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In this series of blogposts, on understanding aiki from a bio-mechanical perspective, I have made reference to key aspects of angular momentum and the behaviors of chains -- particularly the bones considered as a chain of linked rods:
While a popular and instructive video, the author's conservation of momentum theory proposed as an explanation is wrong. The correct explanation is an understanding of the effect of DYNAMIC rigidity that occurs when a chain of rods is lifted by a continuous rotation:
[T]he chain is more like a series of short, rigid 'rods', say the authors, who publish their results today in Proceedings of the Royal Society A1. In their model, each rod is made up of three beads and two connectors. The size of a rod corresponds to the number of beads it takes to turn a section of chain back on itself by 180 degrees (it takes six).
Picking up a portion of rod from the pot with an upward force on one of its ends causes two things to happen, says Biggins. It makes the rod lift, but it also causes it to rotate. The end that is not picked up pushes downwards, and the pot provides a reaction force, he says. "The far end of the rod under those two motions actually goes down, and therefore pushes down. And that gives rise to this extra kick from the pot which drives the fountain.