Now the moments of inertia are 100.083333 = 1/12bd^3+Ad^2.)  This is a very
useful feature, but you must be careful to move the region so that its
centroid is at the origin.

> -----Original Message-----
> From: SEConsultant [mailto:seconsultant(--nospam--at)earthlink.net]
> Sent: Thursday, February 10, 2000 5:06 PM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Off topic, Moment of Inertia
>
>
> Actually, in Autocad (I checked this with Autocad 2000 ADT2) the Principle
> moment of inertia is extracted which is not dependent on the
> location of the
> object - which means that you don't need to move the object if your only
> goal is to find Moment of Inertia.
>
> BTW, thanks to those asking the question. I thought that this feature was
> unique to the S8 add-on in R14 and earlier and had no idea it was
> a standard
> tool with all versions of Autocad. Old dogs can learn new tricks. Thanks
> again for knowledge.
>
> Dennis S. Wish, PE
>
> -----Original Message-----
> From: Peter McCormack [mailto:pmac(--nospam--at)realloghomes.com]
> Sent: Thursday, February 10, 2000 11:04 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Off topic, Moment of Inertia
>
>
> The method I have used with reasonable results is as follows:
>
> 1. Center the shape about the origin (0,0,0) for a rectangle, circle
> etc, the centroid will be at 0,0,0
> 2. Convert the shape to a region.(Using the pulldown menu : Draw-
> >region)
> 3. then use the Mass properties function
>
> I hope this helps.
>
> Peter McCormack
>
> Send reply to:  	seaint(--nospam--at)seaint.org
> From:           	"Fountain Conner" <fconner(--nospam--at)pcola.gulf.net>
> To:             	<seaint(--nospam--at)seaint.org>
> Subject:        	Off topic, Moment of Inertia
> Date sent:      	Thu, 10 Feb 2000 12:45:05 -0600
> Organization:   	http://www.seaint.org
>
> >
> > Does anyone have an ACAD Lisp routine that will reliably calculate the
> > moment of inertia of a shape once it's drawn?
> >
> > I tried one, but it wasn't too accurate -- even on a simple rectanguar
> > section.  Build-up sections were worse.
> >
> > Thank you,
> >
> > Fountain E. Conner, P.E.
> > 1724 Beaver Pond Rd.
> > Gulf Breeze, Fl. 32561
> >    Phone 850-932-5547
> >    Fax      850-934-1916
> >
> >
>
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