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Post by dubiousgolfer on Aug 28, 2020 17:32:07 GMT -5
Here is the answer from Sasho MacKenzie to my questions below:
1st Question: Am I correct that the only forces that can move the COM of the club is the Net force (Linear) and its MOF (Torque) ? The MOF will rotate the COM to align with the tail end of the force vector and that it will rotate around the point of application of that force (just like the screen image below from your 'Intro Kinetics' video)? I must admit I am confused because I posed the same (earlier email) question to Dr Kwon and he has replied back to me below:
"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" But I think his comment and his short piece 'maths' document (I've attached a copy) relates to the COM movement from the 'Mid-Hand Point' perspective. -------------------------------------------------------------------------------
Answer:
By definition, all the forces cancel out in a couple. There is zero net force generated by a couple. A couple does not change the linear motion of the CoM. Perhaps Dr. Kwon has a different meaning for his mid hands couple.
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Question 2: Further to my email above , I plotted the movement of the COM in your 'Intro to club kinetics' video and it showed movement in a straight line and no rotation around the grip handle (see image below ). Therefore I do not understand how there can be a MOF (Torque) acting on the COM that can rotate it to align with the tail end of the linear force.
Am I misinterpreting something here?
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Answer:
The powerpoint animation does not necessarily follow the laws of physics. It is not a simulation. Maybe it does by chance…I’m not sure. It’s just meant to provide a rough visual for the words I’ve written and spoken. If I drew a circle by hand, then you shouldn’t be distracted by the fact that the circle looks like an oval’ish. A force will tend to accelerate the CoM in the direction of the force. It will also tend to rotate the object if the force doesn’t pass through the CoM. In the animation, the CoM moves down and to the left…the force points down and to the left. Qualitatively that makes sense. The club rotates in a direction that has the CoM line-up with the trailing end of the force vector. That makes sense.
Torques act on the club not on the CoM. ---------------------------------------------------------------------
I therefore regard the video as an attempt to provide a pragmatic way of trying to describe the kinetics of the golf swing (even if it is scientifically flawed).
DG
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Post by imperfectgolfer on Aug 29, 2020 10:55:00 GMT -5
Dr Mann "The net force being applied by the moving left hand between P6 and impact actually decreases because left hand speed slows down by 20-25% between P6 and P7."I cannot see that happening in SMK's video below (unless I am misinterpreting something). Look at 15:09 for the P6 position and the purple arrow showing 'F NET' force = 375.3N. Then notice that is progressively increases to 456.3N around P6.5 and then drops slightly to 444N by impact (golfer probably peaked his clubhead speed just before impact). Doesn't that suggest that a larger FNET force is being applied even though the hand speed is slowing? DG I think that you should ask SMK why the net force increases from 375 at P6 to 456 at P6.5 even though left hand speed is decreasing significantly during that time period. Jeff.
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Post by dubiousgolfer on Aug 29, 2020 21:17:28 GMT -5
Dr Kwon's graph explains why the net force is large and that's because the normal force component increases at a greater rate than the tangential force decreases. Both Dr Kwon graphs (if you added the green and blue graph values to get the net force) and SMK video also show a slight drop about 17 msecs before impact. If the tangential force is dropping , maybe that explains why the hands are slowing in the targetwards direction. DG
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Post by dubiousgolfer on Sept 8, 2020 16:20:35 GMT -5
Here is Dr Kwons reply to my email sent several days ago: question asked: When you say the club is rotating and has angular momentum , do you mean about the clubs 'centre of mass'? Am I therefore correct in assuming that : Mmh -is the hands exerted 'moment couple' - which means the club is rotating about its COM (rch x Fmh) - is the MH Force 'moment couple' - which means the club is rotating about its COM
------------------------------------------------------------ Dr Kwon reply:
Sorry for this tardy reply! Our fall semester started and I am struggling with a new graduate course that I picked up this semester.
In general, when I say a body has angular momentum, it is about the center of mass. The COM perspective is the easiest to use.
The net mid-hand torque is the net torque generated by the hand forces acting on the grip and it is a free moment, meaning where you draw the moment vector does not matter. So although the mid-hand torque is computed from the hand forces about the mid-hand point, once it is expressed as a torque, it is a free torque and you can say it rotates the club about the COM. So you are right! I personally don’t like calling the mid-hand torque as couple, as the hand forces are not the same in magnitude and opposite in direction. It is just the net torque generated by the hand forces.
Cheers, Young-Hoo Kwon, Ph.D.
Biomechanics Laboratory Texas Woman’s University
P.O. Box 425647 Denton, TX 76204-5647, USA
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I don't agree with Dr Kwon's last sentence about not calling the mid-hand torque a couple. It IS a couple using the 'inverse dynamics' mathematics (and even in his own equations) but obviously in a real life golf swing the hand forces might not be exactly equal and opposite, therefore the word 'mid-hand-torque' might be more appropriate.
Further a 'moment of a couple' is a free vector not a 'torque' (which is a 'moment of force' and regarded as a 'Force' applied at a distance from a fixed point of rotation).
DG
PS. Just thought I'd add this further comment.
If Dr Kwon is confirming angular momentum is basically around the COM , then any inverse dynamic graphs showing angular momentum trends will not match the angular momentum on the kinematic sequence graphs . The measurements are being made on 2 completely different dynamic motions of the club in space.
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Post by dubiousgolfer on Jul 27, 2021 20:32:48 GMT -5
Dr Mann I've just read an old kinematic sequence article by Phil Cheetham and it helped me realise something about these graphs of Rory and JB Holmes sgpl.ch/Dokumente/Article-KinematicSequenceTransitionDownswing.pdfJB Holmes blue line (his arm) is a lot higher than the brown line (the club) before they cross . Doesn't that mean his PA2 angle was actually decreasing more than Rory before release? So JBH PA#2 loading was better than Rory which explains why his clubhead speed was slightly larger albeit with less arm speed. DG
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Post by imperfectgolfer on Jul 27, 2021 21:16:25 GMT -5
I don't really know what it means when JB Holmes' blue line slope is slightly steeper than the brown line slope before they cross - and I am not sure that it can be correlated with a decrease in PA#2 angle. I can see no evidence (when looking at his swing video) that his clubhead lag angle changes before he releases PA#2.
Jeff.
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Post by dubiousgolfer on Jul 28, 2021 6:40:02 GMT -5
Dr Mann If they were aligned on top of each other (or at the moment they cross each other) , that would mean the lead arm and club were rotating together like a fixed rigid object (ie. fixed angle between lead arm and club shaft). If the blue line is above the brown line and the distance between them increases (for any moment in time during the downswing) that means the angle between the lead arm and club shaft is decreasing. If the blue line is above the brown line and the distance between them decreases (for any moment in time during the downswing) that means the club shaft rotation is catching up with the lead arm rotation . This means the angle between the lead arm and club shaft is increasing which signifies a casting movement. This is what Phil Cheetham says in his document: ---------------------------- Let’s put three examples side by side and examine the relationships of each of the four curves during the downswing phase. From the graphs above it can be seen that “Pro” has relatively smooth accelerations and decelerations with each subsequent segment’s speed peaking higher and later than the previous one. Notice also that the club’s speed peaks at impact and in fact is still going up at impact; this means that the club is still accelerating when it hits the ball. Amateur 1 shows lower speeds; poorer accelerations and decelerations; and the peaks are out of sequence with the arm peaking before the thorax. Amateur 2 has no decelerations of the pelvis and thorax before impact, but has a little deceleration of the arm (followed by a re-acceleration before impact). Both amateurs cast the club and it races ahead of the other body segments; this is evident from the fact that the brown curve is on top of the other curves. This casting action causes the club to peak early and even decelerate a little before impact loosing speed and power. In contrast notice the Pro curves; near the middle portion of the graph the arm (blue) is actually on top of the club (brown). This means that the wrist is cocking more just before release. This action will tend to allow more energy to be supplied to club release from the efficient use of the “stretch-shorten cycle” of the forearm muscles. Since the graphs are scaled equally you can see that the club speed of the pro at impact is higher than that of both amateurs. ------------------------ Although I don't think the difference in clubhead speed is solely due to some stretch-shorten cycle in the forearm muscles . I would suspect that the Pro creates a larger 'In Plane Mof' than the amateurs. This may also explain the residual positive hand couple that we see in some of the inverse dynamic graphs while the 'In Plane MOF ' becomes positive. What that residual positive hand couple may represent is the stopping action of the 'decrease' in the PA2 angle (increasing lead wrist-cock) just before release. Could it be that JBH is using PP1 pressure force differently to increase left hand wrist cock more than Rory in the early downswing and just before release? It could be that his PP1 pressure force is more across the club and that will increase the negative MOF in the early downswing and decrease the PA2 angle . Rory might still be applying PP1 in a more tangential direction to the lead hand path, therefore assisting the left hand/arm speed but creating less negative MOF and larger PA2 angle before release. DG
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Post by imperfectgolfer on Jul 28, 2021 11:06:41 GMT -5
DG, Theoretically, the blue graph being higher than the brown graph would suggest the possibility of float loading and an increase in the clubhead lag angle. However, I can see no significant change in JB Holmes' lag angle between P4 (image 1) => P4.5 (image 2) => P5 (image 3) in the following capture images. We have no scientifically reliable data from instrumented grip studies on the amount of push-pressure being exerted by the trail hand against PP#1, or against the aft side of the club handle at PP#3, between P4 => P5.5 in pro golfers - so any hypothesizing is of no real practical value.
I would love to know if there are differences between Rory McIlroy and JB Holmes - but I do not expect that we will get any scientifically conclusive data from golf researchers in the near future.
Jeff.
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Post by dubiousgolfer on Jul 28, 2021 19:00:32 GMT -5
Dr Mann
Yes, there doesn't seem to be any significant change in the PA2 angle in the early downswing and maybe Phil Cheetham might not be wholly correct saying his 'Pro' kinematic graph demonstrates an increase in lead 'Wrist Cock' (which I assume must be radial deviation). Maybe he has presumed club shaft lag has to be in the plane of lead wrist 'radial <-> ulnar' deviation.
Yes, agreed, no point hypothesising .
DG
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Post by dubiousgolfer on Jul 28, 2021 21:12:41 GMT -5
Dr Mann
This is what Jon Sinclair has told me regarding JBH and Rory from his TPI database.
"JB has for sure more wrist set angle. I am not sure what you consider the release but Rory loses wrist set angle all the way from the top and JB gains.
At shaft parallel JB has about 18 degrees more. "
DG
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Post by imperfectgolfer on Jul 28, 2021 22:23:48 GMT -5
Dr Mann This is what Jon Sinclair has told me regarding JBH and Rory from his TPI database. "JB has for sure more wrist set angle. I am not sure what you consider the release but Rory loses wrist set angle all the way from the top and JB gains. At shaft parallel JB has about 18 degrees more. " DG Interesting! That may account for JB Holmes' ability to generate a faster release of PA#2 between P6 => P7. Jeff.
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Post by dubiousgolfer on Jul 29, 2021 6:30:59 GMT -5
I'm not sure what JS said matches the kinematic graph of Rory where the brown graph is above his blue arm in the early downswing but the distance between them is decreasing and they almost align on top of each other before release. If Phil Cheethams interpretation of the kinematic graphs are correct, it seems that Rory casts at a decreasing rate until release. I know this is hypothesising again , but maybe Rory doesn't use enough PP1 (above or at the coupling point) in the early downswing but then gradually increases it as he moves to point of release. The PP1 (above or at the coupling point) is responsible for assisting negative MOF in the early downswing (to help retain lag). Looking at Jordan Spieth kinematic graph , does it suggest that he is a caster from P4 onwards? DG
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Post by imperfectgolfer on Jul 29, 2021 17:51:28 GMT -5
DG,
If the brown graph being higher than the blue graph in the early downswing is defined as casting, then Jordan Spieth's 3D graph does show casting.
Jeff.
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Post by dubiousgolfer on Jul 29, 2021 18:37:28 GMT -5
DG, If the brown graph being higher than the blue graph in the early downswing is defined as casting, then Jordan Spieth's 3D graph does show casting. Jeff. Many thanks Dr Mann I've looked at these types of graphs for years and now I feel I'm starting to understand them better. DG PS. The Corey Pavin graph is strange in the sense that he is casting at an accelerating rate in the early-mid downswing but then his lead arm accelerates even more than his club maybe in the mid-downswing (ie. the brown and blue graphs get closer together). Does that mean that he has dynamically decreased the rate at which the PA2 angle is increasing or is there an increase in wrist cock? I've asked Dr Phil Cheetham as I am unsure. Also , I shouldn't be using the term cast as that implies a swing fault and active wrist torque . I wouldn't know if JS or CP are using active wrist torque , it could be that the 'In-Plane MOF' is causing the PA#2 release very early in their downswings.
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Post by dubiousgolfer on Jul 31, 2021 19:32:50 GMT -5
Dr Mann One thing is confusing about those PGA pro kinematic graphs in the previous posts. If any of those golfers were DH'ers , the blue and brown graphs would be aligned on top of each other post impact (ie. the lead arm and club shaft would be moving at the same angular velocity from P7-P7.2). Could this be a data sampling frequency issue? See extract from your downswing chapter. Image 1 is at impact, image 2 is at P7.2 and image 3 is at P7.4. Image 4 is a composite image showing the targetwards motion of her left hand and clubshaft/clubhead between P7 and P7.4. The accompanying diagram depicts her left shoulder socket in black, her left arm in red and her clubshaft in green. The blue dot represents her left hand and the orange dot represents the clubhead. Note how Kelli Oride keeps the back of her left hand (GFLW) continuously facing the target between P7 and P7.4 and there is very little counterclockwise rotation of her left hand happening between P7 and P7.4. Also, note that her left hand (represented by the blue dot in the diagram) moves targetwards at an angular velocity that perfectly matches the angular velocity of the clubhead (orange dot) and that action allows Kelli Oride to keep her LFFW intact, and it thereby prevents the clubshaft from bypassing her left arm (from an angular rotational perspective) between P7 => P7.4. That phenomenon defines a DH-hand release action and it allows a golfer to more reliably keep the clubface square to the clubhead arc during that early followthrough time period. DG PS. Those kinematic graphs were taken by the same AMM system that takes only 240 measurements per second . So we are talking again about 1-3 data points from P7-P7.4 .
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