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Post by syllogist on Aug 15, 2020 10:14:28 GMT -5
In the case of a force couple, this might be one instance where it's worth thinking aloud to show the thinker's stupidity.
In his article, Dr. Grober speculated on why the net force couple turns negative near impact. He concluded that it is likely that there are multiple factors and not just one considering the calculated hand forces. One factor that he speculated on involved "the hands cannot keep up with the release of the club."
What does the phrase in quotations actually mean? I think from a practical standpoint it means that they decelerate nearing impact. Why do they decelerate? I think that it is because as the downswing is underway, the combined weight of the club and arms becomes "effectively" greater as they move away from their axis of rotation. There is continuously increasing momentum of the arms and club. A small part of this effective weight is from aerodynamic drag on the clubhead. The greater the force applied during the downswing, the greater is the momentum of the arms and club.
Per Dr. Grober, because the negative force couple increases the radius of the path of the clubhead, "It would be quite spectacular if golfers (tour pros) have learned to harness this natural drag to help them to hit the ball straight."
It seems then that the only way to use the hand force couple to alter the trade off between distance (reduce the radius of clubhead travel) and direction (increase the radius of clubhead travel) is to alter the duration of pivot acceleration.
S
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Post by dubiousgolfer on Aug 15, 2020 11:27:39 GMT -5
Hi S I don't find Dr Grober's article very convincing . He actually says the following which made me even more doubtful. "However, it is interesting to point out that the deceleration of the hands and the orientation of the force at impact highlighted in this paper is reminiscent of an approach to training the golf swing named the ‘Rotor Method’ that was pioneered by Nichols in the 1970s and recently demonstrated in a video by Malaska " I'll need to try and find more information about this 'Rotor Method'. DG Hey presto here it is: www.angelfire.com/realm/moetown/mandrin/golf/Rotor_Method.html
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Post by imperfectgolfer on Aug 15, 2020 11:36:38 GMT -5
DG - whether you are correct in your calculation of the work done by the hand couple - as measured by the rotation of the club around its COM - is unknown to me because I am too physics-illiterate. However, I find it meaningless to think of the hand couple in that way because it is a calculated (and not measured) phenomenon derived from inverse dynamics calculations and neither Kwon or SMK can explain how to biomechanically produce that hand couple phenomenon in a "real life" golf swing action. Consider SMK's vimeo video on his "intro to golf club kinetics". He starts off by using these capture images of Adam Scott's swing. Note that AS is pulling his grip downwards during the early downswing, but the COM of the club is moving upwards between image 1 => image 3. Theoretically, if the only force that was operating is the Mof due to the left hand pulling the club handle downwards, then the COM would move downwards, and not upwards, as the COM attempts to line up with the line of action of the pulling force (due to the left hand pulling the club downards). So, there has to be another force that causes the COM to move upwards. Here is SMK's first explanation. SMK postulates that there are two forces operant, and that the right hand force pushing upwards is greater than the left hand force pushing downwards by 10Nm. That explantion makes sense to me. However, because one cannot easily measure the right hand force, SMK looks at the same situation in a different manner. SMK postulates that there is hand couple and a net force. He derives the value of the hand couple torque by assigning 190Nm of the right hand's 200 Nm force to the hand couple phenomenon (which does not move the club's COM) leaving 10Nm to account for the upward motion of the COM of the club.
Using the hand couple concept may make the mathematics of the inverse dynamics calculations possible, but I think that best explaining the "true reality" in a "real life" golf swing action is dependent on the amount of force being produced by the right hand and the direction of that force at all times during the downswing. In other words, if that right hand force's magnitude-and-vector could be measured, then there would be no need to even think of a theoretical hand couple phenomenon. That's how I presently envisage the forces/torques working on the grip to explain the release of the club between P4 and P7 - I simply think of the left hand pulling the club with a certain amount of force thereby creating a MoF and I think of another force due to the right hand pushing against the aft side of the club (even though I cannot measure the magnitude of that force) in an across-the-shaft manner and also in a force-along-the-shaft manner, and the combination of those two forces produced by the two hands will account for the club's release motion. Both of the forces produced by the two hands work at the level of the club handle to move the entire club at a specific angular velocity and there is no need to imagine a theoretical phenomenon "where the club is rotating around its COM". If a golf researcher placed pressure sensors at PP#1 to measure the amount of push-force being applied by the right palm against PP#1 in a downplane direction down the hand arc path and if the grip was instrumented in a manner that would allow us to measure how much force-across-the-shaft is being produced by the right hand at all times between P4 => P7, then we would potentially better understand how to improve a golfer's biomechanical actions regarding the use of the right arm/hand. The theoretical concept of a hand couple phenomenon, where the two hands are producing a torque that rotates the club around its COM, is totally useless when it comes to teaching a golfer how to use his two hands to produce forces/torques on the club handle.
Jeff.
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Post by dubiousgolfer on Aug 15, 2020 12:18:35 GMT -5
Dr Mann
I am in total agreement with you that the hand couple derived from the inverse dynamics is useless (from a 'practical' instructional perspective). What I find frustrating is that SMK & Dr Kwon have never really made it clear what the effect of this 'hand couple' will have on the motion of the club. Their diagrams and videos can be easily misinterpreted because the physics is so unintuitive.
I have only found out about this myself after coincidentally seeing that physics website and Robert Grobers article , otherwise I would never have 'connected its importance' and continued to (incorrectly) assume that the hand couple was responsible for releasing PA#2 . It's the net torque created via the hands (ie. not a couple with equal and opposite forces) around the mid-hand-point that will release PA#2 .
DG
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Post by dubiousgolfer on Aug 15, 2020 12:50:47 GMT -5
Here is the reply I got from Dr Sasho MacKenzie regarding my diagram Dear Dr Mackenzie I've just found something which I've originally misunderstood. That the hand couple does not move the COM in space and that it can be applied anywhere on the club but still produce the same effect (providing the 'Net Force' and its application point remains unchanged). Correct. A couple has no influence on the linear motion of the CoM.So when you say the hand couple contributes to increased angular velocity of the club do you mean the 'spin' aspect of the angular velocity around its COM and not its 'orbital' aspect? This is a common misunderstanding of angular velocity. There is no ‘orbital’ aspect. The earth’s spin about its axis represents its angular velocity. The earth’s motion around the sun represents its linear velocity. See the red outlined box in the attached document.Is my diagram below (with question and answer) generally correct? I know the position of the COM is probably incorrect and should be situated in space to the right of the shaft Your answers are correct.
PS. I'm not convinced about his explanation for orbital angular velocity as that doesn't match with 'Wikipedia ' . I've sent the same email to Dr Kwon and Dave Tutelman and await their replies (and hope they are consistent with each other)
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Post by syllogist on Aug 15, 2020 13:31:01 GMT -5
Hi Dr. Mann,
Regarding SMK's claim that, from the top, it is the right hand pushes upward more than the left hand pulls downward (thus the clubhead rises above the imaginary horizontal line that represents the position of the club at the top of the swing), I tried an experiment that satisfies me that the right hand does not have to push upward for the clubhead to rise above the imaginary horizontal line.
I placed a club on the ground horizontally in front of me (handle pointing to the left, clubhead pointing to the right). If I pulled the handle with 2 fingers of one hand both leftward and downward (so as to make the clubhead also move leftward), the clubhead rose above a horizontal line on the ground. If I rotated the handle in a circle without moving the club leftward (such that the clubhead remained in place and did not move leftward), the clubhead did not rise.
S
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Post by imperfectgolfer on Aug 15, 2020 14:32:09 GMT -5
Hi Dr. Mann, Regarding SMK's claim that, from the top, it is the right hand pushes upward more than the left hand pulls downward (thus the clubhead rises above the imaginary horizontal line that represents the position of the club at the top of the swing), I tried an experiment that satisfies me that the right hand does not have to push upward for the clubhead to rise above the imaginary horizontal line. I placed a club on the ground horizontally in front of me (handle pointing to the left, clubhead pointing to the right). If I pulled the handle with 2 fingers of one hand both leftward and downward (so as to make the clubhead also move leftward), the clubhead rose above a horizontal line on the ground. If I rotated the handle in a circle without moving the club leftward (such that the clubhead remained in place and did not move leftward), the clubhead did not rise. S You are doing "something" wrong if the clubhead moves upwards when the club handle is pulled leftwards-and-downwards. Also, you are performing the experiment in an incorrect manner because of friction of the club against the ground. Watch the SMK video and you will see that if the butt end of the club is pulled down-and-outwards (away from the target) that the clubhead will move downwards (and not rise upwards) in order to line up with the line-of-action of the pulling force. Jeff.
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Post by imperfectgolfer on Aug 15, 2020 14:57:09 GMT -5
DG, Another point. Consider your diagram.
I presume that the rotation of the club around the COM is "in-and-out" of the page (perpendicular to the page) while the two forces produced by the two hands working as a hand couple are operating parallel to the page. If I am correct in my assumption, then how is it possible? When turning a valve (as seen in the SMK video), the stem of the valve is perpendicular to the wheel where the two hands are placed, so when the two hands turn the wheel it causes the valve stem to rotate about its axis. However, in your diagram, the clubshaft is lying in the same plane as the plane in which the two hand couple forces are operating. If I place a golf club on the ground, and apply a hand couple torque across the shaft at some point where the two forces are working parallel to the ground, but in opposite directions about a midway point between the two forces, then the club (and COM) moves.
Jeff.
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Post by dubiousgolfer on Aug 15, 2020 17:22:23 GMT -5
Dr Mann
The rotation of the club around the red dot (signifying the COM) is anticlockwise within the plane of the page. The opposite 'F' couple forces are also within the plane of the page.
I performed that same experiment after I read that physics website and Robert Grober's comment and couldn't believe it either (which is why I found it unintuitive) . But then decided that I wasn't able to apply a proper couple and always ended up applying forces that were not exactly equal and opposite and will therefore move the COM.
Its best to conduct the experiment with no friction and somehow accurately measure whether you are applying equal and opposite couple forces with the same magnitude and direction (ie. continually perpendicular to the shaft for a period of time).
I think there is an experiment showing this somewhere on you-tube if I can remember what its called.
DG
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Post by dubiousgolfer on Aug 15, 2020 18:28:02 GMT -5
Here is Dave Tutelmans reply:
--------------------------------------------------------------------
I understand why you are confused. Your diagram gets it right, as do Q&A #1 and #2. But it is counterintuitive. Why?
Well, it you apply the couple at MHP-A you FEEL a very different resistance than if you apply it at the CoM.
So what is the difference between "spin" (couple at CoM) and "orbital" (couple at MHP-A, the handle or grip of the club)? Great question. But in order to answer it, we need to disabuse ourselves of the idea of "orbital" motion of the club when the couple is applied at MHP-A. It is still pure spin.
So what would create the orbital motion, if a couple at MHP-A does not? Another great question. Let's look more closely.
If we want an orbital motion, we want the club to spin about the hands, not about its CoM. BUT... that requires the CoM to move. Let's impose on this a great truth of Newton's Laws: the laws of forces and torques act on the CoM. You know that! You just created a diagram and a pair of Q&A that affirm that truth.
But there is a consequence when we try for orbital motion. For the club to orbit around the hands, the CoM has to move. In order to achieve an angular acceleration of A radians/sec/Sec, the CoM has to achieve a linear acceleration L of R*A (where R is the radius, the distance from MHP-A to CoM). That linear acceleration requires a force to produce it. And the only thing in the diagram putting force on the club is the hands. So the hands are not just applying a couple for orbital motion; they are applying a couple PLUS an extra force from the hands.
Don't believe it? Grab a club and extend it and your arms horizontally. Now waggle the club back and forth. Get the following feelings:
(1) FEEL: Let the hands, arms, and shoulders be as "soft" as possible, JUST applying the turning couple. NOTICE: the softer you leave the hands/arms/shoulders, the more the hands move in opposition to the motion of the clubhead. In fact, the closer the center of rotation moves toward the CoM.
(2) FEEL: Put enough firmness into the hands so the hands are still and the movement is the CoM orbiting the hands. NOTICE: the other rotational forces you feel: shoulders, hips, maybe even legs. All these are to pump a lateral (linear) force into the hands to MOVE the CoM in addition to rotating it.
(I don't have time right now to make a video for this, but I will some day.)
Is it a little more intuitive now?
BTW, I'm going to use this explanation in the next few months as part of an article I'm writing on the biomechanics and physics of the swing. I think it needs to be said, for people who don't work with free body diagrams every day.
----------------------
DG
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Post by dubiousgolfer on Aug 15, 2020 19:04:41 GMT -5
I'm now unsure what those 'club angular velocities' represent on the kinematic sequence graphs? Are they measurements of the clubs 'spin' angular velocity (surely not)? DG I found this below from one of Phil Cheethams publications on Biomechanics . Seeing he is the kinematics specialist, I think the angular velocities we see on kinematic sequence graphs are produced as per below. Addendum 25th Aug Reply received from Phil Cheetham: ------------------------------------------ "The angular velocity of the club is very simply calculated. Take the club shaft in on sample and again in the next sample, measure the angle between the two club shafts, and divide by the time (sample rate). That's it. In more technical terms it is the angular velocity in degrees/second of the shaft around a normal to the instantaneous plane of the swing. (But I like my first description better)." ----------------------------------------- I'm therefore assuming that 'inverse dynamic ' graphs measurement of angular velocity (which is mathematically derived) is different to how they measure them in 3D systems (which is done by image comparison). The measurement of a clubs angular velocity in 3D systems is more in line with what I have intuitively believed it to be. Addendum : 5th July 2021 I have now got an answer to the above : Email to Dr Sasho MacKenzie: ------------------------------------------- Dear Dr MacKenzie Sorry to bother you but this is just a quick question. Is the angular velocity graph in your video below different to the angular velocity in the kinematic graphs (in Dr Phil Cheetham's video)? I am assuming that the graph in your video is the angular velocity about the clubs COM while the kinematic graph is about some instantaneous centre of the clubhead swing path. Have I got this correct? vimeo.com/160385937www.youtube.com/watch?v=u43j2o1VNIc&ab_channel=MySwingGolf--------------- Answer from Dr Sasho Mackenzie: ----------------------- No bother. The club angular velocity graphs are calculated in the same way in the two videos. The angular velocity of a rigid body is the same no matter what point is used for a reference. If you take into account the small differences due to shaft flex, then technically the head angular velocity can be different from the grip, but they will be very close. Now, you can calculate components of angular velocity about different axes; however, as long as those axes, are parallel, it doesn’t matter where they pass through the rigid body…angular velocity will be the same. In the videos you attached, both Phil and I are calculating the angular velocity of the club (the grip actually) about an axis that is perpendicular to the swing plane created by the grip’s motion. Hope that helps. --------------------------- So there is no difference in the angular velocities for the kinetics or kinematic graphs (phew!). DG
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Post by imperfectgolfer on Aug 15, 2020 23:18:15 GMT -5
DG,
DT stated the following-: "But there is a consequence when we try for orbital motion. For the club to orbit around the hands, the CoM has to move. In order to achieve an angular acceleration of A radians/sec/Sec, the CoM has to achieve a linear acceleration L of R*A (where R is the radius, the distance from MHP-A to CoM). That linear acceleration requires a force to produce it. And the only thing in the diagram putting force on the club is the hands. So the hands are not just applying a couple for orbital motion; they are applying a couple PLUS an extra force from the hands."
If I understand DT correctly, then for the club to orbit the hands (which is what happens in the downswing) an "extra force" (other than the hand couple) must be in play. If that "extra force" is causing the club to gain angular velocity as the club orbits around the hands, then why is there any need for the hand couple to be in play?
Jeff.
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Post by imperfectgolfer on Aug 15, 2020 23:33:48 GMT -5
DG,
SMK stated the following-: "This is a common misunderstanding of angular velocity. There is no ‘orbital’ aspect. The earth’s spin about its axis represents its angular velocity. The earth’s motion around the sun represents its linear velocity. See the red outlined box in the attached document."
If the earth rotates around the sun, then it obviously has linear velocity along its path. However, if the orbiting path of the earth is circular relative to the sun, then can one not express its motion in terms of angular velocity (radians/second)? That's how I mentally picture the motion of the club during the downswing relative to the hand arc path. The clubhead (or COM) is orbiting the hands as they move down the hand arc path - during the release of PA#2; and surely its motion can be expressed in terms of both linear velocity (relative to its motion along the clubhead path) and angular velocity (relative to the fact that the clubhead is orbiting around the hands).
Jeff.
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Post by dubiousgolfer on Aug 16, 2020 6:32:16 GMT -5
Dr Mann The 'attached document' was from a scientist deplored at some of the poor definitions used in maths /physics for concepts like 'couples/torques/angular velocity'. It seems that SMK sympathises with this scientists point of view and has therefore said there is 'no orbital aspect'. DT seems to infer there is and so does Wikipedia and so does this video below from the well respected 'Khan Academy' www.khanacademy.org/science/physics/torque-angular-momentum/rotational-kinematics/v/relationship-between-angular-velocity-and-speedThe earth is not connected to the sun via some fixed point on the surface of the earth, the 'connection' is the pull of gravity between each objects centre of mass. It is rotating around the sun and does have orbital angular velocity (it also has spin angular velocity around its own axis through its COM). Whatever the various definitions of angular velocity ,angular speed , linear speed, couples , moments/torques , etc , DT has , imho, resolved the issue with his point 1 and 2 'FEEL' examples. To properly implement an inverse dynamics 'theoretical' hand couple in a real life golfers swing , the looser the wrists and arms need to be. Therefore I can only surmise that in a real golf swing , the looser the wrists and arms are , the more likely the golf swing will reflect the inverse dynamics 'maths'. I therefore suspect that the golf swing is basically the 'Net Force' and its created 'MOF' around passive wrists. DG PS. Here is the section in the 'attached document' that SMK referred to:
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Post by syllogist on Aug 16, 2020 6:50:09 GMT -5
Dr. Mann, The animations of the double pendulum on DT's site must be doing something wrong as well. The clubhead seems to rise above the horizontal starting position at the top of the swing. Perhaps the "intentionally created 90 degree wristcock" for purposes of the animation and modeling is the reason. www.tutelman.com/golf/swing/golfSwingPhysics2.phpS
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