John Sapinoso Posted January 21, 2008 Share Posted January 21, 2008 '>http://www.geocities.com/bright_effect/ironcross.htmlbtw newtons = (kg * m)/s^2gravity = 9.8m/s^2so 655 newtons = 66.9 kg67kg = 147 lbsso these numbers were calculated for a 150 lbs gymnast Link to comment Share on other sites More sharing options...
Guest regaman Posted January 22, 2008 Share Posted January 22, 2008 That article got it all wrong!Reasoning that way you couldn't even hold a pound in your outstretched arm, because the tension would still approach infinity....The mistake is applying the reacting forces an armlength away (literaly!) without considering the torque.Actually when the arms are horizontal, the "tension" is ZERO and the torque on the armpit/shoulder reaches its maximum.Here are the formulas:P = weight of the gymnastL = armlengtha = angle between arms and horizontal lineT = tension in the arm (it's actually a compressing force as long as the gymnast is above the iron cross position)M = torque on the shoulderT = P/2 * sin aM = P/2 * L * cos aAssuming the 67kg gymnast has an armlegth of 60cma(deg)---T(N)-----M (Nm)45--------232------13930--------164------17015---------85------1900-----------0-------197The perfect iron cross would place a 197Nm torque on each of the gymnast's shoulders, which is not surprisingly exactly the same as holding a weight half his bodyweight in each hand with arms stretched out to the sides. (33.5kg * 9.81m/s2 * 0.6m = 197 Nm) Link to comment Share on other sites More sharing options...
kbryk Posted January 22, 2008 Share Posted January 22, 2008 Yes, I think Regaman is correct on this one, I'm not to much of a physics savvy person, but I do remember my brother who does know his stuff saying something along the lines of what Regaman has said.One thing I do not completely agree on is e perfect iron cross would place a 197Nm torque on each of the gymnast's shoulders, which is not surprisingly exactly the same as holding a weight half his bodyweight in each hand with arms stretched out to the sides. Simply because holding the Iron Cross is not he same because it is a push force against the rings not a pull force. Link to comment Share on other sites More sharing options...
John Sapinoso Posted January 22, 2008 Author Share Posted January 22, 2008 heh, i thought this was off, because people pull butterflies from hang Link to comment Share on other sites More sharing options...
pistol33 Posted March 4, 2008 Share Posted March 4, 2008 im taking AP Physics right nowthe misconception this article makes is that the tension T generated by the arm is directly out to the side, or only in the x direction or is like the classic tightrope problemin this problem you are to solve for the tension in a tightrope some length, to support a ten pound weight, so that there is no bend in the ropewhen you solve this your answer as h (angle) approches 0 the tension approaches infinityhowever, gymnasts are able to generate a downwards force, or a force in the y direction that undoes thisso basically the misunderstanding is that the gymnast will only be generating force in 1 direction outwards, when in reality he generates in two, which is why that article is so incorrectthe article is correct in the fact that it is impossible to generate a force so great in the x direction that it compensates for the y Link to comment Share on other sites More sharing options...
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