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Post by gmbtempe on Jul 16, 2012 16:48:35 GMT -5
I am not understanding how the change of direction is Parametric Accelration?
I thought it was the tightening spiral, is that caused by change of direction?
Does this video make sense?
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Post by imperfectgolfer on Jul 16, 2012 17:28:42 GMT -5
He is misusing the term "parametric acceleration" (because it applies to the club and not the pelvis) although it is correct to infer that if a golfer creates a greater positive O factor at impact, it can contribute to decreasing the hand arc path radius in the late downswing, and that the shortening hand arc path radius in the late downswing can consequently produce parametric acceleration of the club. Other biomechanical factors that increase the degree of parametric acceleration in the late downswing include i) getting a higher elevation of the left shoulder (through the "throwing a drunk off one's back" swing action) and ii) getting up onto one's toes through impact.
Jeff.
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Post by virtuoso on Jul 17, 2012 12:48:40 GMT -5
jeff, does not the hammer thrower only use parametric acceleration because the force is completely radial (normal/straight pulling force) for the last few revolutions of the throwing motion?
...but on second thought, it couldn't be completely radial could it; there would have to be a small amount of tangential? Otherwise the hammer would quit accelerating along the arc?
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Post by imperfectgolfer on Jul 17, 2012 13:32:35 GMT -5
I don't think that the hammer-thrower ever uses parametric acceleration because the distance between his hands (and therefore handle) and the upper swing center (midpoint between his shoulder sockets in front of the spine) never decreases in the last few revolutions of his pre-release phase. I think that he is applying two forces during the pre-release phase of his throw - i) a tangential force that moves the hands along the circumference of a circular path (of a certain radius) around his rotating body and that causes the hammer-ball to rotate at the same rpm so that the ball acquires a finite level of tangential momentum in a direction that is at a tangent to the circular path and ii) a CP-force (normal force) that resists the CF-loading pull (outward radial pull) of the hammer-ball and that normal force keeps the hand arc path radius, and therefore hammer-ball path radius, constant during the last few revolutions of the pre-release phase of the throw.
Jeff.
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Post by gmbtempe on Jul 17, 2012 14:34:27 GMT -5
This is my understanding as well Jeff, for PA to happen the radius would have to shorten.
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Post by virtuoso on Jul 17, 2012 14:52:16 GMT -5
Ok guys, thanks, gotcha. The radius has to shorten, ie, the hammer thrower can't just step back as he pulls and displace the entire unchanged radius.
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Post by gmbtempe on Jul 17, 2012 15:04:01 GMT -5
Ok guys, thanks, gotcha. The radius has to shorten, ie, the hammer thrower can't just step back as he pulls and displace the entire unchanged radius. Hmmm, I wonder if its not more complex than just the radius. If one said the entire body, arm, clubshaft, etc was one unit, then if the entire unit shifts it would change the radius which could complete the equation.
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Post by virtuoso on Jul 17, 2012 15:09:02 GMT -5
yes Greg, that's where I'm getting confused, but to be fair, I haven't read muira stuff. I've just browsed the forums. I guess I should do my research and go to the source.
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Post by imperfectgolfer on Jul 17, 2012 18:27:44 GMT -5
Virtuoso, Yes - he can step back but that doesn't change the radius because he is moving the fulcrum point backwards to the same degree as he moves the hammer-ball backwards. However, he can also apply parametric acceleration if he also simultaneously moves the fulcrum point upwards by simultaneously standing up more and changing his spinal bend inclination. That's how parametric acceleration is applied - it is due to lifting-up of the fulcrum point via some biomechanical mechanism. Here is a diagram from the Miura paper. The actuator vertically pulls up the torque motor in the late downswing (between P6.5-P7) according to the speed of that "velocity of actuator" curve graph. Jeff.
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Post by virtuoso on Jul 18, 2012 10:19:18 GMT -5
good stuff Jeff, thanks.
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