Imagine a ball tethered to the center via a piece of string and consider the ball in motion. Ignore how the ball got into motion. In other words, ignore the forces that provide energy to keep the ball in constant motion. While the ball is in a state of constant circular motion, the centripetal force keeping the ball moving along a circular path passes through the taut string that tethers the ball to the center point of string attachment.
A good analogy would be looking at the club's behaviour after release of PA#2. At that point, no further energy is inputted into the system and the club is freewheeling in space. If the clubhead follows a circular path (like the ball on a string) then there must be a centripetal force passing through the clubshaft and straight left arm to the fulcrum point (left shoulder socket).
However, consider the clubhead's movement before release. If the clubhead follows a circular path, and the clubhead cannot generate energy independently within itself, then it must be responding to energy derived from the golfer's hand movements in space. The same applies to the orbiting ball. If there is no independent energy source within the orbiting ball, then "something" must be supplying the energy to make the ball keep on moving around in a circle. The energy source is the movement of the index finger and thumb holding the end of the string. To make the orbiting ball keep on moving endlessly in a circle, the hand motion must have a twirling-circular quality where the movement of the hand in its inner circle arc of rotation travels at roughly the same rpm (but different surface speed) than the orbiting ball. If the orbiting hand (in its inner circle rotational motion) abruptly slows down, or abruptly speeds up, or moves in a non-circular manner, then it will immediately disrupt the circular motion of the orbiting ball. In other words, to keep the tethering string taut and to allow the tethering string to apply a constant/unwavering centripetal force directed at the center of the circle of the orbiting ball (and orbiting hand), the hand must keep on moving in a circular fashion. That's the analogy I use for the golf swing - when I state that the hand arc motion must have a circular quality. If the hand arc's motion was constantly along a straight line (and in no sense circular) then how could the clubshaft and clubhead ever travel in a circular arc?
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