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

[joostin]

Posted this in another thread but just wanted to put it out on its own.

I wanted to show where centripetal force is pointing and how much force we're pulling on the clubhead as we're generating and supporting its curved path.  I think it's important to how it relates to body movement and why we see things like Ground Reaction Forces, jumping, squatting, etc.

 

***If unfamiliar with centripetal forces, please check out this 6 min video.  The rest of the thread will make more sense.

 

Jamie Sadlowski's lead arm chicken wing at impact was one of the reasons I got interested in the physics of the swing because initially I figured you want almost full extension at impact, yet this smallish guy was winning long drive with a chicken wing.  It didn't make sense at first look, but then I remembered physics....

 

Just looking at a lot of the long drivers that leave their feet, and even pics of tour players at impact, it's obvious that around impact they are basically pulling (up / away / back), almost like an Olympic hammer thrower (below) but to a lesser degree.  This is because to get the clubhead moving in an arc we need to create angular momentum and centripetal force - force pointed toward the center of rotation.  If you look at slo mo vids, around impact the instantaneous center of rotation of the clubhead is pretty close to the grip end butt of the club (below).

 

I'm gonna calculate the centripetal force of a 200g driver clubhead with its CG 45" from the instantaneous center of rotation at impact, for Brooks Koepka's clubhead speed of 121 mph:

 

Centripetal Force = mass * centripetal acceleration

= mass * radius * angular velocity²

= mass * tangential velocity² ÷ radius

= (200g * (121mph)²)÷45"

= (.2kg * (54.09m/s)²)÷1.143m

= 511.94N

= 115.1lbf <-- 115 pounds of force, just the clubhead alone (more if you calculate the whole club), that Koepka or anyone else is pulling instantaneously at impact swinging at 121mph.

 

IMO the reason Sadlowski and Koepka are chicken winging is not because they're doing it consciously, but because that is part of their natural athletic recruitment of their muscles for this motion in creating and supporting this force.  They're partly recruiting their lead biceps for this pull.  It could possibly stem from a hockey background, or it could just be innate.  Others will recruit muscles differently.  Obviously many don't have a chicken wing and swing just as fast.

 

To create the 121mph swing above, there is significant centripetal force being generated, and that force points away from the clubhead, towards the center of rotation.  Centripetal force governs the physics of the rotating object in any point in the swing arc (just as the sun's gravity pulls on the earth in orbit).  If you look at any point in someone's swing, you can almost tell which way the centripetal force vector pulls by where their weight is shifting or the direction their body momentum is going and anticipating.

 

 

Can even explain Tiger's squat - just part of his muscle recruitment motor pattern

 

 

Wherever the center of the club path arc is at any point in the swing is where the centripetal force is pointing, and is inline with how we are creating, supporting, and anticipating those forces with our body movement.  "Using the ground" has been spoken of so much, but I always felt centripetal forces were mostly not part of the conversation or not really understood  when hearing or reading these things.  We have to push the ground and pull on the rotating object to create these forces.  The more force we can generate, the quicker we can pull, the faster the clubhead will go.  On the pic above, the clubhead is governed by F= mv²/r (clubhead mass and velocity), but you can view the guy pulling his own mass at F=ma.  The dude needs to accelerate his body mass in the direction of the arrow, which constantly changes direction.  We all do this innately, some better than others!

 

Got a little technical, but I think it's worth knowing centripetal force direction as it applies to the swing.  Hope this gives some food for thought, esp. for instructors to put a little more science into reasonings behind swing concepts.

Edited November 30, 2022 by joostin
Updated title to avoid centrifugal confusion

[Zitlow]

I'm no Alfred Einstein but I do believe that's what Mike Austin was referring to when he talked about escape force and the corresponding holding force. 

[getitdaily]

Sacho Mckenzie effectively notes this. He says when you swing an axe into a tree your body doesn't keep moving toward the tree. As the axe comes around, the force of the axe pushes the body away from the tree.

[golfie1]

  The calculation is way more complex than this.  The clubhead is not on a string.  If you with no wrist c0ck then this would apply.

[joostin]

1 hour ago, golfie1 said:

  The calculation is way more complex than this.  The clubhead is not on a string.  If you with no wrist c0ck then this would apply.

Of course, that's why we use instantaneous center of rotation with moving frames of reference in physics classes.  If you look at the pics above, around impact there's a lot less translation around grip end than there is rotation of the clubhead, and the instantaneous center is a little past the hands.  So we can approximate the radius, then we already know instantaneous tangential velocity at impact (measured clubhead speed) and its mass to easily calculate the force at that instance in time.  Centers of rotation constantly change, so do tangential velocities and forces, but we only normally have speed measured at impact, so that's what we can calculate for now.

 

Dr. Kwon's plot of instantaneous centers of rotation shows similar to the slo mo and strobe pics in Figure 4:

http://drkwongolf.info/biom/fsp.html

[joostin]

3 hours ago, Zitlow said:

I'm no Alfred Einstein but I do believe that's what Mike Austin was referring to when he talked about escape force and the corresponding holding force. 

 

2 hours ago, getitdaily said:

Sacho Mckenzie effectively notes this. He says when you swing an axe into a tree your body doesn't keep moving toward the tree. As the axe comes around, the force of the axe pushes the body away from the tree.

Yep.  The thing is, we are creating and anticipating those forces, so we're not as much reacting to the swinging object's pulling force, we are creating all that force.

 

Alfred.. Albert's bro? 😉

[Zitlow]

Alfred, a big hitter was Alberts's cousin. 

 

Edited November 20, 2022 by Zitlow

[jonsnow]

22 hours ago, Zitlow said:

Alfred, a big hitter was Alberts's cousin. 

 

Were you referring to Alfred Einstein the famous musicologist & music editor? 

[DaveGoodrich]

This really demonstrates what it means to "swing within yourself." We may not be swinging 121 mph, but  the forces are significant nevertheless.  Swing too fast, relative to your core strength, and the forces you need to generate will be too high for you to maintain stability and balance. 

[joostin]

1 hour ago, DaveGoodrich said:

This really demonstrates what it means to "swing within yourself." We may not be swinging 121 mph, but  the forces are significant nevertheless.  Swing too fast, relative to your core strength, and the forces you need to generate will be too high for you to maintain stability and balance. 

Adding a chart to show approx. centripetal force around impact for a large range of driver swing speeds:

 

 

The quicker you can move your mass around, the more potential you have for generating force and clubhead speed.  If a smaller person and larger person swing at the same high speed, it's more likely to see the smaller person leave their feet.  But it depends how the person recruits movement for that force.  With the club swinging at an angle to the ground, jumping off the ground indicates the vertical component of the force, but the horizontal component would be indicated by shifting weight away from the clubhead rearward and tilting trail side.

 

 

Side note:  In the chart I put an * because the term lb gets a bit confusing as there's lb(force) and lb(mass).  For our purposes they are numerically equal.  Weight scales as we know them to measure mass are only correct on earth's surface, but they actually measure force in lb(force) or newtons, then basically convert to mass in lb(mass) or oz or kg or g.

[Righty to Lefty]

 Fantastic @joostinbut I a not going to lie, I feel some sort of way towards you right now being that for the longest I felt like I was one of few that was using the math of golf to better understand the athletic side and then you post this showing that you are just as deep down the rabbit hole as I am!  I always knew you were tracking, but I didn't know truly how much you were tracking with what I was posting about the math of golf and how important it is.

 

I was taking a physics class years back and learning about centripetal force when I was exposed to the concept and that is when I saw the planet orbiting around the Sun diagram and while everyone else saw just that, I immediately saw a golf swing. Then the old trash can lid video that I always post truly put things into perspective when I saw that it is no different than the planet revolving around the Sun diagram and that it is just depicting one vector denoting impact at that given moment.  It then made sense to me why everything in the golf swing is so counter intuitive and how you must work in opposition to the force being created to remain stable.  My swing became rock solid stable and takes almost no toll on me physically because I don't fight with the club anymore and it has been quite eye opening to say the least. 

 

I remember the looks that I would get when I would tell people that you are actually trying to "decelerate the club head into impact" but since you are hold onto the butt end of the club that it will stabilize the club face, force rotation, and maintain a stable swing center, which will facilitate an efficient transfer of energy onward to the club head creating a ton of speed and force. This is also why I always harp on low point orientation and that is because if the athlete is aware of their low point, you will play your best golf but you must trust where you are in space in relation to the ball for your swing motion to truly calm down and become its most efficient version. Great stuff...keep it coming!  R to L  

[joostin]

5 hours ago, Righty to Lefty said:

 Fantastic @joostinbut I a not going to lie, I feel some sort of way towards you right now being that for the longest I felt like I was one of few that was using the math of golf to better understand the athletic side and then you post this showing that you are just as deep down the rabbit hole as I am!  I always knew you were tracking, but I didn't know truly how much you were tracking with what I was posting about the math of golf and how important it is.

 

I was taking a physics class years back and learning about centripetal force when I was exposed to the concept and that is when I saw the planet orbiting around the Sun diagram and while everyone else saw just that, I immediately saw a golf swing. Then the old trash can lid video that I always post truly put things into perspective when I saw that it is no different than the planet revolving around the Sun diagram and that it is just depicting one vector denoting impact at that given moment.  It then made sense to me why everything in the golf swing is so counter intuitive and how you must work in opposition to the force being created to remain stable.  My swing became rock solid stable and takes almost no toll on me physically because I don't fight with the club anymore and it has been quite eye opening to say the least. 

 

I remember the looks that I would get when I would tell people that you are actually trying to "decelerate the club head into impact" but since you are hold onto the butt end of the club that it will stabilize the club face, force rotation, and maintain a stable swing center, which will facilitate an efficient transfer of energy onward to the club head creating a ton of speed and force. This is also why I always harp on low point orientation and that is because if the athlete is aware of their low point, you will play your best golf but you must trust where you are in space in relation to the ball for your swing motion to truly calm down and become its most efficient version. Great stuff...keep it coming!  R to L  

You're not alone!  Thanks for sharing the interest.
In fact no one does the math better than Tutelman, who writes about this:  https://www.tutelman.com/golf/swing/gripPressure.php

Tutelman runs circles around myself and everyone else as far as the math, but the view I'm taking with this, as you are, is about swing implications.
(BTW Tutelman's centripetal force numbers are a little off from mine because he's using a center of rotation point between the 2 hands, which I don't see as reality in the swing, but he does include the shaft contribution.)

Here's why I think this subject is missing in the greater golf world.  I think people understand centrifugal force - an object pulling on you, because it can be felt.  Yes, centrifugal is technically a fictitious force pointing outward.  Centripetal is the "real" force pointing inward, but the centrifugal is felt relatively tugging on you.  Because of that, it seems most people see centrifugal force as something that we have to counterbalance with ourselves - a reaction to the centrifugal force.  However my stance is that it's really the opposite:  We create inward pointing force as centripetal force tells us:  The clubhead is "pulling" only because that's what we are pulling on it - in order to have it rotating in the first place and getting it to speed up.

 

Here's another way of putting it.  On force plates around impact, some players are seen going weightless.  I'm probably doing similar myself looking at video.  The clubhead pulling 100+ lbs doesn't add to our own weight (let's say 200lb) as we are swinging on the plates (to make it 300lb).  We're not even counterbalancing it to make it a wash (200lb+100lb-100lb = 200lb).  Nope, we're not even putting our own full weight on the plates at that time - it can be weightless (0lb).  We have to create centripetal force pointing up and away around impact - vertically and horizontally - in order to keep the clubhead moving in its arc at impact at any speed.  More centripetal force, more speed.

At the top of the backswing we have to create angular momentum with torque to get the clubhead moving into the downswing.  Then we have to create centripetal force pointing down in order to keep the clubhead moving in an arc.  Then sideways.  Then up.  We have to constantly create centripetal force pointing from the club towards ourselves during the entire swing - as the strobe light picture with arrows shows.

Edited November 22, 2022 by joostin

[joostin]

I wanted to start showing other athletic movements, because I believe that generating centripetal force is powering a lot of sports, and that our bodies know to do it innately. 

 

Here's an article from Rod Cross, a physicist from Sydney University on swinging a tennis racquet:

http://www.tennisindustrymag.com/articles/2006/07/wrist_snap_in_the_serve.html#:~:text=Such a force is called,forward in a straight line.

 

For a guy serving a tennis ball at 115mph (max racquet speed at 103mph) he approximates centripetal force at 151 lb.  Cross even relates the golf swing to tennis in that torque dominates the swing at first (introducing angular momentum like I said above), then centripetal force dominates afterwards.  Also how in the kinetic chain, limbs have to slow down to allow the next pieces of the chain to speed up - just like golf.

 

I wanted to show a tennis serve because it's opposite a golf swing with the ball being hit overhead, yet the physics are the same.  All the pros jump to serve the ball.  They are basically "using their weight", in the same manner that golfers "use the ground".  Here's Andy Roddick, one of the most prolific servers ever, around impact:

 

I drew horizontal lines to show that he is no longer accelerating up, basically at the top of his jump, and the last hinge in the kinetic chain - the wrist - is pulling down.  The centripetal forces acting on the racquet point like this:

 

Gravity is pulling on his mass (green arrow F=m[body]*g), supporting his downward centripetal force pull on the racquet (F=m[racquet]*v²/r) to speed its velocity.  As everyone serves, tossing the ball with the lead hand up, that arm quickly tucks down to help speed rotation and to help get body mass moving down for this motion.  Rear leg kicks back also to move mass in support of these forces and for balance.

 

***In essence, at the end of the kinetic chain (at impact or the end goal position) we want our net bodily force pointing from the object being swung towards ourself, propelling that object.***

 

The slower the speeds and the bigger the person's mass, the less we may notice this pulling effect.  For instance a wedge head at 300 grams swinging at 20 mph at ~40" instantaneous center of rotation is only requiring 5 lb of centripetal force - doesn't take much body movement to support that force.  More to come...

Edited November 25, 2022 by joostin

[Socrates]

On 11/22/2022 at 6:49 AM, joostin said:

You're not alone!  Thanks for sharing the interest.
In fact no one does the math better than Tutelman, who writes about this:  https://www.tutelman.com/golf/swing/gripPressure.php

Tutelman runs circles around myself and everyone else as far as the math, but the view I'm taking with this, as you are, is about swing implications.
(BTW Tutelman's centripetal force numbers are a little off from mine because he's using a center of rotation point between the 2 hands, which I don't see as reality in the swing, but he does include the shaft contribution.)
 

 

I really liked the Tutelman article.  It always baffled me when people said you need to grip the club like you were holding a bird.  It just seemed wrong as I knew that the clubhead at 100 mph was very heavy and you needed a lot of grip pressure to hold onto the damn thing or it would fly out of your hands.  Tutelman's dissertation (I didn't read every word or formula because my brain would have exploded) answered a lot of my questions.  The big thing that I took out of it, was that having strong hands allowed you to grip the club with more pressure while not being tensed up.  Excerpt from Tutelman article.   (I'll note that Monte is posting in a Biomechanics Forum, which is amazing in it's own right.)  

 

...

Which brings me to the real value of hand strength. Between writing the previous paragraph and this one, I took a lunch break and caught up with a biomechanics forum where I hang out. What was freshly posted there came from a past long drive champion (Monte Scheinblum, 1992 US champion). It was so spot-on for this point, let me splice together a couple of sentences from the thread so it stands on its own:

The player has to be capable of withstanding a great deal of outward force from the clubhead’s momentum near impact, which is why I believe my ability to create high speeds is greatly attributed to my 100^th percentile hand strength.

I responded:

Yes, but cause and effect get a little muddied here. The hand strength, rather than helping create the speed, allows you to hang onto the club. BUT... Yeah, it also helps create the speed, because your hands, wrists, and forearms can be more relaxed when exerting the force necessary to hang on.

Monte's reply was that he agreed 100%.

The significance is that Monte can use a much smaller fraction of his strength capability to prevent the club from flying off. With all that hand strength left in reserve, he can relax his forearms, wrists, and hands at a higher speed than you or I could while exerting the force necessary to hang onto the club.

 

 

[joostin]

42 minutes ago, Socrates said:

With all that hand strength left in reserve, he can relax his forearms, wrists, and hands at a higher speed than you or I could while exerting the force necessary to hang onto the club.

Agreed...  And sorry for the math, it's just a simple version of what Tutelman presents 😉

 

I honestly think so many things can be explained with the kinetic chain coupled with centripetal force.  With the wrists being the last link in the kinetic chain, it makes sense you want them to be relaxed even with the math.  F=mv²/r says that with mass and velocity fixed, the smaller the "r" radius, the higher the force.  So even mathematically we want that center of rotation of the last link in our chain to be as close to the wrists as possible, not increasing that radius because of stiff wrists.  It kinda contradicts each other:  The faster you swing, the more force and gripping strength you need, but the more you want to take advantage of a relaxed wrist hinge.  Thinking of the guys that sometimes release their trail hand (Couples, Singh..) it actually makes sense.

 

Rod Cross, in the tennis link above, mentions this about wrists:

The fact that a racquet can indeed be swung on the end of a length of string proves that a wrist torque is not needed to swing a racquet. A wrist torque helps to get things started, but it’s really the centripetal force that matters most. 

 

Speaking of biomechanists...  I think centripetal force is not in everyone's normal vernacular (like "using the ground", GRF...), because in large part they've been looking elsewhere, maybe because of what they measure, not sure.  Dr. Sasho MacKenzie says there are 4 things determining clubhead speed:

- Hand path length

- Force along hand path

- Amount of rotation

- Torque applied around that rotation 

[Fit For Golf podcast ep.13]

 

What he's saying is everything inline with the clubhead and hand paths, which is fine, but without centripetal force IMO that's missing a big piece of the puzzle.

 

Another Fit For Golf podcast, ep.17... Group discussion on:  Does speed create the GRF or does GRF create the speed?

 

Dudes... our own bodies generating centripetal force is creating GRF and speed.  The physics is staring us right in the face.

 

I have to believe we will hear more of this.  Seed planted here on golfwrx!

 

[glk]

Centripetal force causes the curvilinear path of the club head   That is it   Has nothing to do with grf, etc.  

 

you seem interested so id recommend taking dr kwon certification classes    Next sessions are in march/april  30 hours of classes    Centripetal force is one slide - centrifugal is more but that is the basis just for telling you to ignore anyone who talks about centrifugal having anything to do with the swing.

 

 http://drkwongolf.info/courses.html

 

class flyers  with outline   http://drkwongolf.info/documents/Flyer 2023-03 Global.pdf

Edited November 26, 2022 by glk

[joostin]

6 hours ago, glk said:

Centripetal force causes the curvilinear path of the club head   That is it   Has nothing to do with grf, etc.  

 

you seem interested so id recommend taking dr kwon certification classes    Next sessions are in march/april  30 hours of classes    Centripetal force is one slide - centrifugal is more but that is the basis just for telling you to ignore anyone who talks about centrifugal having anything to do with the swing.

 

 http://drkwongolf.info/courses.html

 

class flyers  with outline   http://drkwongolf.info/documents/Flyer 2023-03 Global.pdf

Thanks, I would consider the classes.  I do think Dr. Kwon is the best in the industry and have watched and read probably hours of his technical explanations.  I took a look at the latest course outline http://www.drkwongolf.info/documents/Level I Outline 2021-03.pdf, and as a lowly mechanical engineer do have at least a cursory understanding of all the technical bulletpoints.

 

The most I've seen to date on centripetal force in the golf swing was in reading of K. Miura's parametric acceleration paper from 2001.

 

As far as CF causing the curvilinear path of the clubhead, yes.  As far as it having nothing to do with GRF, I honestly haven't seen anything to prove that it doesn't.  I look specifically for this, which is why I started this thread, because signs point to it being so.

 

CF of course obeys Newtons 3 laws.

- 1st law:  Center pointing force keeping the clubhead rotating.  Without CF it will just go off in a straight line, tangent to its arc.

- 2nd law:  F=ma, where centripetal acceleration = v²/r

- 3rd law:  There's no centrifugal force acting on the clubhead rotating, however there is an equal and opposite force acting on the object holding it in rotation - the shaft undergoing tension.

 

It's the 2nd law that I'm saying is body created.  F=mv²/r (club mass, velocity, instantaneous radius of rotation), created in large part by F=ma (body mass & acceleration).  Everyone agrees we create torque.  Torque from rotating the torso; torque created by a moment arm between the CM and COP force vectors; etc.  We also know that we're creating all this centripetal force.  We would physically need to move body mass just to support all that force.  For an Iron Byron machine, it needs a lot of weight and anchoring to support the forces.  Also, I've seen the double link model driven by torque that's been around for a long time, and simply have a feeling that all the focus has been on torque creation but nothing to find in the way of supporting all this CF.  That's why I believe otherwise - that these torques and GRFs have everything to do with CF.

 

No I don't know better than Kwon, MacKenzie, others, not close.  However I've dug enough as an outsider golf science nerd to see that CF is not spoken of much, and have taken enough engineering classes that I can't help but see it acting in plain view. 

[Soloman1]

Some comments in random order:

 

1. I don't see a relationship between centripetal force and the jumping movement in a tennis serve. Impact with the ball at a height creating a steeper downward ball flight is the goal, not as a power move. The longer path and radius of the racquet creates the angular velocity.

 

2. The vector of centripetal force (and phantom centrifugal force) doesn't contribute to vectors of angular velocity or force vector at the ball. I think it should be considered more of a stabilizing effect and not a cause.

 

3. Vertical GRF is the last force after horizontal and rotational forces and peak vertical GRF is before impact. The handle raises approaching impact and "jumpers" have a straighter lead arm to maximize radius/velocity when rotational braking action has reached a limit earlier than non-jumpers.

 

4. When considering if a movement or force contributes to faster angular velocity, does pulling harder on the handle increase velocity? Does pulling less decrease velocity? No. It contributes to the 3D club head position at impact.

 

Again, I think looking at centripetal force as an effect is correct. There is no centripetal component in the angular velocity equation.

[Socrates]

18 hours ago, joostin said:

......

The faster you swing, the more force and gripping strength you need, but the more you want to take advantage of a relaxed wrist hinge.  Thinking of the guys that sometimes release their trail hand (Couples, Singh..) it actually makes sense.

 

A long time ago (the '80's) I remember watching video of my swing and I noticed my right hand came off like Singh and Couples.  Many would tell me that was wrong and I needed to fix that.  Good thing I never really listened to that and now I truly understand why that happened.

 

All the math is fine and I usually trust the math as long as the conclusion reached is logical based on what I know about Physics (1st year University level - you know - like DeShampoo 😉).  Love it when it all ties together and you can follow the process until the end.  (Light bulb comes on!) 

 

I first ran across Tutelman in the 90's and downloaded his stuff off an ftp site (this was before html was a thing).  He helped me understand a lot of what I knew and forced me to correct the things I thought I knew, but were wrong.  Keep up the good work.

[joostin]

Critique is welcome.  I realize it's a "new" concept that's unproven/unmeasured, and biomechanists and instructors aren't on board at this time.  I'll try to rationalize with science.  Here are responses.

22 hours ago, Soloman1 said:

There is no centripetal component in the angular velocity equation.

There actually is.  Centripetal acceleration is radius times angular velocity (w) squared, so CF = m*r*(angular velocity)²

22 hours ago, Soloman1 said:

2. The vector of centripetal force (and phantom centrifugal force) doesn't contribute to vectors of angular velocity or force vector at the ball. I think it should be considered more of a stabilizing effect and not a cause.

CF vector is not inline with the tangential velocity because that's not how it works.  It would only create the velocity of the clubhead.  From there we have to take conservation of energy and momentum formulas to determine the collision physics (KE=1/2mv² for both bodies before and after collision and m1v1 = m2v2)  But for the force of the clubhead, it is the CF equation above because the only acceleration is pointing to the instaneous center of rotation (changing), not tangent to the arc.

 

22 hours ago, Soloman1 said:

4. When considering if a movement or force contributes to faster angular velocity, does pulling harder on the handle increase velocity? Does pulling less decrease velocity? No. It contributes to the 3D club head position at impact.

It does, this is exactly centripetal force!  Watch an Olympic hammer thrower.  There's only a chain so they can't apply any torque or hand path forces.  The only real force they can apply to speed up the ball/hammer is centripetal force.  Those that can apply more force throw it farther as they generate more angular and tangential velocities.  How do you speed up a ball on a string?  Only by pulling CF.  There's no torque, no path forces that can be applied to it in rotation. 

 

It is exactly this why I'm posting after a long time thinking of this.  I might attach a clubhead to a string just to test, but we can swing it, dangerously, at a decent speed.  Again we won't be able to apply any torque or hand path forces - only centripetal.  Once we install a shaft, yes, we can apply these forces and torque, but that doesn't mean centripetal force magically disappears.  Hammer throwers are obvious CF users, but I think the CF concept gets confusing for golf because of the complexity of the swing.  Hand path forces and torques on the grip make sense intuitively, centripetal not so much for most.  Doesn't mean these large centripetal forces should be ignored.

 

*Edit:  Adding that CF is the basis for parametric acceleration (K. Miura), where they showed that pulling up on the handle right before impact increases speed.

 

22 hours ago, Soloman1 said:

1. I don't see a relationship between centripetal force and the jumping movement in a tennis serve. Impact with the ball at a height creating a steeper downward ball flight is the goal, not as a power move. The longer path and radius of the racquet creates the angular velocity.

All true.  Of course I'm arguing there is CF involved in it as a power move.  But again with some object in rotation, there is always centripetal force involved in the angular velocity.

 

 

 

22 hours ago, Soloman1 said:

3. Vertical GRF is the last force after horizontal and rotational forces and peak vertical GRF is before impact.

I can only surmise the the body is anticipating and starting to create the forces ahead of it being transmitted to clubhead velocity.  The horizontal and rotational forces would be supporting the more horizontal centripetal forces during the swing.

 

With centripetal forces generated in excess of 100lb, not pointing along the clubhead path, those forces are coming from somewhere and being supported by the body somehow.  Scientifically I can't prove that it's a stabilizer to clubhead tangential momentum.  It's not being canceled by a fictitious outward centrifugal force on the clubhead.  If it was only a "reaction" force to the rotating club (angular velocity created only by torques and hand path forces), the person swinging would have to balance CF by moving their body mass toward the ball at impact, which is not what we see.  The only scientific explanation I have is that CF is constantly generated by body movement at different magnitudes and different directions during the swing.  Parametric acceleration was only describing an upward pull at impact.  I'm saying it's generated during the entire swing.

 

If others can prove otherwise with sound physics, by all means chime in.  Check out centripetal forces, check the math, and add to the conversation!

Edited November 27, 2022 by joostin

[joostin]

In a long shot I wrote to Dr. Kwon suggesting if he can provide some general understanding of centripetal forces in his content to the masses, and shared some of my thoughts.  I was ecstatic to see a response at all, and he genuinely comes across as a nice guy with interest.  Here's his response, first stating the obligatory centrifugal fallacies, then a word on the centripetal subject:

 

Mechanically speaking, centrifugal force is one of the most commonly misunderstood concept in the golf instruction. Centrifugal force is basically a fictitious force that appears to act only when the motion was observed in a rotating reference frame. So explaining the golf swing motion in the perspective of centrifugal force is incorrect regardless of the context.

 

As you correctly pointed out, what causes a curved motion is the centripetal force. 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.

 

Cheers,

 

Young-Hoo Kwon, Ph.D.

Biomechanics & Motor Behavior Laboratory

Texas Woman’s University

 

I asked for permission to post this, and promised to not bother him anymore lol.  Thank you Dr. Kwon, you're the best!

[clevited]

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 

[Zitlow]

@joostin The way it was showed to me was with a ball whirling in a circle on a string around my hand. When I let go of the string the ball and string would fly in a straight line.

 

When the whirling ball is pulling away from the resisting string and hand, is the pulling force centrifugal force and the resisting string and hand forces centrifugal force?

[joostin]

20 hours ago, 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 

Thanks for doing that with Tutelman!  That link was actually posted above already 😉

[joostin]

16 hours ago, Zitlow said:

@joostin The way it was showed to me was with a ball whirling in a circle on a string around my hand. When I let go of the string the ball and string would fly in a straight line.

 

When the whirling ball is pulling away from the resisting string and hand, is the pulling force centrifugal force and the resisting string and hand forces centrifugal force?

The only real force is centripetal force.  You're pulling centripetally, inward, and the ball wants to keep flying off in a straight line but you're overcoming that, and it has no choice but to rotate in a circle.  Pull harder and it will spin faster.

 

The fictitious centrifugal force would be perceived if you were a like a tiny person standing on the ball on the side of the string, like on a fast spinning carousel ride, feeling like you were getting pushed against the ball.  But then if the string were let go, the ball and you would go flying off tangent to the circle, not pointing outward opposite the centripetal force:

[joostin]

Just noting that I reached out to Dr. Kwon in part because he has a degree in astronomy, and centripetal forces are just a fact of life with the bodies in the universe due to gravity.  It seems a lot of biomechanists come from a background in strength and conditioning.  I admire that, and they've obviously had to take physics for their degrees, but the physics may be more deep rooted with Kwon, so I "gravitated" to him.

 

The downswing is essentially the art of generating the centripetal force using the ground and leg action.

-Kwon

 

Centripetal forces would have everything to do with GRFs.

My stance is still that it is what the body is driving throughout the entire swing.

[baudi]

On 11/26/2022 at 4:09 PM, Soloman1 said:

 

1. I don't see a relationship between centripetal force and the jumping movement in a tennis serve. Impact with the ball at a height creating a steeper downward ball flight is the goal, not as a power move. The longer path and radius of the racquet creates the angular velocity.

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. 

 

Quote

The remark the vector of centripetal force (and phantom centrifugal force) doesn't contribute to vectors of angular velocity or force vector at the ball. I think it should be considered more of a stabilizing effect and not a cause. 

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).

 

[clevited]

4 hours ago, joostin said:

Thanks for doing that with Tutelman!  That link was actually posted above already 😉

 

Ah, my bad.  I must have missed it.  Yeah, it was a pleasure picking his brain and seeing his thoughtful and thorough analysis.  

[joostin]

I jumped the gun without introducing centripetal force for those unfamiliar.  Here's a 6 min video that Kahn Academy does a good job of explaining, which I'll add to the 1st post too.  Highly recommend watching if the concept, equations, and calculations above didn't make sense. 

 

[jholz]

Interesting stuff. I don't have the math or physics background necessary to fully understand the math, but intuitively, this makes sense.

 

Feeds into a lot of things I've been working on lately. Appreciate it, y'all.