Pivot center

[Jeff]

Yoda - that's a very good question.

Consider this diagram - from Wikipedia.



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?

Jeff.

[12 piece bucket]

That a real golf swing in that add?

Certainly no hands slowing going into impact there...

(bobby jones's driver swing either for that matter)

Mandrin!!! What's up man!

[12 piece bucket]

12PB

I agree that SG is a great ball striker. However, I would not automatically conclude that it is primarily due to his having a leftwards-centered backswing action. He has many other great golf swing skiils.

I also don't mind if you prefer a leftwards centered backswing action. Sam Snead and Mike Bennett and Aaron Baddeley (new swing) use that type of backswing action to great effect.

However, other great golfers - Tiger Woods, Ben Hogan, Anthony Kim - use a rightwards-centered backswing action with great results.

Jeff.

Not ready to let you put Hogan in that list . . . we can find swings indicating otherwise . . . Hogan did EVERYTHING.

[Yoda]

Yoda - you wrote-:

"As I've already explained twice in this thread, the Clubshaft is responding to the Sweetspot's Centrifugal Line of Pull. It is NOT creating it (as you continue to insist). In fact, this 'bass-ackwards' thinking is your fundamental error.

The Sweetspot is orbiting, and in so doing, is creating a Centrifugal Force (Clubhead Inertia resisting a change in its direction). The Clubshaft is supplying the Centripetal Force that enables that orbit."

I appreciate your input, but I have a different explanation for observed events.

I agree that the clubshaft is not creating the centrifugal line of pull. (The clubshaft is creating another force - which I will explain later).

I think that you are wrong to state the clubshaft is supplying the centripetal force. The CP force is created by the hands holding the clubshaft, and the clubshaft is simply the connecting structure between the clubhead and the hands.

Here is my explanation. I created this model.



Imagine that a person is twirling a ball (attached to a piece of string) around his head. Imagine that he grasps the string between his right index finger and his thumb in a pincer grip and imagine that he holds his right hand vertically above his head and moves his right hand in a constant small circular motion. That circular motion is represented by the small inner circle.

Imagine that the string length is 18" and the red ball is attached to the end of the string.

The ball will travel in a constant circular path (represented by positions 1 and 2 and 3). The CP pull is exerted by the hands and the pull is along the length of the string and the CP force is at right angles to the ball position (right angles to a tangent at the circumference) at any point in time. The ball wants to travel in a straight line (at a 90 degree tangent to the circumference of the circle) at every moment in time, but it is prevented from that action by the CP force that is directed towards the center of the circle. The string transmits the CP pulling force from the ball to the hands (inwards pull towards the center). The string doesn't create the CP force. The ball travels in a perfect circular motion because the CP force (directed inwards towards the center via the string) balances the CF force (hypothetical outward -directed force).

[Bold emphasis by Yoda.]

Jeff,

You have made two statements that are in direct conflict:

Behind Door #1:

"The CP pull is exerted by the hands and the pull is along the length of the string . . . "

Behind Door #2:

"The string transmits the CP pulling force from the ball to the hands (inwards pull towards the center)."

So which is it, Jeff?

Does the hand -- it really should be singular as defined by your model -- create the CP pull (hand to ball) or does the ball create it (ball to hand).

Hint:

We need a third door.

We are concerned here with a mass orbiting about an axis of rotation and exhibiting both a centripetal force (a force 'seeking the center' and whose origin we are now debating) and a centrifugal force (a force 'fleeing the center' and that is reactive to the centripetal force). An orbiting mass constantly accelerates towards its axis of rotation. This centripetal acceleration demands an equal and opposite force that opposes the centripetal force and creates an outward centrifugal reaction directed away from the axis.

In your model, the ball is the orbiting mass and the hand is the axis of rotation. The hand is not the centripetal force (as you incorrectly state). It is, after all, the axis! Instead, the centripetal force (acceleration) is exerted on the ball by another object (in your model, the string). Then, the centrifugal reaction is exerted by the ball on the object that originated the centripetal acceleration (the string).

The string, Jeff.

The string.

[Jeff]

Yoda - I think that this debate is getting unnecessarily complicated and unproductive.

I think of it this way.

The orbiting ball receives all its energy via the string. The source of the energy is the hand moving in a circular manner. The energy travels outwards from the hand to the orbiting ball- along the string. That energy is required to perform two roles - i) keep the ball moving at a constant speed and ii) keep the ball moving in a circle at its constant surface speed.

When a orbiting object travels at a constant speed it may not need much energy to keep it moving at a constant speed (if there is little frictional resistance to its movement in space), but it requires constant energy input to constantly change direction (in order to move in a circular path). In other words, although the orbiting ball is traveling at a constant surface speed, it needs a constant source of energy to centripetally accelerate (accelerate towards its center of rotation). That energy comes from the hands and it is transmitted via the connecting string.

If you understand that viewpoint - then door 1 is correct.

Door 1

Behind Door #1:

"The CP pull is exerted by the hands and the pull is along the length of the string . . . "".

However, one can look at this orbiting ball scenario from a different perspective, and look at the scenario from the orbiting ball's perspective. Then door 2 applies.

Door 2

Behind Door #2:

"The string transmits the CP pulling force from the ball to the hands (inwards pull towards the center)."

The ball experiences a centripetal force that pulls it towards its center of rotation. The pull is in the direction of centripetal acceleration. The ball doesn't know where the CP force is coming from - it only knows that it is being pulled by the string, and it happily thinks that the string is providing a CP force that keeps it a state of constant centripetal acceleration.

However, ultimately the centripetal force that keeps the orbiting ball traveling in a circle is derived from the orbiting hand's movement in space, and the energy must travel from the hands to the ball - travels outwards along the string. Some of that energy makes the ball move, and some of that energy supplies a centripetal force that conceptually travels back down the string and keeps the orbiting ball from flying into space. I find it meaningless to think of the string providing that CP force - because the string doesn't really create energy. It only allows part of the energy created by the orbiting hands to be translated into a CP force that pulls the orbiting ball towards the center.

I actually try to avoid using the terms "centripetal" and "centrifugal" in my thinking about the golf swing. It gets way too semantic and too complicated. The idea of a centrifugal force is merely a conceptual idea used to see the CP force as being balanced by an equal and opposite force. Many people state that it not really a force - because it cannot exist alone. I am sympathetic to the idea that it is not useful to conjure up the idea of a CF working in the opposite direction to a CP force.

I much prefer to think in terms of vectors of movement and force, and not use the terms CP force or CF force if I can avoid the terms. That's why I like nmgolfers explanation of the release phenomenon.

See - http://perfectgolfswingreview.net/Ne...%20Science.htm

When I think of the golf swing, I never think of a rotational center. I simply see different body parts moving in 3-D space, and I simply think of the forces that move those body parts and their vector of movement. I think the same way with respect to the golf club. The golf club is only moved by pull-forces or push-forces exerted at grip level.

Jeff.

[Jeff]

Here is another perspective on the orbiting ball situation.

I prefer not to even think of CP forces and/or CF forces.

I simply know that the hand moves in a small circle, and that the orbiting ball rotates in a larger circle, and that they rotate at the same rpm. I know that they are connected together by a piece of string. I know that the energy source is the hands (rotating in space). In that sense, the hands are pulling the ball via the string. I know that the orbiting ball follows a circular path because the hands follow a circular path, and that the string will remain taut if the hands move smoothly and at an even pace. I know that the whole system breaks down if the hands suddenly accelerate, or suddenly decelerate, or if the hands move in a straight line.

I don't need to invoke concepts of CP force or CF force to understand what is happening in this orbiting ball situation.

The same applies to the pivot. I don't need to think of a pivot center or a pivot axis. I simply think of how the body moves in 3-D space.

Jeff.

[Yoda]

Yoda - I think that this debate is getting unnecessarily complicated and unproductive.

Jeff . . . this praise . . . coming from you . . . I'm not worthy. But thanks anyway . . . you've made my day!

I think of it this way.

The orbiting ball receives all its energy via the string. The source of the energy is the hand moving in a circular manner. The energy travels outwards from the hand to the orbiting ball- along the string. That energy is required to perform two roles - i) keep the ball moving at a constant speed and ii) keep the ball moving in a circle at its constant surface speed.

When a orbiting object travels at a constant speed it may not need much energy to keep it moving at a constant speed (if there is little frictional resistance to its movement in space), but it requires constant energy input to constantly change direction (in order to move in a circular path). In other words, although the orbiting ball is traveling at a constant surface speed, it needs a constant source of energy to centripetally accelerate (accelerate towards its center of rotation). That energy comes from the hands and it is transmitted via the connecting string.

If you understand that viewpoint - then door 1 is correct.

Door 1

Behind Door #1:

"The CP pull is exerted by the hands and the pull is along the length of the string . . . "".

Buzzer Sound: EEEEEEEEEEEEEEEEEEEENNNNNNNNNNNNNNNHHHHH!!!!

Sorry, Jeff. Door #1 is not the correct answer. It never has been the correct answer, and it never will be the correct answer, despite your heroic attempts to obsolete every physics text ever written. Simply put, the axis of rotation is not the centripetal force. Try again . . .

However, one can look at this orbiting ball scenario from a different perspective, and look at the scenario from the orbiting ball's perspective. Then door 2 applies.

Door 2

Behind Door #2:

"The string transmits the CP pulling force from the ball to the hands (inwards pull towards the center)."

The ball experiences a centripetal force that pulls it towards its center of rotation. The pull is in the direction of centripetal acceleration. The ball doesn't know where the CP force is coming from - it only knows that it is being pulled by the string, and it happily thinks that the string is providing a CP force that keeps it a state of constant centripetal acceleration.

However, ultimately the centripetal force that keeps the orbiting ball traveling in a circle is derived from the orbiting hand's movement in space, and the energy must travel from the hands to the ball - travels outwards along the string. Some of that energy makes the ball move, and some of that energy supplies a centripetal force that conceptually travels back down the string and keeps the orbiting ball from flying into space. I find it meaningless to think of the string providing that CP force - because the string doesn't really create energy. It only allows part of the energy created by the orbiting hands to be translated into a CP force that pulls the orbiting ball towards the center.

Buzzer Sound: EEEEEEEEEEEEEEEEEEEENNNNNNNNNNNNNNNHHHHH!!!!

Sorry, Jeff. Door #2 is not the correct answer. The string doesn't transmit the centripetal force -- as you continue to maintain and would have us believe -- the string is the force! Try again . . .

I actually try to avoid using the terms "centripetal" and "centrifugal" in my thinking about the golf swing. It gets way too semantic and too complicated. The idea of a centrifugal force is merely a conceptual idea used to see the CP force as being balanced by an equal and opposite force. Many people state that it not really a force - because it cannot exist alone. I am sympathetic to the idea that it is not useful to conjure up the idea of a CF working in the opposite direction to a CP force.

I much prefer to think in terms of vectors of movement and force, and not use the terms CP force or CF force if I can avoid the terms. That's why I like nmgolfers explanation of the release phenomenon.

See - http://perfectgolfswingreview.net/Ne...%20Science.htm

Buzzer Sound: EEEEEEEEEEEEEEEEEEEENNNNNNNNNNNNNNNHHHHH!!!!

Sorry, Jeff. Attempting to divert the argument is poor tactics and just plain bad manners. Besides, the usual smoke and mirrors won't work here. Try again . . .

When I think of the golf swing, I never think of a rotational center.

Bad idea.

I simply see different body parts moving in 3-D space, and I simply think of the forces that move those body parts and their vector of movement. I think the same way with respect to the golf club. The golf club is only moved by pull-forces or push-forces exerted at grip level.

Yes, I can see how such a simple approach might work. After all, this whole centripetal-centrifugal thing is way too complicated. Especially when we're confronted with the truths taught in Physics 101. Best that we redefine the way the world works, right Jeff?

[Jeff]

mb6606

You sent me a link to this video of Jason Zuback's swing which has a number of slow mo sections.

I made the following series of images of his downswing



First of all, regarding the clubshaft. I see this problem of double images frequently - even when examining slow mo videos. This makes it difficult to know what is real versus camera distortion. Image 4 shows two images - one of the images has forwards shaft lean. Image 5 shows double images at impact with the shaft bent back in the one image, which would be expected post-impact.

I therefore do not know whether Jamie Sadlowski's forwardly bent shaft post-impact is "real" (possibly due to using a more flexible shaft) or due to camera distortion.

Secondly, look at Jason's swing. He has a centralised pivot action, and he reverse pivots slightly with a large hip turn (like Sam Snead) by the end-backswing - image 1. Note how he then has to reverse his spine tilt in the downswing so that he acquires a rightwards tilt-secondary axis tilt in the early downswing - image 3. I don't think that it is a biomechnaically efficient action. Note how he loses his clubhead lag angle - image 3. I think that it is due to the fact that he has to throw his upper torso and therefore arms backwards (away from the target) at the start of the downswing to reverse the spine tilt. The reversal of hand movement (going backwards away from the target) predisposes to an early release.

Here is another example of that phenomenon in an excellent golfer.



In the first swing, he reverse pivots slightly. Note his loss of clubhead lag angle. In his second swing I think he has a better pivot action with a reverse-K look, and a much better retention of his clubhead lag angle in the early downswing.

That's why I prefer Jamie's swing over Jason's swing. Jamie has a traditional swing with a reverse-K look at the end of his backswing, and he retains his clubhead lag angle well into the downswing.

Jeff.

[Jeff]

Yoda - I have been deciding whether to respond to your last post. Your tone is demeaning and ill-conducive to an ongoing debate. Why can't you simply state your disagreements without pontificatiing, and implying that you are the final arbiter of the "truth"?

I don't think that you understand my perspective.

I will rephrase it.

The hand's movement is the source of energy that gets the ball to move in space. The string transmits that energy to the ball by means of a continuously taut string - any hand movement in a circular manner causes the ball to move in a circular manner (at the same rpm but along a wider circle of greater radius). The energy imparted to the ball gets the ball to perform two actions - i) acquire a surface speed of a finite value; and ii) centripetally accelerate - which merely represents a constant change in direction so that the ball follows a circular path.

The string is continously taut. That is necessary to counteract the tendency of the ball to fly away (due to centrifugal forces). Therefore, you are correct to state that the taut string represents the centripetal pull towards the center to counteract the tendency of the ball to fly away in a straight line direction at a tangent to the circle. However, the centripetal pull is not created by the string - the string is merely reactive to the dynamic situation of an orbiting ball that has acquired enough energy to fly away unless the string remains taut and provides a centripetal pull towards the center.

My personal perspective is focused on the following facts.

1) The source of energy for the orbiting ball is the continuous movement of the hand in a circular motion.
2) The hand constantly pulls the orbiting ball via a taut string - so that the orbiting ball revolves at the same rpm as the hand.
3) The ball acquires motional energy from the hand via the taut string, and this energy includes a centrifugal tendency which requires an equal counteracting centripetal pull by the taut string to ensure that the orbiting ball moves in a circular orbit.
4) It is true that the taut string is a constant reflection of the centripetal pull towards the center. However, the string doesn't independently create that centripetal force. The centripetal force only becomes necessarily operant because the ball has acquired motional energy from the motion of the hand that would cause it to fly off into space unless there was a centripetal pull from the taut string to counteract/equalize that centrifugal tendency.

Jeff.

[Yoda]

Yoda - I have been deciding whether to respond to your last post. Your tone is demeaning and ill-conducive to an ongoing debate. Why can't you simply state your disagreements without pontificatiing, and implying that you are the final arbiter of the "truth"?

I don't think that you understand my perspective.

Yes, Jeff, I suppose my post was a bit much. But, you know what? For some reason, you inspire that in me!

Oh, and I completely understand your perspective on centripetal force, both from the 'hand's' point of view (Door #1) and the 'ball's' point of view (Door #2). Goodness knows you've spent enough time educating me. And your writings do indicate that you understand the essence of the forces involved. Yet, you then totally deep-end and restate to your own end basic laws of physics that have been accepted since the time Isaac Newton wrote his Philosophiæ Naturalis Principia Mathematica (1687). You put up a lot of good stuff, Jeff, but man, you make it difficult to sit idly by as you reinvent concepts that have served mankind well for centuries.

To me, that's arrogance.

And when I see it, I call it.

Then again, that's why I've created this Golf By Jeff Forum and given you domain. Here you can pick things apart to your heart's content and enjoy relative freedom from my comment. But, it's also why I've put a 'caveat emptor' sign at the front door and stated that your presence here does not imply endorsement of your opinions by LBG.