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Post by dubiousgolfer on Aug 20, 2020 10:40:30 GMT -5
Hi S
I have no issues with the MOF taking over , but its not just caused secondarily by the 'Net Force' he implies in his 'Intro Kinetics' video . I'm assuming there is another force in play which he hasn't mentioned and I think its an additional torque.
It's important because we need to know what actual forces are releasing PA#2.
DG
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Post by syllogist on Aug 20, 2020 15:09:51 GMT -5
Hi DG,
One way to find out whether the positive MOF or whether some other force that wasn't described as an in-plane force causes release to happen.
I would be surprised if an active or passive straightening of the right arm causes release. My reasoning is that such straightening causes hand travel along the hand path and I don't know why mere hand travel would trigger the release. Also, I'm guessing that such straightening inhibits passive right palmar flexion which occurs during release.
Do you have some thoughts as to why the negative couple at shaft horizontal wouldn't induce positive MOF to cause release?
S
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Post by dubiousgolfer on Aug 20, 2020 17:32:41 GMT -5
Hi S
Not sure what you mean by "why the negative couple at shaft horizontal wouldn't induce positive MOF to cause release" .
Back to square 1 on that physics website as one of the comments didn't make any sense to me. I'm going to give it a few days and then reflect on it all again.
DG
PS. I've spent days looking through the internet to find something similar to SMK's intro kinetics video and got zilch ! There is not one 'article/video/comment' even though I know it works in real life. You tie a string to one end of a ruler on a smooth desk , then place the ruler and string at an angle, pull the string about a foot and watch the ruler line up with the string. There is one 'expert' in physics and maths who has decided to add my question to his list of viewer requests to explain (Michael Van Biezen - who finds it very interesting).
ADDENDUM: After many hours today I think I've figured it out myself. There does need to be a 'Force' applied at the 'mid-hand-point' on grip to stop it from being rotated due to the 'spin ' 'moment of a couple ' about the clubs COM. Only then will SMK's 'MOF' effect which rotates the COM to align with the 'Net Force' happen as per his 'Intro kinetics video' .
Addendum: I repeated that string/ruler experiment and it wasn't as expected. The ruler did 'spin' around its COM as I pulled the string , but as the attached end met the the increased tension of the string (as ruler end tried to pull the string around) , it stopped the attached end from rotating . As soon as the attached end of the ruler was stopped, the COM of the ruler rotated to align with the string.
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Post by dubiousgolfer on Aug 21, 2020 20:32:27 GMT -5
If anyone is interested in how the MOF in SMK's 'Intro Kinetics' video moves the COM to align itself with the 'Net Force' , I've answered my own question on the physics website. But I doubt it will make sense to anyone who doesn't have some understanding of physics and I could be completely wrong (because I've forgotten most of the physics I've learned over 40 years ago). The physics of torques , pseudo forces , circular motion , free body diagrams , etc was one of many difficult non-intuitive topics.
What it does suggest is that a force across the shaft is required to stop the grip end of the club spinning around its COM. Only then will the COM move and align itself with the 'Net Force' (as per SMK's video).
In a real life golf swing , we stop the handle from spinning around uncontrollably, so SMK's video has assumed those 'handle spin retarding forces' are already in place when the 'Net Force' is being applied. Therefore the MOF idea works and so does the maths calculating the 'Moment of Force' acting on the COM.
So what is the point of my exercise? I think by knowing that there needs to be another force to stabilise the grip handle might also help explain some of the force graphs shown in Koike and Choi research.
DG
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Post by syllogist on Aug 22, 2020 0:46:49 GMT -5
Hi DG,
Would you please elaborate. Both ends of the club rotate around the COM of the club. What part of the swing or action are you describing where force across the shaft is necessary and what action results in force across the shaft? I take it that the golfer need not actively do something.
Thanks. S
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Post by dubiousgolfer on Aug 22, 2020 11:01:17 GMT -5
Hi S The force across the shaft is the one that prevents the handle of the grip from spinning around the clubs COM. I don't know the biomechanics involved in applying the force but it needs to be there if you wish to rotate the clubs COM about the hands during the downswing. The action of the force and its direction is shown in my answer on the physics website . physics.stackexchange.com/questions/574405/what-forces-are-causing-the-com-to-align-with-the-tail-end-of-force-vectorBut I'm wondering theoretically if 'Release' could actually mean allow the 'spin' of the club around its COM (while obviously moving it 'linearly' by the 'Net Force') ? Difficult to convey in words but as the lead wrist starts to momentarily uncock (by the clubs 'spin' around its COM) , can the lead arm be moved in such a way as to 'passively' prevent the PA2 angle from increasing? Without the need for applying a push force across the shaft by the right hand to create a negative MOF to promote a lagging effect in the early downswing? The above might explain the Jack Nicklaus comment that "You can't release the club too soon". Here is Dr Kwons reply below (between the dotted lines) which has confused me again . It's the poor physics definition of 'Torque/Couple/Moment/Moment Of Force' that complicates matters. As far as I am aware , a Torque is 'Not A Free Vector' but a 'Couple' is but scientists sometime use 'Torque' and 'Couple' interchangeably but its not actually correct. A 'Moment Of Force' is a 'Torque' and acts about a point . A 'Couple' doesn't necessarily need to be about a specific point (ie. a free vector) therefore if he means 'Couple' instead of Torque , then he's correct. When he and the other golf scientists say 'rotating the club' they need to be more specific . Does it mean about the MID-HAND-POINT or about its COM or some other 'point/axis of rotation'? ---------------------------------------------- First, try this article: drkwongolf.info/technotes/mh_kinetics.pdf. According to Newton’s 2nd law, the forces acting on the club accelerates the COM of the club linearly. SO both gravity (club’s weight) and hand force participate in this. Likewise, the torques acting on the club together cause the club to accelerate angularly. Torque is a free vector so both the moment produced by the MH force and the MH torque participate in rotating the club. ------------------------------------- DG PS. I've read Dr Kwon's article and gone through the maths - which make sense . But note what he says about particular variables in his equation: On page 3 it states: hc in Eq. 8 is the angular momentum of the club about its COMOn page 4 it states the below: If you look at the right hand side of the equation 11 , that dot above the hc means it's the derivative with respect to time (ie. net rate of change of angular momentum = 'Net Torque'). I don't know why he is using terms I've never heard of before (ie. 'Inertial Moment') , but MOI x angular acceleration = rate of change of angular momentum = Torque. So basically he is adding the 'hand couple' MMH + 'Sasho Mackenzies Moment of Force' (rch x FMH) = 'Net Torque' about the clubs COMI am fairly confident that the 'Net Torque' his equations are defining is the rate of change of 'net spin angular momentum' of the club about its COM. In my opinion its not the rate of change of angular momentum of the clubs COM about mid-hand-point.
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Post by dubiousgolfer on Aug 24, 2020 20:41:30 GMT -5
For your information , I have raised emails to Dr Kwon, Dave Tutelman, Phil Cheetham , Dr Sasho Mackenzie : awaiting replies
Dr Kwon -A second email- 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
Dave Tutelman - question asked: One thing that's still confusing me and it relates to Dr Sasho Mackenzies 'Intro Kinetics' video (link below - from 0:00 - 06:01).. He seems to be suggesting that an eccentric linear force will cause an MOF across the clubs COM and 'move' it so that it aligns itself with the tail end of the linear force vector. But I thought an eccentric force can be decomposed to an equivalent translational force through its COM and a 'Couple' (which will 'not move' the COM). If I am correct , then shouldn't there be another force added in that video which stabilises the grip end (ie. stop it from 'spinning' around its COM)? Otherwise I cannot fathom how that COM can be moved into alignment with that linear force.
Phil Cheetham - question asked: Am I correct in assuming that the 'Moment Of Force' caused by the 'Net Force' is a 'couple' that can only cause rotation around the clubs COM?
Sasho MacKenzie- question asked: 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)?
My last question is just to check whether the impression I had after seeing his video is what he intended to communicate.
DG
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Post by dubiousgolfer on Aug 25, 2020 8:39:45 GMT -5
Dr Mann I can now categorically prove that SMK intro kinetics video may have given the wrong impression: At 3:30 in that video he says : "It's also going to produce some rotation. So this force is going to cause some rotation of this club, this COM is going to line up with this force vector" While he made that comment above he positioned his screen pointer on the COM and made a circular path to line up with the force vector line as if the 'point of application of the force on the grip' was the centre of that circle. Also at 3:32 he placed his screen pointer on that 'point of application' and made a circular movement . So I plotted the movement of that centre of mass from 3:40 -3:43 and the COM moved in a straight line , it didn't rotate on some arced path (see my image below). This proves to me that the club is only rotating about the COM and not about the 'mid-hand-point'. DG PS. There is a webinar on the 27th August with Sasho Mackenzie that anyone can register for (not sure if there is a cost but people can pose questions on the registration link). On his twitter account , he says that ROC could be the first item on the agenda. us02web.zoom.us/webinar/register/2215976746568/WN_CibUJgZ0SFK5ju3mZ8NMAgADDENDUM 26TH AUG Obviously , there is still an issue as to how the handle is not rotating in the image of my previous post . That means there must be a force being applied to the handle to prevent it moving in a circular path (around its COM) and therefore a component of that force is being used to move the COM to the left . 13/05/22 : The spinning of the club around its COM should cause the point of application to deviate off its straight path . Another force is required in that animation to act on the point of application to keep the force moving on its linear path.
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Post by imperfectgolfer on Aug 25, 2020 9:05:49 GMT -5
DG,
You wrote-: "This proves to me that the club is only rotating about the COM and not about the 'mid-hand-point'."
That statement may be true in your mental universe, but it is not true in my mental universe.
There are "facts", and "alternative facts".
Jeff.
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Post by syllogist on Aug 26, 2020 14:10:49 GMT -5
Hi DG,
Perplexing, but in the end I don't think that there's any value in showing the SMK club kinematics; I don't think that it describes anything but forces at play. There's nothing to conclude from it.
I'm wondering if the center of mass is not a point on the club, but rather, a point on the golfers chest. Does the center of mass of an object have to be a point on that object? For example, if the object were a circular ring, would the center of mass be a point in the middle of the ring and not on the ring? If the center of mass is a point in the "center of a circle around the golfer" and represents the end of the handle of the club (where the "circle" moves as illustrated in SMK's video), then the center of mass merely represents a point that spins (and does not accelerate (as noted by Grober).
Release:
As the force couple becomes negative at about shaft parallel in the downswing, the hands decelerate. SMK states that, at this point, the moment of force (or net force as he alternatively called it), becomes positive. I think that the positive moment of force is not meant to describe release itself. Release is a predicated on the mass of the object, which is a variable in angular momentum. In other words, the clubhead constantly accelerates (and more so during release) and if the center of mass of the club is a few inches from the clubhead, then obviously the center of mass of the club constantly accelerates.
Release will happen so long as the club has sufficient mass and the only way to increase the velocity of release is to "add more force by pushing the shaft" when the force couple turns negative, if you are able in a full swing.
S
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Post by dubiousgolfer on Aug 26, 2020 18:06:15 GMT -5
Here is Dave Tutelmans reply related to the answer I gave on the physics website.
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"Yup, you got it! The 'A' force isn't the only force at the handle, if the handle is constrained to move in the A direction. You need a little crosswise force to keep the handle on the line, which also brings the CoM into line.
BTW, they will line up, one way or the other. The question is whether the CoM moves to line up (as you showed), or the handle moves due to rotation (pure 'A' force, with no constraint for the handle to move in the 'A' direction). Either way, the tendency is to line up. ------------------------------------
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Post by dubiousgolfer on Aug 26, 2020 18:53:53 GMT -5
Hi DG, Perplexing, but in the end I don't think that there's any value in showing the SMK club kinematics; I don't think that it describes anything but forces at play. There's nothing to conclude from it. I'm wondering if the center of mass is not a point on the club, but rather, a point on the golfers chest. Does the center of mass of an object have to be a point on that object? For example, if the object were a circular ring, would the center of mass be a point in the middle of the ring and not on the ring? If the center of mass is a point in the "center of a circle around the golfer" and represents the end of the handle of the club (where the "circle" moves as illustrated in SMK's video), then the center of mass merely represents a point that spins (and does not accelerate (as noted by Grober). Release: As the force couple becomes negative at about shaft parallel in the downswing, the hands decelerate. SMK states that, at this point, the moment of force (or net force as he alternatively called it), becomes positive. I think that the positive moment of force is not meant to describe release itself. Release is a predicated on the mass of the object, which is a variable in angular momentum. In other words, the clubhead constantly accelerates (and more so during release) and if the center of mass of the club is a few inches from the clubhead, then obviously the center of mass of the club constantly accelerates. Release will happen so long as the club has sufficient mass and the only way to increase the velocity of release is to "add more force by pushing the shaft" when the force couple turns negative, if you are able in a full swing. S Hi S Biomechanics is not just about kinematics but also relates to kinetics, therefore the understanding of forces/torques and their effect on movement is important (to me anyhow). The conclusion regarding SMK's video is that it gave the impression that the club (including its COM) was being angularly rotated around the grip handle area . I had that image in my head for a few years now and was using that concept to try and understand the kinematics . To now find that the impression given was flawed is very frustrating but at least I have a better grasp of what the inverse dynamics really mean when they talk about 'hand couples' and the 'moment of force' caused by the 'Net Force'. My issue is not related to the 'COR' (centre of rotation of the swing), just the kinetics side and how forces on the club via the hands can explain the kinematics. I don't understand what you mean by the below: "If the center of mass is a point in the "center of a circle around the golfer" and represents the end of the handle of the club (where the "circle" moves as illustrated in SMK's video), then the center of mass merely represents a point that spins (and does not accelerate (as noted by Grober)."Grober merely said a couple doesn't accelerate the clubs COM. I also do not understand what you meant below: "I think that the positive moment of force is not meant to describe release itself. Release is a predicated on the mass of the object, which is a variable in angular momentum. In other words, the clubhead constantly accelerates (and more so during release) and if the center of mass of the club is a few inches from the clubhead, then obviously the center of mass of the club constantly accelerates.
Release will happen so long as the club has sufficient mass and the only way to increase the velocity of release is to "add more force by pushing the shaft" when the force couple turns negative, if you are able in a full swing."Mass is not a variable of angular momentum , do you mean 'Moment Of Inertia' ? Yes, the COM is obviously accelerating but its not due to the 'Moment Of Force' as I've found out in the posts above and confirmed by Dave Tutelman. Note that the MOF will cause the club to have 'spin' and that will angularly accelerate the clubhead and the handle (around its COM). This is the completely unintuitive part of the golf swing because you have to superimpose many different movements caused by the kinetics - such as the 'hand and MOF couple effects ' , the extra 'side force effect' to stabilise the club handle and the 'linear force effect' (and maybe others). Why should 'Release' happen as long as the club has sufficient mass ? How and when would you be adding more pushing force to the shaft when the force couple turns negative ? I am assuming you mean when the 'hand couple' turns negative. DG
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Post by dubiousgolfer on Aug 26, 2020 21:21:39 GMT -5
I'm also finding it difficult to imagine that Sasho Mackenzie has made some mistake concerning the screen image below from his 'Intro Kinetics' video (there must be a logical reason why he did this). The red arrows are what he should be portraying should happen to the club during rotation (around its COM), not his black arrows. But his MOF figures values are correct inasmuch as they are identical to the 'MOMENT OF THE COUPLE' that the club experiences around its COM. If the above is wrong than his concept below is wrong too. DG
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Post by syllogist on Aug 27, 2020 10:47:14 GMT -5
Hi DG,
Angular momentum = mass x velocity x radius ....... Momentum = mass x velocity
The club needs sufficient mass to cause release (uncocking of the wrists and where such mass does not include the weight of the hands themselves). I have given an example in the past involving swinging a club made of balsa wood. Such a light weight instrument does not induce uncocking. Note that if you were to swing a club shaft without a head, you would actively have to uncock your wrists to speed up the tip of the shaft.
When I referred to the force couple, yes I meant SMK's term "hands couple." Bear in mind that the hands themselves do not apply force; they merely hold the club. The true force couple can be either the chest, shoulders, or arms, or any combination.
When I mentioned the point in the middle of a circle, I was referring to SMK's graphics on his kinematics video. I got the impression from the video that his showing of the club swing kinematics used a frame of reference where the club swings around a point in the middle of a circle.
I think that the concept of a double pendulum as representative of arms and club is sufficient to understand the kinematics of swinging a club. It doesn't matter in terms of kinematics whether the pendulum is driven or merely under the force of gravity. In a double pendulum there is tensile force as well as length and mass of the segments of the double pendulum. Release in a good swing is all about momentum
S
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Post by dubiousgolfer on Aug 27, 2020 11:13:45 GMT -5
Hi S
Yes, apologies , you are correct about angular momentum having a variable of mass.
I normally use L = Iw (Moi x angular velocity) but I = m (r x r) for a point mass 'm' with a radius of rotation 'r'.
There is no problem using the double pendulum as a model concept to understand the golf swing but it cannot fully explain the full reality of the golf swing. Who knows whether the DP type swing is more optimal than what we see in PGA pro tour swings but unfortunately real golfers are not built like DP models.
DG
PS. Note that any 'couple' alone applied to the club via your hands (whether it be via your chest, shoulders, arms) will not accelerate the COM of the club.
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