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Post by dubiousgolfer on Aug 18, 2020 16:38:33 GMT -5
DG, You wrote-: " Look at the diagram below where the NET force (I've not shown it or the MOF effect for the sake of simplicity) is responsible for moving the clubs COM down the dotted line. While this is happening imagine a 'Hand Couple' being applied (ie. the F forces) . Can you see how the 'club' is being 'angularly accelerated' around the COM? The 'Hand Couple' is not moving the COM but responsible for spinning the club around its COM." No! I cannot see that happening. First of all, the club's COM does not move downwards vertically, but at angle dependent on the shape of the hand arc path and the degree of lag. Secondly, I cannot envisage the club being angularly accelerated around the COM because the proximal end of the club is attached to the hands which prevent it from rotating around the COM. The club handle has to go where the hands are going and any angular momentum gained by the club must come from forces operating at the level of the hands, while producing an effect at the level of the left wrist hinge joint. Jeff. That diagram was just to show you how the club can have angular momentum but not move the COM. Yes, I agree with your last paragraph which is why I think it's futile to use inverse dynamics 'hand couple' to try and explain what's happening in a real golfers downswing. This is why I said that DT's 'feel' example was as close as one could get of replicating a 'hand couple' (ie. with soft hands/arms). If one wants to theorise about using a 'hand couple' then one will also have to explain how to biomechanically facilitate the movement of the proximal end of the club in the downswing. DG
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Post by dubiousgolfer on Aug 18, 2020 16:52:07 GMT -5
Hi DG, I agree with your diagram in that the center of mass of the club will try to line up in the direction of the force. What makes this challenging for me is that we have a double pendulum that is not only rotating around an imaginary point on the chest but is also rotating around the hands at some point. To think through a force couple and moment of a force, I visualize a 12" cylindrical bar placed on a table. I push one end of the bar with my hand. My hand acts like a force couple and the push provides the force to rotate the bar around its center of mass (found exactly 6" from the end of the bar). The center of mass does not move. Since I am pushing the extreme end of the bar, the moment of a force is greater than if I were to push the bar at a point 1" from the center of the bar. The moment of a force does not rotate the bar but describes how easy it is to rotate the bar. Does the moment of a force mean something different in the double pendulum model? S Hi S In the 'non-driven DP' there is no 'MOF' because there are no external forces and torques acting on the model- it just rearranges its 'moment of inertia' and the angular momentum (that it already had at the beginning of the downswing) In a 'driven DP' there is an MOF because external forces/torques are acting on the model, just as we've been talking about in the 'inverse dynamics' world , but there is no conservation of angular momentum in the golfer/club system. DG
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Post by dubiousgolfer on Aug 18, 2020 19:05:50 GMT -5
Here is Dr Kwons reply
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The answer to your question is: Yes, the hand couple participate in moving the COM of the club by rotating the club around the mid-hand point.
Please find attached a short piece regarding the MH-centric perspective.
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Now that has thrown another spanner in the works!!!
I've sent him another email to clarify his reply, just in case there is a communication issue. He uses a lot of maths and 'vector cross products' and is also assuming that one is well acquainted with Newtons Laws as it pertains to Torques and angular motion.
I have a feeling he is using a different 'Frame Of Reference' in his maths and that may explain his comment above.
DG
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Post by imperfectgolfer on Aug 18, 2020 22:09:24 GMT -5
DG,
You quoted Kwon as follows-: "The answer to your question is: Yes, the hand couple participate in moving the COM of the club by rotating the club around the mid-hand point."
That's exactly the point that I have repeatedly been trying to make - although I believe that the club is actually rotating around the left wrist joint in the plane of radial => ulnar deviation (if one maintains an intact LAFW/GFLW).
Jeff.
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Post by dubiousgolfer on Aug 18, 2020 22:23:41 GMT -5
Dr Mann
I've copied Dr Kwon's comments and attached document to SMK for a response. Unsure of its relevance to PA#2 release , but Dr Kwon's mathematical equations have factored in the weight of the club causing an Mof across mid-hand-point.
DG
PS. Dr Kwon's document says the following:
"Both the hand couple (M) and the torque produced by the weight of the club participate in rotationally accelerating the club."
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Post by syllogist on Aug 19, 2020 6:08:01 GMT -5
Hi DG,
Where Dr. Kwon replied to you, "Yes, the hand couple participate in moving the COM of the club by rotating the club around the mid-hand point."
It seems to me that the hand couple participated in rotating the club around its handle by causing the club to gain momentum. The COM of the club obviously has to move since the hand couple moves (rotates) the club and the COM is a point on the club. I think what SMK points out is that the COM (and the hands) continuously rotates around an axis of rotation that is a distance away from the club.
I believe that if the torso were considered the force couple, then the mass being rotated is the combination of the arms plus the club.
I'm just taking an "educated" guess.
S
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Post by dubiousgolfer on Aug 19, 2020 8:18:10 GMT -5
Hi S
A hand couple will not move the COM. If you were able to truly apply a pure hand couple on the club, it would only cause it to rotate around the COM . SMK confirms that and so does Dave Tutelman but Dr Kwon seems to disagree (and I'm trying to find out why).
DG
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Post by syllogist on Aug 19, 2020 12:44:17 GMT -5
Hi DG,
You misunderstood my point about the movement of the COM. To explain, at the top of the swing, the COM of the club can be 6 ft. from the ground and, at impact, it is a couple of inches from the ground. Du to the force couple, the COM rotated around a perpendicular axis to the plane located through the vicinity of the sternum. The COM of the club is "orbiting" around the torso.
S
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Post by dubiousgolfer on Aug 19, 2020 13:06:31 GMT -5
I'm going to stick my neck out here and say that the MOF is a 'couple' just like the 'hand couple' and if couples cannot move the COM , then both the MOF and 'Hand Couple' can only 'spin' the club around its COM.
Therefore
(1). Dr Kwon is correct 'mathematically' using a local frame of reference around the Mid-Hand-Point to say the 'hand couples' move the COM around the 'Mid-Hand-Point'. (2). SMK's 'Intro Kinetics' video is misleading (because it seems to imply that the COM is rotating around the application point of the force, but I don't think so) . In fact he says the COM is 'pulled' into alignment by the 'Net Force' components . (3) Because I've spent hours looking at the physics below:
3.1 - Complex motion of any rigid body , in this case the golf club, can be reduced to a 'Force' through its COM and a 'moment of a couple' around its COM (Chasles Theorem)
3.2 - Once you have point 3.1 , there is a requirement to move that 'Force' going through the COM to a new point to reflect a closer reality to where the golfer would apply that force (Mid-Hand-Point). Leave the 'moment of a couple' around COM unchanged until later (ie. 3.4)
3.3 - The method for doing point 3.2 is by using a mathematical technique for replacing the original force (applied to its COM) with an 'equivalent force + couple ' applied 'at/around' the mid-hand point .
**** But a couple applied anywhere on the golf club will not cause the COM to move****
3.4 - We can now move the couple in point 3.1 (ie. originally around the COM) to the mid-hand point because its effect on the golf club remains the same. 3.5 - What we have at the mid-hand-point is a 'Force' + 2 couples (the original in point 3.1 + point 3.3) .
**** But a couple applied anywhere on the golf club will not cause the COM to move****
Question: So why does the 'Net Force' in SMK's 'Intro Kinetics' video show the COM of the club seemingly aligning with the tail vector of the 'Net Force' ? Theoretically , this cannot happen with the 2 'hand couples. It is the 'Net Force' components that are moving the COM to align with the tail end of the vector while the hand couples are spinning the club around its COM.
Dr Mann might think my reasoning is insane and I don't blame him.
DG
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Post by imperfectgolfer on Aug 19, 2020 18:20:14 GMT -5
DG,
You stated-: "Dr Mann might think my reasoning is insane and I don't blame him."
Actually, I think "insane" is not a strong enough adjective to reflect what I really think of your reasoning and I cannot easily think of an adjective that can really reflect my negative opinion regarding your expressed reasoning in that post.
However, as moderator of this golf forum, I will never ban any individual even if they express opinions that I (as moderator) think are nonsensical or "wild". So, please continue to express any opinion that you want to express regarding any topic re: golf swing biomechanics/mechanics/physics.
Jeff.
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Post by dubiousgolfer on Aug 20, 2020 8:20:18 GMT -5
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Post by dubiousgolfer on Aug 20, 2020 8:38:22 GMT -5
Hi DG, Where Dr. Kwon replied to you, "Yes, the hand couple participate in moving the COM of the club by rotating the club around the mid-hand point." It seems to me that the hand couple participated in rotating the club around its handle by causing the club to gain momentum. The COM of the club obviously has to move since the hand couple moves (rotates) the club and the COM is a point on the club. I think what SMK points out is that the COM (and the hands) continuously rotates around an axis of rotation that is a distance away from the club. I believe that if the torso were considered the force couple, then the mass being rotated is the combination of the arms plus the club. I'm just taking an "educated" guess. S Hi S The issue that I am basically perplexed about is SMK's 'Intro Kinetics' video up to 6:01 where he claims a 'Force' (not passing though the COM of the club) will create a 'Moment Of Force' that will move the COM. From a physics standpoint that doesn't make sense to me because a 'Force' applied to an object that is not directed through its COM causes 'translation and rotation' . The translation part is that the COM of the object continues to move in the same 'direction' as the original 'Force' . The rotational part is that the object 'spins' about its COM . It therefore will not move to become aligned with the original 'Force' that was applied to the object. I am claiming (relating to SMK's video) that there is another force/torque that must act on the club (via the hands) to make its COM move and align with the original 'Net Force'. DG
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Post by syllogist on Aug 20, 2020 8:57:18 GMT -5
Hi DG,
I watched SMK's kinetics video again that described the force couple and MOF. This time I paid close attention. :-) I think that the following summary will answer your questions in your previous post.
1) Forces acting on the club were simplified to include "one net force" and "one net couple." Torque due to the "one net force" which SMK referred to as "MOF." The sum of "one net couple" and "MOF" equals the total torque that affects the club.
2) The forces presented are "in-plane." The frame of reference is a plane represented as a circle with the end of the handle always in its center. During the swing of the club, the reference circle moves (not in a rotational sense). The club and thus the clubhead rotate around the rim of the circle during the swing.
3) The couple causes in-plane rotation of the club. The couple is dominant when the motion of the club is slow. The couple is negative during the backswing and becomes positive at shaft vertical to slow the club during the end of the backswing. During the downswing, the couple becomes negative at about shaft horizontal. This is the point where the golfer cannot apply force to increase the angular velocity of the club. (Note that this represents "release.")
4) The MOF is positive during the backswing. The MOF becomes negative at about shaft vertical which causes the club to complete the backswing. The MOF becomes positive at shaft horizontal. It is the MOF that accelerates the COM of the club.
From this summary of the video, one can ascertain that acceleration of the COM of the club occurs during wristcock and wrist release. These motions are due to the MOF and not the couple. One can say that the couple plays a part in the acceleration of the COM of the club because it becomes negative at shaft horizontal, which allows the MOF to take over (and MOF is what accelerates the COM of the club).
S
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Post by dubiousgolfer on Aug 20, 2020 9:07:56 GMT -5
Hi S
I understand what is being said in your post but its the MOF part that doesn't make sense . In his intro video he seems to imply its the 'Net Force' alone that causes the 'MOF' that angularly accelerates the COM of the club. I am saying that doesn't make full sense from a physics perspective and that there must be another force/torque acting (not fully explained in his video).
DG
PS. Think about this a bit more deeply. If the 'Net Force' alone is causing the COM to move in an arc path (via an MOF that it creates), then there must be a radial force (ie. an instantaneous 'Centripetal Force') acting on its COM. For a centripetal force to exist , there must be a torque that causes it to rotate first (a torque creates rotation while the centripetal force sustains it). Therefore, there can't just be a linear 'Net Force' acting through the hands to make the COM move in an arced path, there must also be a 'Torque'.
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Post by syllogist on Aug 20, 2020 10:16:46 GMT -5
Hi DG,
SMK stated that when the net couple becomes negative at shaft parallel in the downswing, the MOF becomes positive and "takes over." I think that the only way to reconcile what SMK stated and your point is to consider that the couple becomes negative, allowing the MOF to become positive, accelerating the COM.
If you think about it, if one were able to continue to have the net couple positive well beyond the release point, the MOF would remain negative and the wrists wouldn't release - which would happen if you were swinging a rod made of balsa wood.
S
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