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Old 12-07-2003, 12:21 AM   #1
Tyler Hass
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I read this article and it sounds like BS to me: http://www.geocities.com/bright_effect/ironcross.html

Could any of you physics experts take a look and shed some light on this?
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Old 12-07-2003, 02:30 AM   #2
Paul "The Viking"
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I'll give it a go. The picture doesn't come through for me, but I get the idea.

The main problem with this argument is the assumption that the human arm acts like a rope, only supporting tension forces. If that were true, then the discussion there would be right.

However, the human body can support forces other than tension forces. The gymnast, in essence, is trying to make his/her body into something rigid. Think about hanging a metal cross on the rings. Obviously, a metal cross can support itself even at 90 degrees because it is rigid enough.

I could do the math, but it essentially comes down to the fact that the gymnast is applying a torque to his/her arms, which are also receiving torques from the ring. So its the torque balance, not the tension balance that's important in this case.

-Paul
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Old 12-07-2003, 05:21 AM   #3
mark a. blakemore
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I'm not a physicist but I did stay at a Holiday Inn Express lastnight;-)
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Old 12-07-2003, 06:39 PM   #4
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Yeah, the guy who wrote it was not a ring man. I can tell you this with high confidence.

The move is interesting/complex though and requires what is for many a brand new motor recruitment pattern.



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Old 12-07-2003, 07:19 PM   #5
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I'm laughing. Now I understand why when you lower towards the cross all of a sudden like the forces become infinite and the gymnast's arms explode/collapse/shatter.

Those of you who have missed that did so only because the gymnasts you've see were careful enough not to actually go to 90.

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Old 12-07-2003, 10:45 PM   #6
Tyler Hass
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Yeah, I figured it was crap. One more reason I thought this was because there is a move where you go from a hang and pull UP to a cross without bending your arms. I hear that there are a few guys, like Jovtchev, that can do it. But it's rarely competed. Has anyone seen it done before?
Btw, if it took infinite force to hold a cross (as per the article), would the universe explode if someone actually did it? I assume infinite force requires infinite energy.
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Old 12-08-2003, 10:12 AM   #7
Roger Harrell
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The argument is totally invalid. This argument makes some significant assumtions that are flat out wrong such as "In fact, one could substitute two solid surfaces or two bars for the rings.)" Doing a slightly high iron cross between two rigid structures is FAR easier than doing the same on rings. Little tip, the rings move. In order to do an iron cross a gymnast must engage their shoulder complex in such a way as to create direct downward force at their hands. The shoulder of a gymnast can then be equated to a rigid junction, not cables under tension, nor a hinged bar. Place a T made out of wood between two objects. That's a closer model for a cross. I didn't check the guys equasions at all because his basic assumtions on his model are wrong.

BTW I know many folks that can hold a cross anywhere from suppord down to about 20 degrees BELOW horizontal. And the move pulling up to a cross from a hang with straight arms is called a butterfly. It's a D value move. (Gymnastics moves are rated A-E, E being the hardest, so a butterfly isn't even in the toughest category of skills).
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