Post by dubiousgolfer on Aug 12, 2020 12:14:08 GMT -5
Oh dear, I've just found out something that I was unaware of concerning the 'Hand Couple' and its completely unintuitive (see physics forum question below).
www.physicsforums.com/threads/couples-and-forces.431946/
Question:
I have a general question. I just do not understand two concepts:
1. The idea of a couple being a free vector: If a couple produces a moment around the midpoint of the distance between the two forces that create a couple then how come if we move it to a different point of a body it creates the same effect? Wouldn't it rotate the body a round new point?
2. How can a couple and a force be resolved in to a single resultant force. I have seen questions in a text books that have a bunch of forces and couples acting on a body and they ask for the resultant force without a couple. If the couple can be moved to any point and force cannot , than how come we can find the resultant force without a couple? As I understand, the moment of a force cannot cancel a couple since it is fixed around a point and a couple is not? Thank you very much for al of you answers. I would really appreciate if someone could explain these two things for me.
Source www.physicsforums.com/threads/couples-and-forces.431946/
Answer:
Your confusion arises because, I think, you're assuming the couple's moment is going to produce a rotation about the point midway between the lines of action of the forces. It'll actually cause a body to rotate about its center of mass.
The moment due to a couple is a free vector because you can shift the pair to wherever you want on the body and it will produce the same motion. The net force of a couple is 0, so the object's center of mass won't accelerate regardless of where the forces act. The moment due to the couple is independent of the axis, so the rotational acceleration doesn't depend on the relative displacement of the pair of forces from the center of mass. When a single force F acts on a body, it will always cause the same linear acceleration of the center of mass regardless of where it acts; however, by moving the force around, you can get different moments. So by moving the force to the right place, you can get the same moment as what the couple would have produced, and the linear acceleration will remain unchanged.
-------------------
The above is also reiterated in Robert Grober's recent article below page 2
"Forces and Torques Near to Impact in the Golf Swing" dated 23rd June 2020
arxiv.org/pdf/2006.11778.pdf
" A force couple generates a torque, but does not accelerate the center of mass "
Gulp !!!!
Doesn't this mean that the 'Hand Couple' does not help release PA#2 ?
Correction: Does this mean 'release of PA#2 angle happens' but the 'hand couple' itself does not contribute to increase clubhead speed?
I don't know what it means (yet!!).
The Grober article has a chapter that Dr Mann might find interesting (I haven't read it yet).
"The final section of the paper speculates about various mechanisms by which the negative force couple can be generated"
DG
PS. A lot of maths in this article and he uses a DP model as it seems a close fit near to impact of a real golfer :
His conclusion:
"Motivated by MacKenzie’s observation of a negative couple near to impact [1, 2], this paper has explored a model for how the golf club moves near to impact. It assumes the club is moving as the distal arm of a double pendulum and that at impact the club head is moving straight down the target line, at its maximum speed, on a path of defined curvature.
From this model,the forces and torques required to move the club near to impact are calculated. The results obtained from this model are shown to be quantitatively consistent with data reported by Mackenzie to within a few percent. Indeed, the negative couple near to impact is found to be a robust feature of this model. It balances torques resulting from the forces that drive the center of mass of the golf club. These torques reduce the radius of curvature of the path of the club head as it moves through impact. By applying a negative couple the golfer is able to achieve a larger radius of curvature. This reduces the difference between the path of the club head and the target line as the club head moves near to impact. Because the negative couple can also serve to reduce the rotational speed of the club, its presence in the golf swing manifests a trade between distance and direction."
Notes :
Looks like he hasn't found an answer to how such a large negative torque can manifest itself in the golfers hands nearing impact.
The negative couple is quite large 70lbs force per hand just before impact but over a very short period of time
Option A : Negative Torque using hands: If this torque were due to the fact the hands can not keep up with the release of the club, it might be difficult for the golfer to perceive this applied torque. It would be quite spectacular if golfers have learned to harness this natural drag to help them to hit the ball straight
Claims too much force via hands considering they are also applying hundreds of Newtons of linear force too.
Option B: Aerodynamic Drag Of the Clubhead - That doesn't make sense to me because the shaft is in forward bend not lagging bend.
Option C: Inertia of the squaring of the clubface - That doesn't make sense because golfers like Phil Mickelson don't experience much club squaring inertia and what about Jamie Sadlowski with his strong grip (virtually no club squaring applicable) ? They both have forward shaft bend and therefore negative torque in their hands just prior to impact .
Dr Mann's impedance of the trail elbow is another option but do JS & PM impede their trail elbows ? I thought they swing more along their turned shoulder planes with some distance between their upper arms and torso but still have negative torque in their hands prior impact.
I am tending towards the bolded sentence above and I can also see some logic in the idea of using the negative hand torque to increase the swing radius of the clubhead so that it tracks closer to the ball target line for a longer period of time , but that doesn't mean that the clubface is still square to the clubhead path. One still has to use the biomechanics of the Drive-Hold hand release to keep the clubface square to the clubhead path (whether tracking closer to the ball target line through impact or not).
There are still significant differences between Koike's research , Choi and the inverse dynamics findings.
Koike 1st/2nd Research + Choi - shows virtually symmetrical opposing left/hand force graphs
Koike 1st Research - shows a negative lead hand couple of approx -14 Nm, trail hand slightly positive +2 Nm.
Koike 2nd Research - shows a negative lead hand couple of -3 Nm, trail hand virtually zero (maybe slightly positive) .
Choi Research - shows a lead hand torque of -3Nm , trail hand - 10Nm .
Inverse Dynamics - Show negative hand couple torque of -50Nm
Koike 1st Research - shows large forces for both lead/trail hands up the axis of the shaft towards the butt end approaching impact (lead =330N, trail=100N)
Koike 2nd Research- shows lead hand force up the shaft 500N towards butt end approaching impact , trail hand hovering mostly in the -40N mark then moving to +20N into impact.
Choi research - shows lead/trail hand forces up the shaft of about 100N each just before impact (Total 200N). Note that the article did mention a large variation in the force up the shaft in the butt end direction and you can see that variation in graph G (the shadowed regions).
I don't understand why Robert Grober is using such a high negative torque figure of -50Nm when Koike shows -12Nm and -3Nm (1st/2nd Research respectively) and Choi shows -13Nm.
Theoretically, there might be no need for a hand couple if the 'Net Force' was produced between the boundaries of the hands in such a way as to create a 'moment of force' that could mimic the hand couple required. But the fact that Koike and Choi have actual measured graphs of 'couples' in each hand show how complicated the real life situation is compared to the mathematical 'inverse dynamics' approach.
www.physicsforums.com/threads/couples-and-forces.431946/
Question:
I have a general question. I just do not understand two concepts:
1. The idea of a couple being a free vector: If a couple produces a moment around the midpoint of the distance between the two forces that create a couple then how come if we move it to a different point of a body it creates the same effect? Wouldn't it rotate the body a round new point?
2. How can a couple and a force be resolved in to a single resultant force. I have seen questions in a text books that have a bunch of forces and couples acting on a body and they ask for the resultant force without a couple. If the couple can be moved to any point and force cannot , than how come we can find the resultant force without a couple? As I understand, the moment of a force cannot cancel a couple since it is fixed around a point and a couple is not? Thank you very much for al of you answers. I would really appreciate if someone could explain these two things for me.
Source www.physicsforums.com/threads/couples-and-forces.431946/
Answer:
Your confusion arises because, I think, you're assuming the couple's moment is going to produce a rotation about the point midway between the lines of action of the forces. It'll actually cause a body to rotate about its center of mass.
The moment due to a couple is a free vector because you can shift the pair to wherever you want on the body and it will produce the same motion. The net force of a couple is 0, so the object's center of mass won't accelerate regardless of where the forces act. The moment due to the couple is independent of the axis, so the rotational acceleration doesn't depend on the relative displacement of the pair of forces from the center of mass. When a single force F acts on a body, it will always cause the same linear acceleration of the center of mass regardless of where it acts; however, by moving the force around, you can get different moments. So by moving the force to the right place, you can get the same moment as what the couple would have produced, and the linear acceleration will remain unchanged.
-------------------
The above is also reiterated in Robert Grober's recent article below page 2
"Forces and Torques Near to Impact in the Golf Swing" dated 23rd June 2020
arxiv.org/pdf/2006.11778.pdf
" A force couple generates a torque, but does not accelerate the center of mass "
Gulp !!!!
The Grober article has a chapter that Dr Mann might find interesting (I haven't read it yet).
"The final section of the paper speculates about various mechanisms by which the negative force couple can be generated"
DG
PS. A lot of maths in this article and he uses a DP model as it seems a close fit near to impact of a real golfer :
His conclusion:
"Motivated by MacKenzie’s observation of a negative couple near to impact [1, 2], this paper has explored a model for how the golf club moves near to impact. It assumes the club is moving as the distal arm of a double pendulum and that at impact the club head is moving straight down the target line, at its maximum speed, on a path of defined curvature.
From this model,the forces and torques required to move the club near to impact are calculated. The results obtained from this model are shown to be quantitatively consistent with data reported by Mackenzie to within a few percent. Indeed, the negative couple near to impact is found to be a robust feature of this model. It balances torques resulting from the forces that drive the center of mass of the golf club. These torques reduce the radius of curvature of the path of the club head as it moves through impact. By applying a negative couple the golfer is able to achieve a larger radius of curvature. This reduces the difference between the path of the club head and the target line as the club head moves near to impact. Because the negative couple can also serve to reduce the rotational speed of the club, its presence in the golf swing manifests a trade between distance and direction."
Notes :
Looks like he hasn't found an answer to how such a large negative torque can manifest itself in the golfers hands nearing impact.
The negative couple is quite large 70lbs force per hand just before impact but over a very short period of time
Option A : Negative Torque using hands: If this torque were due to the fact the hands can not keep up with the release of the club, it might be difficult for the golfer to perceive this applied torque. It would be quite spectacular if golfers have learned to harness this natural drag to help them to hit the ball straight
Claims too much force via hands considering they are also applying hundreds of Newtons of linear force too.
Option B: Aerodynamic Drag Of the Clubhead - That doesn't make sense to me because the shaft is in forward bend not lagging bend.
Option C: Inertia of the squaring of the clubface - That doesn't make sense because golfers like Phil Mickelson don't experience much club squaring inertia and what about Jamie Sadlowski with his strong grip (virtually no club squaring applicable) ? They both have forward shaft bend and therefore negative torque in their hands just prior to impact .
Dr Mann's impedance of the trail elbow is another option but do JS & PM impede their trail elbows ? I thought they swing more along their turned shoulder planes with some distance between their upper arms and torso but still have negative torque in their hands prior impact.
I am tending towards the bolded sentence above and I can also see some logic in the idea of using the negative hand torque to increase the swing radius of the clubhead so that it tracks closer to the ball target line for a longer period of time , but that doesn't mean that the clubface is still square to the clubhead path. One still has to use the biomechanics of the Drive-Hold hand release to keep the clubface square to the clubhead path (whether tracking closer to the ball target line through impact or not).
There are still significant differences between Koike's research , Choi and the inverse dynamics findings.
Koike 1st/2nd Research + Choi - shows virtually symmetrical opposing left/hand force graphs
Koike 1st Research - shows a negative lead hand couple of approx -14 Nm, trail hand slightly positive +2 Nm.
Koike 2nd Research - shows a negative lead hand couple of -3 Nm, trail hand virtually zero (maybe slightly positive) .
Choi Research - shows a lead hand torque of -3Nm , trail hand - 10Nm .
Inverse Dynamics - Show negative hand couple torque of -50Nm
Koike 1st Research - shows large forces for both lead/trail hands up the axis of the shaft towards the butt end approaching impact (lead =330N, trail=100N)
Koike 2nd Research- shows lead hand force up the shaft 500N towards butt end approaching impact , trail hand hovering mostly in the -40N mark then moving to +20N into impact.
Choi research - shows lead/trail hand forces up the shaft of about 100N each just before impact (Total 200N). Note that the article did mention a large variation in the force up the shaft in the butt end direction and you can see that variation in graph G (the shadowed regions).
I don't understand why Robert Grober is using such a high negative torque figure of -50Nm when Koike shows -12Nm and -3Nm (1st/2nd Research respectively) and Choi shows -13Nm.
Theoretically, there might be no need for a hand couple if the 'Net Force' was produced between the boundaries of the hands in such a way as to create a 'moment of force' that could mimic the hand couple required. But the fact that Koike and Choi have actual measured graphs of 'couples' in each hand show how complicated the real life situation is compared to the mathematical 'inverse dynamics' approach.