Physics- The Bottom Arm in the Immediate Post Take Off

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Physics- The Bottom Arm in the Immediate Post Take Off

Unread postby willrieffer » Wed Apr 25, 2018 3:33 pm

Physics of The Bottom Arm in the Immediate Post Take Off

The immediate post take off is regulated by the dispersion (not loss) of initial kinetic energy into 3 areas. 1) residual kinetic energy (horizontal veloocity) 2) pole energy and 3) potential energy. The lead arm can play a role in the direction of that dispersion. Since most vault coach/analysts don't consider gravity and the resulting potential energy curve, this relation is often ignored or misunderstood.

Of first consideration is the swing/pendulum of the vaulter in the immediate post take off. Since the pole produces a braking force the resultant activity is that the vaulter as mass will swing forward and more importantly UP unless some other force/resistance is used. If they swing forward/up kinetic energy is transferred into potential energy with a resulting loss in velocity. The equation is KE(kinetic starting) = KE(kinetic residual) + pE(pole energy) + PE(potential energy) which shows that for any increase in height/potential energy there is a corresponding loss in velocity. This lurks behind why there are questions about high angle takeoffs (pre and FTO) as well in that to achieve a reduction in pole braking force by driving pole rotation it takes an investment in converting kinetic energy into potential energy with again a resultant loss in horizontal velocity. So there are TWO braking forces in the early post take off in the vault, the pole, and gravity. It's notable here that vault coaches are often fixated on angular/rotational energy concerns. That moving the Center of Mass out/away from the rotational axis slows a rotating system. This idea is correct, but the physics of this is such that this situation can only SLOW a rotating system and NOT make it stop. What stops (and slows) the vault system is gravity. Think of a moving car meeting a hill (lets ignore friction). The car will slow down and stop and without putting on the brakes roll back down the hill. IF it meets a steeper hill it slows faster. But whether it meets a steep or gradual hill it stops where its initial kinetic energy equals potential energy. If we visualize these two hills, gradual and steep, as triangles they would have the same height but different base lengths where the base length is relative to the cars horizontal velocity where gradual is longer than steeper. This is all relative to the vault in that IF the vaulter does something to turn the potential energy curve up they will lose horizontal velocity. They "make the hill" steeper, turn up too fast, and stall out in gravity.

If we go back to the lead arm it lies in a triangle with top arm, and the pole between the two hands. And its one of the few places/ways the vaulter can effect the placement of the CoM/potential energy curve and thus regulate steepness of the curve(hill). Putting force/pressure on with the bottom arm can regulate the swing slowing or stopping the shoulders/hips/CoM from pundularly rotating forward and UP and the resultant premature transfer of kinetic energy into potential energy.

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