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Old 01-02-2009, 12:38 PM   #8
Erick Mead
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Dojo: Big Green Drum (W. Florida Aikikai)
Location: West Florida
Join Date: Jun 2005
Posts: 2,619
Re: Physical Theory of Aiki?

David Henderson wrote: View Post
Here's a specific discussion that I found useful:

One quote in particular reminded me of things Erick is writing about:

"[T]ension and compression often operate at 90 degrees to each other. Tense a rope and its girth goes into compression. The fibers crowd closer and closer together. Load a pillar, and its girth widens slightly, as the weight it is supporting pushes its molecules apart into tension. If you pull on a rope, its fibers compress together. If you push on a pillar, it gets (maybe only a very little) wider in the middle."
... And exactly the same thing happens on the bias in torsional shear, which you have probably recognized from the torsion tube blog post. Oversimplifying it a bit (but not too much) tensegrity is just compression and tension structures laid out in the torsional shear pattern, but with the tube itself missing -- much like the stress members of truss are in the pattern of a bending/shear stress in a beam, but with the rest of the beam missing. Tensegrity uses almost pure shear to support or resist loads -- it is just decomposed into its positive and negative phases.

I like the anatomy-trains site, but whereas it works on tension and compression as primary quantities and shear as secondary, I tend to think in shear terms as the non-decomposed primary quantity, because it preserves the necessity to see the rotations occuring in all axes.

A very interesting site worth going through and understanding on the placement of fulcrums (fulcra?) and relative mechanical advantage gained by USING the shear in a mechanism, vice trying to compensate it away (the way we do in "normal" muscular joint leverage) is found here:

I often illustrate this intentional use of leverage-induced shear by placing my student on the ground with a good grip on my gi lapel and the opposite shoulder (in a good judo form) and then I C-curve my spine (putting shear out the back) and there is absolutely not a damn thing I can do to keep him from tossing me right over him with almost no effort. After he does that, I then let him have the same grip, and then keep my spine in proper posture with upper and lower lordosis intact (shear now going out through the belly ("filling the dantien qi" in Chinese terms). He can lift himself off the floor and hang bodily from my gi for all I care -- but he can't throw me. It generally impresses them enough to begin to pay attention to the form of the body in moving.

What does this have to do with oscialltion you may say? Well, moment (in classical terms) is just the static potential form of angular momentum (the hanging pendulum or balanced broom handle -- depending on the positive or negative phase), so, pretty much it has everything to do with it. The placement of the shear in the dantien orientation maximizes my static moment, and placement the other way just about eliminates it ...

Last edited by Erick Mead : 01-02-2009 at 12:49 PM.


Erick Mead
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