Centripetal Force: pulling 115lb on a 200g clubhead at 121 mph. Swing implications. Biomechanists, physicists, where u at? Dr. Kwon reply!

[airjammer]

Can anyone explain to the slow kid, how this information makes you better at golf?

[Socrates]

On 11/29/2022 at 7:17 AM, clevited said:

Not sure if this will be of any interest, but it is kinda related.  I attempted to explore some thoughts about golf club force but I only focused on needed grip strength to swing fast.  I was able to get Dave Tutelman to do a much more in depth analysis which I found pretty interesting. 

 

https://www.tutelman.com/golf/swing/gripPressure.php 

Somehow I missed that whole thread in 2020.  Maybe it too much for my puny brain to comprehend during golf season.  At least now I've read the Tutelman article and gained some understanding.

[Soloman1]

4 hours ago, baudi said:

If I follow correctly, mainly as a result from rotary forces. But jumping also leads to full extension of the body which may lead to a bigger radius and thus acceleration to the max. 

 

This makes me think. If elements are not necessary better leave them out of the equation. If elements are necessary, then analyze and define how they operate and contribute. 

In this case, if there is reinforcement of the net result (stabilizing); then the phenomena is to be addressed as an influence.     

 

I like the fact that Joostin comes with hammerthrow and a tennis serve. (I studied Roger Federer's serve for many hours 🙂 and still have pictures of hammerthrow somewhere).

 

 

The forces are derived from body motion.

 

Yes, that is what I meant about a longer radius.

 

Hand strength, grip forces and centripetal force need to be enough to keep the clubhead moving along the intended angular path to counteract forces developed in angular velocity and acceleration. 

 

 

The faster your swing, the more force is needed to counteract the external forces. Your body creates the internal forces.

 

Pulling harder than necessary will pull the clubhead closer to you and affect the clubhead path. It’s a balancing act that we do with no thought.

 

You can make it part of the equation of a golf swing, but I still see it as an effect, not something that is a conscious action.

 

Pulling harder with the same clubhead speed will not result in a better shot because of the effect on the 3D position of the head at impact.

 

I guess that sums up my thoughts on it.

 

 

[J.Bex]

2 hours ago, Soloman1 said:

 

The forces are derived from body motion.

 

Yes, that is what I meant about a longer radius.

 

Hand strength, grip forces and centripetal force need to be enough to keep the clubhead moving along the intended angular path to counteract forces developed in angular velocity and acceleration. 

 

 

The faster your swing, the more force is needed to counteract the external forces. Your body creates the internal forces.

 

Pulling harder than necessary will pull the clubhead closer to you and affect the clubhead path. It’s a balancing act that we do with no thought.

 

You can make it part of the equation of a golf swing, but I still see it as an effect, not something that is a conscious action.

 

Pulling harder with the same clubhead speed will not result in a better shot because of the effect on the 3D position of the head at impact.

 

I guess that sums up my thoughts on it.

 

 

I honestly agree for the most part.  The previous response to you was mainly to correct that centripetal force and angular velocity are indeed part of the same equation.

 

I think what makes things confusing is that pulling harder doesn't necessarily mean the handle is pulling the head's 3D position out of whack.  It would pull it out of whack without rotation.  It would also happen in rotation if intended, but I think our body learns what to do to balance the handle/head arcs to maintain the clubhead's path.  So balancing act, feedback loop, yes.  Stabilizing... I'm not sure on that other than the body having to keep in balance while pulling without falling over.  We're not countering any centrifugal force, just the club's inertia which came from our torque and forces to begin with.

 

[Anecdotally, and I've heard others say the same, after I did some speed training without hitting balls, once I started hitting balls I was topping like crazy.  That could be a case of pulling to recruit more centripetal force for speed alone without a ball and not having to adjust for ball striking (no feedback loop).]

 

Like in the Khan Academy video, the centripetal force is really changing the velocity vector position of the object rotating.  The rotation is started with torque and more linear forces, but then CF comes into play.  

 

Where I differ is the belief that CF is not a reaction to the forces that the club creates and GRFs, but whole the reason for them (once it gets in some kind of rotation from the torques and linear forces).  Going back to Kwon's statement: "When the centripetal force is gone, the object simply starts moving straight due to the inertia. The downswing is essentially the art of generating the centripetal force using the ground and leg action."  We have to create a curved motion with our hands which transmit a curved motion to the club path.  The reason we are generating ground reaction forces in the first place is because we innately have to generate centripetal forces.

 

Our middle ground may be that there is a feedback loop from the body creating CF, without us thinking, but the body also needing to adjust for proper ball striking mechanics.  So definitely a balance.  We can train to swing faster (and pull greater CF) but it needs to be in a feedback loop with golf performance.

[Ifelloffladder]

3 hours ago, airjammer said:

Can anyone explain to the slow kid, how this information makes you better at golf?

I'm with you on this.  WITB: Driver, irons, calculator, etc

[Ifelloffladder]

7 minutes ago, joostin said:

I honestly agree for the most part.  The previous response to you was mainly to correct that centripetal force and angular velocity are indeed part of the same equation.

 

I think what makes things confusing is that pulling harder doesn't necessarily mean the handle is pulling the head's 3D position out of whack.  It would pull it out of whack without rotation.  It would also happen in rotation if intended, but I think our body learns what to do to balance the handle/head arcs to maintain the clubhead's path.  So balancing act, feedback loop, yes.  Stabilizing... I'm not sure on that other than the body having to keep in balance while pulling without falling over.  We're not countering any centrifugal force, just the club's inertia which came from our torque and forces to begin with.

 

[Anecdotally, and I've heard others say the same, after I did some speed training without hitting balls, once I started hitting balls I was topping like crazy.  That could be a case of pulling to recruit more centripetal force for speed alone without a ball and not having to adjust for ball striking (no feedback loop).]

 

Like in the Khan Academy video, the centripetal force is really changing the velocity vector position of the object rotating.  The rotation is started with torque and more linear forces, but then CF comes into play.  

 

Where I differ is the belief that CF is not a reaction to the forces that the club creates and GRFs, but whole the reason for them (once it gets in some kind of rotation from the torques and linear forces).  Going back to Kwon's statement: "When the centripetal force is gone, the object simply starts moving straight due to the inertia. The downswing is essentially the art of generating the centripetal force using the ground and leg action."  We have to create a curved motion with our hands which transmit a curved motion to the club path.  The reason we are generating ground reaction forces in the first place is because we innately have to generate centripetal forces.

 

Our middle ground may be that there is a feedback loop from the body creating CF, without us thinking, but the body also needing to adjust for proper ball striking mechanics.  So definitely a balance.  We can train to swing faster (and pull greater CF) but it needs to be in a feedback loop with golf performance.

And why Ben Hogan was not only athletic but a genesis when it came to the golf swing. That is why his downswing created so much hip and leg action to create whatever you guys are talking about. He was ahead of his time.

[J.Bex]

4 hours ago, airjammer said:

Can anyone explain to the slow kid, how this information makes you better at golf?

 

37 minutes ago, Ifelloffladder said:

I'm with you on this.  WITB: Driver, irons, calculator, etc

LOL

Sorry it doesn't help you get better at golf!

 

The whole subject is really just an explanation of why we see forces that we see during the swing.  If anything it's food for thought for instructors, biomechanists, or people interested in these physics and how it can relate to swing mechanics, body movement, force plate data, stuff like that.  Possibly it can help as people apply methods for speed training.  Or it can simply bring light to why we see certain things in swings of people of different builds and abilities, and help with the narrative for those that try to explain the swing.

 

Centripetal force is just physics.  There's nothing new in physics.  I'm just trying to bring light to it because it's a topic that, looking from the engineering/physics side, seems obvious, yet it's hardly mentioned in golf/sports and highly unknown or misunderstood.  We've seen "ground reaction forces" all over golf media for years, and it can be a bit perplexing without this context even for the experts (like mentioned earlier about a discussion whether GRFs create speed or speed creates GRFs).  I just hope people can start getting centripetal forces into the conversation with an understanding if it, and that Dr. Kwon can possibly shed some light to a bigger audience with his technical background and reach.

[nkurz]

Nice topic!  I've been reading through William McNally's Master's thesis "Forward Dynamic Simulation of a Golf Drive: Optimization of Golfer Biomechanics and Equipment", and there is a lot of overlap with what's being said here.  One of the studies he did involved figuring out the forces on the grip throughout the swing well enough to recreate it with a reasonably accurate simulation model.  Here's a paragraph that seemed particularly apt:

 

Quote

The simulated grip forces provide some interesting insights. Firstly, the application of force by each golfer is remarkably consistent. Secondly, the translational forces in the grip frame indicate that nearly all the grip force at impact is acting along the shaft axis ZG. In other words, the force is nearly completely centripetal. In a way, this result concurs with the idea of “free-wheeling” through impact, as the golfer does not apply significant forces in any direction but the one required to keep the club moving in an arc. Lastly, there is minimal torque applied about the shaft axis throughout the whole swing. This is surprising because it is natural to think that the act of closing the clubface originates from a torque applied about the shaft axis. Evidently, this is not the case.

 

 

This matches up really well with Kwon's reply "The downswing is essentially the art of generating the centripetal force using the ground and leg action."  There's some fine control stuff at the top of the swing to get things set up correctly, but after that, it's all about pulling along the shaft!  

 

It's a wide ranging thesis, and worth the time to read if you are interested in these issues.  McNally published a number of the sections as individual papers.  I'm hoping to make some posts about the conclusions of some of these when I understand them better, particularly his treatment of "gear effect".  Spoiler:  he makes a good case that the forces on the shaft during impact make a considerable difference to the spin for off-center impacts.  Both the thesis and the paper are currently available for free download, but I'll attach them to this post so there's a copy if they should disappear in the future.  

Forward Dynamic Simulation of a Golf Drive.pdf The Golf Shaft’s Influence on Clubhead-Ball Impact Dynamics.pdf

Edited November 30, 2022 by nkurz
Typo s/Kwan/Kwon/

[J.Bex]

15 minutes ago, nkurz said:

Nice topic!  I've been reading through William McNally's Master's thesis "Forward Dynamic Simulation of a Golf Drive: Optimization of Golfer Biomechanics and Equipment", and there is a lot of overlap with what's being said here.  One of the studies he did involved figuring out the forces on the grip throughout the swing well enough to recreate it with a reasonably accurate simulation model.  Here's a paragraph that seemed particularly apt:

 

 

 

This matches up really well with Kwan's reply "The downswing is essentially the art of generating the centripetal force using the ground and leg action."  There's some fine control stuff at the top of the swing to get things set up correctly, but after that, it's all about pulling along the shaft!  

 

It's a wide ranging thesis, and worth the time to read if you are interested in these issues.  McNally published a number of the sections as individual papers.  I'm hoping to make some posts about the conclusions of some of these when I understand them better, particularly his treatment of "gear effect".  Spoiler:  he makes a good case that the forces on the shaft during impact make a considerable difference to the spin for off-center impacts.  Both the thesis and the paper are currently available for free download, but I'll attach them to this post so there's a copy if they should disappear in the future.  

Forward Dynamic Simulation of a Golf Drive.pdf 3.52 MB · 1 download The Golf Shaft’s Influence on Clubhead-Ball Impact Dynamics.pdf 918.64 kB · 1 download

Awesome, thanks for contributing this thesis.

Looks like 1st post... Welcome!

[J.Bex]

Saw some of Kwon's new videos, and he dropped a quick bit of centripetal force (day after responding to me 🙂) around the 5:30 mark:

 

 

This one was another example of a top/thin after starting to "use the ground" more - 1:07:20 mark (sign of greater centripetal force but not adapted yet due to lack of strike feedback?):

 

 

[glk]

9 hours ago, airjammer said:

Can anyone explain to the slow kid, how this information makes you better at golf?

It doesn't but the motion dr kwon promotes can create higher grf torque and better sequencing to generate higher club head speed.  Centripetal force only acts to guide the club head on it's curved path - too much and you  miss the ball inside (shortened radius) as noted previously by soloman.  Too little and you'll likely hit it fat since the club head will move in a straight path - like a car on slick roads - too little tire friction so car slide off the turn. 

[Insidious2]

This is all great, but for most people pretty useless. Who can explain it to an idiot like me and tell me what im supposed to do with it?

[Socrates]

1 hour ago, GoTime said:

This is all great, but for most people pretty useless. Who can explain it to an idiot like me and tell me what im supposed to do with it?

A lot of it just explains what is happening from a physics point of view.  It may allow one to better understand what is going on and how the forces are helping.  I think it might be more important to a high level instructor to know so that they understand that what they are doing is correct and not just BS.  Part of their job is to dumb it down and make it useful to the instructee.

 

The biggest thing I got from all of this (YMMV) is how important strength is in the hands and forearms and how it allows one to control the club and control the speed that can be generated.  The stronger your hands are, the easier it is to control and hold onto the club without becoming tense and losing the ability to create speed.  There obviously is a lot more here, but I wouldn't sweat it.

 

I think I succeeded if I got just one thing out of this...

 

Edited December 1, 2022 by Socrates

[J.Bex]

16 hours ago, glk said:

Centripetal force only acts to guide the club head on it's curved path - too much and you  miss the ball inside (shortened radius) as noted previously by soloman.  Too little and you'll likely hit it fat since the club head will move in a straight path - like a car on slick roads - too little tire friction so car slide off the turn. 

So I'm not going to convince everyone but here's more info.  McNally's research and simulations showed force on the grip at impact to be almost completely centripetal, not surprisingly.  Similar to Tutelman and of course what this thread is suggesting. 

 

I calculated a wedge head swing at 70 mph corresponds to approx 65 lb of centripetal force.  The same head swung at the same instantaneous radius at 35 mph corresponds to 16 lb of CF.  What's too much or too little CF?  We golfers swing all types of clubs at full speed, 3/4 speed, chip shots - all of different centripetal forces.  We can learn to swing faster or with finesse.

 

CF does not require a whole lot of distance to pull. I think this is where people are getting hung up.  You can technically pull 16lb of force on a club in rotation without moving the instantaneous center of rotation.  You can do the same pulling 65lb of force or 100+lb.  The point of that force is to control the angular velocity and clubhead speed higher or lower, not change the radius.  Yes, we can pull and create too much of a pull to displace the head path (like I was saying about topping), but just like we learned to find the bottom of the swing with a slow chip swing and a fast full swing, we can learn to make it work at all different forces.  Little centripetal force in the swing just means little speed.  Large CF means the body is moving faster - hips clearing, larger GRF, etc. - to create speed.  We can't create any speed without some kind of force.  Our forces create speed, not the other way around, and the driving force is centripetal. 

[glk]

5 hours ago, joostin said:

So I'm not going to convince everyone but here's more info.  McNally's research and simulations showed force on the grip at impact to be almost completely centripetal, not surprisingly.  Similar to Tutelman and of course what this thread is suggesting. 

 

I calculated a wedge head swing at 70 mph corresponds to approx 65 lb of centripetal force.  The same head swung at the same instantaneous radius at 35 mph corresponds to 16 lb of CF.  What's too much or too little CF?  We golfers swing all types of clubs at full speed, 3/4 speed, chip shots - all of different centripetal forces.  We can learn to swing faster or with finesse.

 

CF does not require a whole lot of distance to pull. I think this is where people are getting hung up.  You can technically pull 16lb of force on a club in rotation without moving the instantaneous center of rotation.  You can do the same pulling 65lb of force or 100+lb.  The point of that force is to control the angular velocity and clubhead speed higher or lower, not change the radius.  Yes, we can pull and create too much of a pull to displace the head path (like I was saying about topping), but just like we learned to find the bottom of the swing with a slow chip swing and a fast full swing, we can learn to make it work at all different forces.  Little centripetal force in the swing just means little speed.  Large CF means the body is moving faster - hips clearing, larger GRF, etc. - to create speed.  We can't create any speed without some kind of force.  Our forces create speed, not the other way around, and the driving force is centripetal. 

The driving force is the grf and pivoting torques.    

 

The point of centripetal force is to alter the direction not control the angular velocity - CF is a function of angular velocity (and mass as you have posted the F=MA for it).     Angular velocity is angular displacement divided by delta time.    Little centripetal force  does not mean little speed - it means that it may not be sufficient to have the object continue on a curved path - like my car sliding on a slick surface example - unless the car slows down it won't stay on the road - just not enough friction between the tires and the road to support higher speeds so driver has to slow to a speed that will not have them slide off.

 

And the art is that given that a golfer is accelerating the club head in the downswing the CF has to be increased to produce an arc that results in impact - and it is more complex than just CF cause all the while this is happening the trail arm is extending, the left side of the body is rotating and extending - so the arc height of the swings is being managed too - poor job of managing the arc height and even though one can have sufficient CF  one can hit it fat, swing over it, thin it, etc.

[glk]

Dont know if you have  watched these videos from sasho but they address the club kinetics, as applied  to the grip , that create club angular velocity.  Notable by absence is centripetal force since it does not contribute to angular velocity    Dr kwon has done a similar study with like results - he mentioned in his last certification class that he has another paper coming out that addresses the "alpha wars" (jacobs and manzella contend that a positive couple can be applied thru impact against the rest of the golf science community - but refused to share their data claiming proprietary ) - i suspect this paper is another blow to jacobs, etc

 

 

 

Edited December 2, 2022 by glk

[J.Bex]

14 hours ago, glk said:

videos from sasho but they address the club kinetics, as applied  to the grip , that create club angular velocity.  Notable by absence is centripetal force since it does not contribute to angular velocity  

Yes, it's missing from his presentation... Or is it???  Look at these purple net force arrows, circled, that are applied by the golfer on the club.  Why are they pointing in the directions they are pointing?  Why are they pointing towards the center of rotation - centripetally?  At magnitudes similar to what I posted (466.5N is 105lb)?  Because they are centripetal forces!

What do the net forces indicate about the golfer's movement and GRFs, as this is what he's applying to the club?

 

The body pushing weight around in 3D space, and the GRFs associated with that, manifest in net forces being applied to the club.  The body innately knows it needs to create these forces to get the speeds desired. 

 

I had seen these before, but thank you for posting, because I've wanted to hear Dr. MacKenzie make some kind of meaningful statement about centripetal forces but haven't run across it yet.  His explanations seem to only revolve around torque, moments of force, couples, and end there.  That's good, but honestly it's pretty basic.  That's learned in statics classes with forces on beams.  Obviously he is knowledgeable, likable, and presents convincingly well.  But again, looking from a perspective in dynamics it's still very much missing a narrative on CF that's not just brushing it off in passing.

 

He's correct with pointing to the area under the curve in the torque+time graph which indicates a change in angular momentum.  However going back to the hammer thrower, it would be impossible to use the moment of force graph trying to explain a hammer thrower's changing angular velocity, without CF like this, when the forces would be inline with the COM - no moment arm no torque for those angular velocities.

 

15 hours ago, glk said:

sufficient to have the object continue on a curved path - like my car sliding on a slick surface example - unless the car slows down it won't stay on the road - just not enough friction between the tires and the road to support higher speeds so driver has to slow to a speed that will not have them slide off.

The car analogy and tire-to-road friction is correct to CF, and I get what you're saying.  The relation to golf though would only be analogous to grip strength, because otherwise we have a physical tether to the centers of rotation and body with the shaft, that the car does not.  Unless our grip gives out we can't help but move the club centripetally.  The car+driver is creating its own forces and speed; the club cannot without external forces from us.

 

15 hours ago, glk said:

Little centripetal force  does not mean little speed

It does because F = mv²/r.  As force goes down, so does velocity.

 

Look I'm not trying to brush off your thoughts and that Dr. MacKenzie sucks.  He doesn't.  I'm trying to promote the thought and have us think for ourselves.  CF is there, they're big forces, and I feel like it's brushed off.

Edited December 2, 2022 by joostin

[glk]

I never said centripetal doesnt exist, i said it doesnt add to velocity - it doesnt increase the velocity magnitude just changes the direction.     And yes in sasho's presentation the hand couple force decreases significantly thru impact with the moment being dominate - this is the wrist as a free hinge   idea - we lose the ability to continue to apply a positive force via the hands and are hands actually are working to slow the club - but the centripetal force remains.    Long ago i was practicing lightening my grip while taking practice swings in my garage - result was once i stuck a 5i into the ceiling and on a another i shattered my graphite driver shaft when it launched  into the back wall - both had plenty  of velocity just insufficient cf, lol.

 

and hammer thrower continue to add velocity since they continue to spin - believe after they complete 3 or 4 revolutions before they release the hammer.    Im no expert on the hammer throw but from video they certainly appear to create moments which is just another term for torques and like the golf swing the ground is used to generate torque which is translated to momentum of the hammer.   An eccentric force is applied at the hammer handle similar to a golf swing .   A difference is the golf swing doesnt require full body 360 rotation and the objective is quite different unless you are rory and hit a shot into a lake.

 

you have things backward as velocity decreases the necessary centripetal force to continue circular motion decreases and likewise when the velocity magnitude increases so does the required centripetal - whether or not one  can generate the needed cf is not guaranteed.

 

like soloman, i'm bowing out of the discussion.

 

 

[J.Bex]

@glk I respect your and @Soloman1difference in opinion.  At least we're talking about it because it's a conversation worth having that no one else is.  And at least we agree that centripetal forces are there.

 

As you know my opinion is that it does contribute significantly to the angular velocity and clubhead velocity because it's literally in the equations F=mrw² = mv²/r.  This is on top of the torques and force moments that Sasho presents.

[betarhoalphadelta]

4 hours ago, joostin said:

@glk I respect your and @Soloman1difference in opinion.  At least we're talking about it because it's a conversation worth having that no one else is.  And at least we agree that centripetal forces are there.

 

As you know my opinion is that it does contribute significantly to the angular velocity and clubhead velocity because it's literally in the equations F=mrw² = mv²/r.  This is on top of the torques and force moments that Sasho presents.

 

I think the disconnect is cause and effect. The force needed to keep the club on its arc is proportional to the square of the velocity. 

 

But I think this means that the more velocity you have, the more force you NEED. Not that the more force you HAVE, the more velocity you get.  At least at impact.

 

This has already been touched on with the discussions of speed training sometimes leading to topping problems. Once you've developed clubhead speed if you don't apply enough centripetal force, you'll chunk it. If you apply too much, you'll top it. But IMHO the relevant aspect here isn't velocity of the clubhead, it's the radius. Not enough force applied means the radius increases (and you chunk it). Too much force applied and the radius decreases (and you top it). 

 

I think the kinetic chain leading to high clubhead speed is a completely different topic from the centripetal force needing to be applied to stabilize the club on its arc through impact. I think that force is essentially a result of clubhead speed, not a generator of clubhead speed.

 

Full disclosure: I did not stay at a Holiday Inn last night, and I'm only a gEEk, i.e. an electrical engineer, not a mechanical engineer or physicist. So I'm about 25 years removed from my last statics/dynamics classes.

[J.Bex]

14 hours ago, betarhoalphadelta said:

 

I think the disconnect is cause and effect. The force needed to keep the club on its arc is proportional to the square of the velocity. 

 

But I think this means that the more velocity you have, the more force you NEED. Not that the more force you HAVE, the more velocity you get.  At least at impact.

 

This has already been touched on with the discussions of speed training sometimes leading to topping problems. Once you've developed clubhead speed if you don't apply enough centripetal force, you'll chunk it. If you apply too much, you'll top it. But IMHO the relevant aspect here isn't velocity of the clubhead, it's the radius. Not enough force applied means the radius increases (and you chunk it). Too much force applied and the radius decreases (and you top it). 

 

I think the kinetic chain leading to high clubhead speed is a completely different topic from the centripetal force needing to be applied to stabilize the club on its arc through impact. I think that force is essentially a result of clubhead speed, not a generator of clubhead speed.

 

Full disclosure: I did not stay at a Holiday Inn last night, and I'm only a gEEk, i.e. an electrical engineer, not a mechanical engineer or physicist. So I'm about 25 years removed from my last statics/dynamics classes.

Electrical engineers need some love too!  Yes, the disconnect is cause and effect.  The more the thread goes on the deeper I'm looking to be more informed.  I'll say this:  As someone "of science" I'm open minded enough to be OK if I'm wrong.  It's possible I'm wrong.  It's possible I'm not.  It's possible things there's a whole mix of things that we're all somewhat right and somewhat wrong.  Things just have to be proven.  So I was thinking of presenting the following.

 

Let's look at Dr. MacKenzie's data in the vid.  We need some kind of force to be eccentric to the center of mass (COM) to create a moment of force (M).  That force is what the golfer applies at the grip. This is on top of the torque couple created by the hands (C).  The whole idea in the downswing is to create positive total torque (T).  The data shows that nearing impact C becomes more and more negative - meaning we're torquing with the hands the "wrong" way (which isn't intuitive at first, and also sounds like the debate in the "Alpha War").  But the M becomes bigger, ovecoming C.  Like this approaching impact with the clubhead speed increasing (CHS):

Ok, all net Torques are positive.  Good.  The progression of T is a little weird because they decrease then increase. But ok... it's just 1 swing sample of a guy getting up to 119.5mph.

 

What creates angular velocity according to Dr. MacKenzie and everyone?  Torques M and C.

What creates M?  The F NET force magnitude times its moment arm from the COM based on its direction.  [In the last pic, moment arm = 37.5Nm/466N = .08m = 3.17"]

What direction is F NET pointing in these frames?  Centripetally. They're literally centripetal forces creating M, which create angular velocity.  That's even without looking at the CF equation.  That's my dilemma with seeing CF as simply a reaction, or enough or not enough.  You guys know my thoughts already on body movement and Dr. Kwon's statement supporting this.

 

Let's say we don't want a centripetal pointing force, but instead, as most might believe, we want to apply force along the path of the club or target (which seems intuitive) - like this yellow arrow:

Or this:

The problem with these is that it will create a negative Moment of force.  We would have to overcome that with a large C torque with the hands, which is completely opposite of what all the data shows and Dr. MacKenzie presents.  Not good.  You want centripetal pointing force with a little lead to the club COM according to the analysis.  The bigger the lead, the bigger the moment arm, the bigger the Moment.  Again to me this explains body movements seen like long drivers pulling up/rearward/away at impact. 

 

Weird things happen right before and at impact:

Dr. MacKenzie didn't discuss this other than saying there's noise at impact, so the Torque data in the last pic supposedly isn't reliable:  Torque goes from positive to negative pretty drastically not even looking at impact.  Yet the clubhead is still picking up speed with negative Torque and decreasing forces.  This would be where wrist hinge can go "free".  But can we explain the momentum of the clubhead doing this with a lag in effect?  Is it just the lateral speed of the hands (moving frame of reference) adding a little speed at this point?  Centripetal forces remain high,  supporting speeds, though they're decreasing... Maybe a change in radius to the centripetal pull?  Parametric acceleration?  Just our body's attempt to stop limbs to allow the clubhead's momentum to whip through as the last link in the kinetic chain?  Combination of above?  Would love to hear thoughts.

[betarhoalphadelta]

Something struck me about this today. When you're looking at all of the net force diagrams in @joostin's post, it's a 2D representation from a face-on view. 

 

This causes a mental thought of dealing in an x-y plane where x is the force positive/negative in relation to the target line and y is the force positive/negative in relation to vertical. 

 

At impact, that force looks very vertical, making you think that your centripetal force is pulling "up". 

 

And to an extent it is... But not entirely. It's also pulling "away", in the z dimension (forward/back relative to the ball). 

 

If you're swinging properly, and have basically an in->out->in swing pattern, you have two things that centripetal force is doing. First, it's keeping the clubhead on the vertical portion of the arc so it doesn't drive into the ground before you get to the ball. Second, it's keeping the clubhead on the horizontal portion of the arc so it doesn't swing OUTSIDE the target line before you get to the ball. 

 

Joostin talks about 115 lb of force that you must apply to keep the clubhead on the arc... But the club isn't vertical, so the arc isn't vertical. It's also pulling the clubhead *away* from the ball on the axis defined as "z". 

 

This is important. When we talk about all the problems we have as ams with things like early extension, throwing the trail hip at the ball and not gaining lead hip depth, it's all about having the ability to pull the clubhead on its arc away from the tangential path. And if you can't pull "backwards", you only get one of the two axes covered. 

 

 

Edited December 8, 2022 by betarhoalphadelta

[J.Bex]

10 hours ago, betarhoalphadelta said:

also pulling the clubhead *away* from the ball on the axis defined as "z". 

 

This is important. When we talk about all the problems we have as ams with things like early extension, throwing the trail hip at the ball and not gaining lead hip depth, it's all about having the ability to pull the clubhead on its arc away from the tangential path. And if you can't pull "backwards", you only get one of the two axes covered. 

Exactly.  The pic I had on the first page of Justin Thomas DTL showed this with vertical and horizontal arrows.

 

If you think in terms of what your body does to create centripetal force, it makes things apparent.  For example Kyle Berkshire pulling his lead foot away, an amplified clearing of the hips, on top of going weightless at impact:

 

[J.Bex]

Just adding a few more pics here because I saw Kyle Berkshire broke his ball speed record again, 236.2 mph, ridiculous...

 

DTL showing of the force vectors, again paying attention to the direction he's producing force at this moment and where the body is moving is very apparent with him.  Approx centripetal force of the clubhead alone calculates to 185lb here:

 

With JT his force generation is visually most apparent vertically going on his toes:

 

With Scheffler his force generation is visually most apparent horizontally with his feet sliding back:

 

With Sadlowski he visually has a lot of tilt at impact.  Face on view this is apparent along with his "chicken wing".  Also the loading of the ground on the downswing:

 

With Rory he doesn't jump up or back visually, but of course the forces are still there and will show up on the force plates.  Mr. Smooth:

 

The main forces are centripetal forces around impact, same as Dr. MacKenzie's diagrams shown before, pulling towards the body, where all their body movements are indicating... especially Kyle!

[J.Bex]

Here's some application in regards to muscle and strength training implications in light of centripetal forces, especially the pulling aspect around impact:

 

Core:

Generating so much force so fast in rotation we obviously want our core to be strong and resilient for force transmission between the lower and upper body.  Rory has mentioned he consciously would brace the core when setting up for a big drive.

- Some exercises:  planks, anti-rotation like Pallof press, loaded trunk rotation, med ball twists, side bends, leg raises, hollow body holds, even the out of fashion crunches...

 

Posterior chain:

Many including long drivers talk about the importance of posterior chain strength.  From a centripetal force view it's obvious with the forces generated - the highest of which around impact is basically pulling as shown in the previous post.

- Some exercises:  Deadlifts, RDLs, hip thrusts, bridges, rows, pullups, pulldowns, face pulls, back extensions... everything for calves, hamstrings, glutes, back

 

Triple Extension:

Of hips, knees, ankles - which in large part consists of posterior chain.  Calves, hamstrings, quads, glutes.

- Some exercises:  Jumps, sprints, DLs, squats, Olympic lifts, high pulls...

- Here's Tiger going from his "squat" (triple flexion... which I might post about in a future post) to triple extension around impact, as apparent by the angles of the joints:

 

(Btw hitting without legs, knees on the ground, there's still lots of explosive hip extension to generate force in rotation)

 

We know young Tiger would snap his lead leg for more power (perhaps to it's demise), but it's really easily explained why he did that when you think in terms centripetal force direction and magnitude.  With his added mass and changes in mechanics, he doesn't rely on that snap anymore, though there still is knee extension.

 

It's like a young kid who can hit a mile:  They're probably jumping out of their shoes because they have to move all their mass as fast as possible to pull all that centripetal force.  When they grow up and put more mass on, they don't have to jump out of their shoes because they have more mass to work with that just has to move a little (but still quickly) to generate the same amount of force.  Best case you have high fast twitch muscle composition, mass, strength, and of course great mechanics.  Easy speed!

 

Edited December 20, 2022 by joostin

[nkurz]

I apologize that I don't know how to phrase this clearly and haven't thought it through fully, but reading through this thread, I'm wondering whether there is an important difference between force applied along the shaft (let's call it 'axial' force) and force inline with the center of gravity (call it 'centripetal' force).   Some people seem to be treating these two as close enough to be equivalent, but I'm not sure that they are.  

 

At the top of the backswing, the club is bent backward and the COG is probably several inches behind the shaft.  If the golfer simply "pulls" along the shaft, doesn't this apply a torque to the head that accelerates it not just down but foward?  As the downswing continues, the reaches a point where it is straight (with COG still an inch or so behind), and then just before impact the club is bowed forward, but presumably with the COG still a bit behind, more so for a stiffer shaft.  

 

How should this difference affect our interpretation of Mackenzie's force model?   If I'm remembering right, I think he's using a coordinate system that defines an axis from the COG to the middle of the golfers grip.   He then splits things up into a centripetal force and torque.  Which is a fine way of viewing things, but I'm not sure if it produces the right intuition. 

 

Is my mental model of the axial force including a built in torque correct?    How large is this effect?  How much difference does the rearward position of the COG of a driver make?    What if it was twice as much (or if one used a shaft that had double the bend)?  Are the forces on an iron (with the COG much closer to inline with the club) more purely centripetal?  

 

 

[J.Bex]

3 hours ago, nkurz said:

At the top of the backswing, the club is bent backward and the COG is probably several inches behind the shaft.  If the golfer simply "pulls" along the shaft, doesn't this apply a torque to the head that accelerates it not just down but foward?

Yes, but the data shows we're not pulling along the shaft at this point.  Here's the club after it already started moving out of the top position.  Net force is at an angle to the shaft (circled):

 

3 hours ago, nkurz said:

just before impact the club is bowed forward, but presumably with the COG still a bit behind, more so for a stiffer shaft.

Yes the forward bend indicates that we're pulling somewhat axially (data shows net force leading the club a little), and the CG is attempting to align with the direction of pull through the hands.

 

3 hours ago, nkurz said:

How should this difference affect our interpretation of Mackenzie's force model?

 

3 hours ago, nkurz said:

Is my mental model of the axial force including a built in torque correct?

It seems you're pretty much saying what MacKenzie presents!  Just the direction of net force isn't necessarily inline with the shaft axis or with the club CG all the time.  Preceding impact it looks mostly axial.  Whether the force is inline with the shaft axis or with the CG or a little leading (as data shows), it's all really centripetal pointing close to the instantaneous centers of rotation.  The shaft locks our hands in position, along the shaft axis, but I believe we swing in reaction to the feel of the CG - not locked into pulling along the shaft axis. 

 

3 hours ago, nkurz said:

How large is this effect?

The induced torque pretty much dictates the dynamics according to the MacKenzie analysis.  I personally don't think it's the whole story as centripetal force is being pulled even without the force vector leading the CG for that torque, carrying angular velocity. 

 

3 hours ago, nkurz said:

How much difference does the rearward position of the COG of a driver make?    What if it was twice as much (or if one used a shaft that had double the bend)?

I'd imagine we adapt intuitively to a change in CG with our biomechanics.  However, all things equal, to MacKenzie type analysis, the more rearward the CG and the stiffer the shaft (to restrict the forward bend), the better as you will have a bigger moment arm.  Double the moment arm, double the moment of force while applying the same net force.  I don't know how true that is in practice though.  Not sure how close CG gets to aligning to the shaft axis at the hands, but that all affects dynamic loft, lie, and face angle.  You also see long drivers bomb it with XX shafts and "regular" shafts alike (Kyle B). 

 

8 hours ago, nkurz said:

Are the forces on an iron (with the COG much closer to inline with the club) more purely centripetal?  

I haven't seen iron data, but there would definitely be less forward bend due to it's CG, so possibly the force would be more inline with the CG to the hands.  I always thought it would be interesting to see a driver that had it's shaft inline with the head CG - like an Orange Whip or a LAB putter - no forward bend, no toe droop.  It would be illegal but fun as a practice club.

[RoyalTroon]

What is the centrifugal piece called if not a true force, a centrifugal property?

[betarhoalphadelta]

2 minutes ago, RonJon said:

What is the centrifugal piece called if not a true force, a centrifugal property?

 

No. As stated, it is something that flat out doesn't exist. As we say in golf, feel isn't real. 

 

When you get a golf club moving around an arc, the clubhead has a velocity that is tangential to the center of the arc. The centripetal force is what is PULLING on the club to continue changing its direction and keep it along the arc. 

 

It *feels* like the club is pulling away from you, but it's doing no such thing. If you let go of it, it won't go in the direction of the "feel", which is away from your hands, but will continue in the direction it's going. 

 

I.e. if we're talking about impact, you're pulling up and away from the target line. If there was true centrifugal force, letting go of the club would send it down and away from you (into the ground and across the target line). But that's not what happens. if you let go of it, it will fly towards the target because that's where its momentum is headed. 

 

 

[RoyalTroon]

Some impact grenades are armed in flight using spiral rifling.  The centrifugal property created by the spin moves internal weighted pins outward, or away from center,  toward the shell casing thus exposing the detonator to the firing pin.