Ball Actually "Pinched" Against Ground or Not

[Lihu]

The drawing above shows the two forces (ignoring the gravitational force) acting on the golf ball at impact.

One of the forces is the normal force (call it N) and the other (the one omitted by iacas) is the frictional force (call it f). Let @ represent the angle theta, and k the coefficient of kinetic friction. The sum of the y-component of those two forces is given below:

N sin @ - kN cos @

If this sum is negative, there will be a net downward force on the ball, which means the ball will be accelerated toward the ground. In order that the sum above be negative, the coefficient must be greater than the tangent of the angle @.

k > tan @

In the case of a 31 degree angle, k would have to be greater than 0.6. Rare indeed are coefficients this large, and non-existence they are for golf balls. Thus, no pinching will ever be observed, but that doesn't absolve iacas of the responsibility to offer correct physics to support his view.

Iacas, if my diagram and associated mathematics is in error, please explain exactly why..

Nice diagram showing the forces affecting the ball. However, I think the frictional force you are talking about translates into rotational movement of the ball as opposed to pinching the ball into the ground? The net force upwards seems to be a lot bigger than this downward motion, so the ball continues up. . .

[saevel25]

Nice diagram showing the forces affecting the ball. However, I think the frictional force you are talking about translates into rotational movement of the ball as opposed to pinching the ball into the ground? The net force upwards seems to be a lot bigger than this downward motion, so the ball continues up. . .

Yea, Joe Mama is incorrect. Kinetic friction is only there when an object is moving, hence the term kinetic. The golf ball will always be sliding up the clubface when concerning a properly hit golf ball.

The only instance it would not slide up the clubface is if the static friction was high enough to maintain the ball's position on the clubface. The ball literally sticks on the clubface. With any sort of golf swing other than one with probably an impossible downward angle of attack with an extremely low delivered loft you are not going to see the ball stick to the clubface. The static friction is way to low for that to happen.

[Joe Mama]

Yea, Joe Mama is incorrect. Kinetic friction is only there when an object is moving, hence the term kinetic.

All that is necessary for kinetic friction to exist is for RELATIVE motion between ball and club face to occur. For example, if you snap the tablecloth off the table, the cups and saucers remain stationary. In such a case, kinetic friction is acting on the saucers. There is relative motion between ball and clubface as the face moves downward and the ball either slides or rolls up the face, or in the extremely unlikely case I described, also moving downward. For this reason, it is proper to speaking the kinetic friction acting on the ball.

[Joe Mama]

I am still waiting for someone to explain the flaws in my diagram. Have I incorrectly represented the forces. Did I apply Newton's Second Law incorrectly? Iacas? I respect your opinion regarding physics, since you earlier responded to a challenge from a poster in this thread (Augusta Hacker) to you to study Newton's Laws: You wrote, "I probably took a lot more physics classes than you. I don't need to re-read Newton's Laws of Motion in order to debate someone who seemingly forgot some of what he learned in Physics 101." I hope my analysis won't similarly lead you to believe I forgot what I learned in high school. Maybe I haven't taken a lot of physics courses as you, but I still stand by my conclusions, so far. Did I forget something, or did you?

[saevel25]

All that is necessary for kinetic friction to exist is for RELATIVE motion between ball and club face to occur. For example, if you snap the tablecloth off the table, the cups and saucers remain stationary. In such a case, kinetic friction is acting on the saucers. There is relative motion between ball and clubface as the face moves downward and the ball either slides or rolls up the face, or in the extremely unlikely case I described, also moving downward. For this reason, it is proper to speaking the kinetic friction acting on the ball.

Kinetic friction only exists when there is motion. Hence the use of the word kinetic. That is a physics basic.

The ball is at rest, static. Then it slides and rotates up the clubface in the time it takes for the ball to deform and bounce off from the impact.

The only way the ball would stay attached to the club is if the ball deforms so much that it stays attached to the clubface long enough for the ball and clubface to hit the ground. In that case the ball would probably go no where significant. So it is physically impossible to do so in the realm of applicable golf.

Kinetic Friction is derived by the normal force. The normal force is derived by the orientation of the force vectors. The only way you are getting a kinetic friction to point towards the ground is if you end up with a case where the clubface is pointing towards the ground. Which would change the direction of the vertical force vector. Besides that the ball will always slide up the clubface as long as there is enough force to get it above the static friction force.

You might end up with a situation where you have a 1 degree delivered loft, with a 10 degree downward angle of attack, producing -1.65 launch angle. Even then the ball would still slide up the clubface producing backspin.

Honestly as a golfer, I don't care about theoretical extremes.

[Joe Mama]

I think the frictional force you are talking about translates into rotational movement of the ball as opposed to pinching the ball into the ground? .

Place a ball on the floor and apply a sideways (tangential) force to it, perhaps by brushing along its side with your foot. Does it rotate? Does it also translate (move forward)? The frictional force on the golf ball is responsible for the torque that rotates the ball, and that force would, if there were not a competing force, cause the ball to translate (move downward). In every realistic situation involving our golf ball the competing force wins, and the ball moves upward. That we all agree on. What some of us don't agree with is the force diagram iacas presented. We need to wait to hear from him if he wishes to defend it.

[Ernest Jones]

Do you have a lot of cats @Joe Mama ?

[Hatchman]

"There is relative motion between ball and clubface as the face moves downward and the ball either slides or rolls up the face, or in the extremely unlikely case I described, also moving downward. For this reason, it is proper to speaking the kinetic friction acting on the ball."

I think your onto something. "Extremely unlikely case". 80 years ago grooves were put on blade forgings. Do you really think they were designed and spaced to clear grass or added to keep the ball from sliding up the face? The resulting bite on the ball, especially the non conforming groove, played to really good golfers advantages. The likelihood of downward movement in negligible, if any.

[Joe Mama]

For readers who may have joined this thread only recently, and have read only recent posts, they may be under the impression that I am defending the proposition that golf balls are driven toward the ground following a strike by the iron. Let me say that I've never stated, nor have I ever implied in a post, that such was the case. All videos I know about do not show the so-called "pinching" or "trapping'," or compression of the ball against the ground, and I've never seen a golf-ball size crater in photos of Tiger's near-perfect divots. I have presented what I believe is a correct analysis based on force vectors and their components, and Newton's Second Law, and demonstrated that the golf ball-clubface interface would have to have an improbably high coefficient of friction in order that the ball be driven downwards. We all agree that the ball is not pinched--well, most of us. What we seem not yet to agree on is whether my force diagram and subsequent derivation of the "tangent" equation is correct. I have further claimed that iacas left off one of the forces acting on the ball when he presented his force diagram. Does he still believe that the frictional force does not belong in the diagram? If he agrees it SHOULD have been shown, why was it omitted in his drawing? Based on what he learned in the many physics classes he has taken, does he find fault with my derivation of the tangent equation, which shows that, theoretically, with a large enough coefficient, the ball WOULD be driven downward?

[Joe Mama]

I think your onto something. "Extremely unlikely case". 80 years ago grooves were put on blade forgings. Do you really think they were designed and spaced to clear grass or added to keep the ball from sliding up the face? The resulting bite on the ball, especially the non conforming groove, played to really good golfers advantages. The likelihood of downward movement in negligible, if any.

Yes. Of course.

[arturo28mx]

If the ball were "pinched" against the ground, what happens when the ball is on a tee?

Slow motion video would clarify this, wouldn't it?

I really don't think the ball pinches against the ground. It starts lifting from the ground the moment the club touches it.

[Fourputt]

For readers who may have joined this thread only recently, and have read only recent posts, they may be under the impression that I am defending the proposition that golf balls are driven toward the ground following a strike by the iron. Let me say that I've never stated, nor have I ever implied in a post, that such was the case.

All videos I know about do not show the so-called "pinching" or "trapping'," or compression of the ball against the ground, and I've never seen a golf-ball size crater in photos of Tiger's near-perfect divots.

I have presented what I believe is a correct analysis based on force vectors and their components, and Newton's Second Law, and demonstrated that the golf ball-clubface interface would have to have an improbably high coefficient of friction in order that the ball be driven downwards.

We all agree that the ball is not pinched--well, most of us.

What we seem not yet to agree on is whether my force diagram and subsequent derivation of the "tangent" equation is correct.

I have further claimed that iacas left off one of the forces acting on the ball when he presented his force diagram. Does he still believe that the frictional force does not belong in the diagram? If he agrees it SHOULD have been shown, why was it omitted in his drawing?

Based on what he learned in the many physics classes he has taken, does he find fault with my derivation of the tangent equation, which shows that, theoretically, with a large enough coefficient, the ball WOULD be driven downward?

What you don't seem to understand is that we are golfers, so we don't care what happens as long as it doesn't affect what we are doing on the golf course.

I certainly don't care if you are right or wrong, and I don't have the physics background (my last physics class was 52 years ago as a senior in high school) to prove it one way or the other.  I just know that when I strike the ball properly with a slightly descending blow, it goes up, and the backspin imparted by the friction between the clubface and ball makes it go even higher than it would just from the loft of the club.  This is all I need to know about it from a player's perspective.

[iacas]

Here's the thing…

We all agree that the ball is not pinched--well, most of us.

What we seem not yet to agree on is whether my force diagram and subsequent derivation of the "tangent" equation is correct.

Who cares whether your diagram is correct or not? The ball is not pinched against the ground. This is a golf forum. Once the golf question is answered, it's really not worth the point to continue. It's not a physics forum.

I have further claimed that iacas left off one of the forces acting on the ball when he presented his force diagram.

You keep writing this, but @Joe Mama , let me ask you this: where in the heck did I even post a diagram?

The fact: I have not posted a diagram. You keep stating this, and yet never - not once - seemed to think it appropriate to go back to see if you weren't talking out of your rear end. I never posted a diagram.

Having said that, I wish to make the point that the purpose of my post was to correct what I think is incorrect physics applied by iacas to justify his claim that no pinching occurs. He's right--as most all of us are--that no pinching occurs, but his opinion is biased by looking at videos that correctly show no pinch, as well as on his vector diagram, which is incomplete and therefore incorrect, for the reasons I gave earlier. The actual physics involved WILL allow a golf ball to be pinched, but it would have to have a coefficient of kinetic friction on the golf club face greater than 0.6.

Again, I state that iacas physics is incorrect.

Nope. No diagram from me.

Please explain why the "physics" in the force diagram below is wrong, iacas. The force diagram you provided in a previous post omitted the frictional force. Why did you omit it? Is DOES exist, doesn't it?

I never posted a diagram, so I suppose in one sense I omitted ALL of the forces from my diagram, huh?

A) Club Face Motion is nowhere near that severe. We all know this.

B) Frictional force actually opposes motion. So if you're arguing that friction forces the ball down, you should draw your arrow in the other direction. As I pointed out seven years ago in this thread, if you slide a block to the left, the frictional force vector points to the right. So which is it? :)

At the end of the day, it's very simple: whether you're talking about horizontally or vertically, the launch angle physics are the same. The ball cares primarily about the clubface to the tune of ~70-85%. The other 15-25% comes from the ball and friction and the club's path (in this case, downward slightly).

So again, let's assume we are hitting a 6I. A Titleist 714 MB 6I with 31° loft. Let's assume that we're going to get 75/25 (which is pretty close to accurate for a PGA Tour player hitting a 6I). They also hit the ball with an AoA of -4.1°.

PGA Tour players will launch this 6I at about 14.1°. At the first glance, that's still closer to 31° than it is to -4.1°, so even without consider anything else the clubface (loft) is contributing more than the path (-4.1°). Yet that's not even considering the dynamic loft delivered. At the end of the day, the PGA Tour pros are delivering close to 10-11° less loft to the ball than the static loft of the club. At 20°, with a -4.1° AoA, that's 24.1° spin loft. That's about 18° - 25% from the dynamic loft toward the path, or 75% of the way from the path toward the dynamic loft. That's 6° from the clubface loft (dynamic, in this case), or… 14°. The launch angle of the average PGA Tour 6-iron.

Those are the real-world physics. I couldn't care less to figure out the physics of the golf world, because honestly it's too complex and it's not my job. Urethane balls launch higher because they have a lower frictional coefficient. They spin less, too, for similar reasons. The contact location matters. The CG of the clubface matters. Clubhead speed matters (among other things, it changes how much normal force exists between the clubface and the ball).

At the end of the day, unless you swing down with a negative AoA with very little loft OR you have negative loft, the ball immediately goes up. On that we all agree, and that is the point of a thread on a GOLF forum.

Did I forget something, or did you?

You seem to have forgotten that I didn't even post a diagram.

---

@Joe Mama , please stop addressing me. I'm not really participating here, and only really saw fit to join here to point out that you cannot even recognize who posted a diagram or not. I know how a ball behaves off the clubface. I have Phantom high-speed camera videos of even PUTTS being hit, let alone actual golf balls being hit.

For example…

The second is even a bit thin… which is going to launch the ball lower, yet it goes up, up, up!

[Mac62]

I just wanted to wish this thread a happy anniversary, after being resurrected from seven years ago. Talk about necroposting.....

[Joe Mama]

What you don't seem to understand is that we are golfers, so [U] we don't care [/U] what happens as long as it doesn't affect what we are doing on the golf course.   I certainly don't care if you are right or wrong, and I don't have the physics background (my last physics class was 52 years ago as a senior in high school) to prove it one way or the other.  I just know that when I strike the ball properly with a slightly descending blow, it goes up, and the backspin imparted by the friction between the clubface and ball makes it go even higher than it would just from the loft of the club.  This is all I need to know about it from a player's perspective.

Fair enough. I respect your right not to concern yourself with things that don't interest you. I'm sure you likewise respect the right of others to wish to discuss things that are of interest to THEM. It is enough for you enough for you to know that the ball is not compressed against the ground. I hope you can understand that the engineering mentality some of us have--the need to know how and why things work--compel us to ask WHY. In this case, it was possible to answer quite definitively the question of how and why and under what conditions it could ever be possible to compress the ball against the ground.

[Fourputt]

Quote:

Originally Posted by Fourputt

What you don't seem to understand is that we are golfers, so we don't care what happens as long as it doesn't affect what we are doing on the golf course.

I certainly don't care if you are right or wrong, and I don't have the physics background (my last physics class was 52 years ago as a senior in high school) to prove it one way or the other.  I just know that when I strike the ball properly with a slightly descending blow, it goes up, and the backspin imparted by the friction between the clubface and ball makes it go even higher than it would just from the loft of the club.  This is all I need to know about it from a player's perspective.

Fair enough. I respect your right not to concern yourself with things that don't interest you. I'm sure you likewise respect the right of others to wish to discuss things that are of interest to THEM. It is enough for you enough for you to know that the ball is not compressed against the ground. I hope you can understand that the engineering mentality some of us have--the need to know how and why things work--compel us to ask WHY. In this case, it was possible to answer quite definitively the question of how and why and under what conditions it could ever be possible to compress the ball against the ground.

Just FYI, my brother is a PHD chemical engineer, and he would tell you the same thing, but have fun if that's what floats your boat.

[Joe Mama]

Here's the thing…   : where in the heck did I even post a diagram? The fact: I have not posted a diagram. You keep stating this, and yet never - not once - seemed to think it appropriate to go back to see if you weren't talking out of your rear end. I never posted a diagram.

My mistake. It was saevel25 who posted the vector diagram below. Since you earlier had described your background in physics, I attributed the incorrect drawing (see below) to you. [quote] B) Frictional force actually opposes motion. So if you're arguing that friction forces the ball down, you should draw your arrow in the other direction. As I pointed out seven years ago in this thread, if you slide a block to the left, the frictional force vector points to the right. So which is it? :) [/quote] Of course: slide a block to the right across a table, the frictional force acting on the block points to the left. We agree completely. Slide a club face under the ball and the frictional force provided by the ball on the club face will oppose the club face's motion. By Newton's Third Law, the club face exerts and equal but [b]opposite[/b] force on the ball. That's the frictional force arrow in my diagram (below) that you apparently incorrectly believe is pointing in the wrong direction. Because the vertical component of the frictional force on the ball is less than the vertical component of the normal force (contact force) the club face exerts on the ball, the fall moves upward immediately, never downward (provided the coefficient of friction is less than 0.6). You WERE the poster who said he took "a lot more physics classes" than others, weren't you, or did I mis-identify that poster as well? Do you agree with my description of what happens, and why, or do you not care, because as a golfer, as long as you know the ball is never compressed against the ground, that's all you need to know?

[Lihu]

[quote name="iacas" url="/t/15607/ball-actually-pinched-against-ground-or-not/72#post_1179652"] Here's the thing…   : where in the heck did I even post a diagram? The fact: I have not posted a diagram. You keep stating this, and yet never - not once - seemed to think it appropriate to go back to see if you weren't talking out of your rear end. I never posted a diagram.

My mistake. It was saevel25 who posted the vector diagram below. Since you earlier had described your background in physics, I attributed the incorrect drawing (see below) to you. [quote] B) Frictional force actually opposes motion. So if you're arguing that friction forces the ball down, you should draw your arrow in the other direction. As I pointed out seven years ago in this thread, if you slide a block to the left, the frictional force vector points to the right. So which is it? :) [/quote] Of course: slide a block to the right across a table, the frictional force acting on the block points to the left. We agree completely. Slide a club face under the ball and the frictional force provided by the ball on the club face will oppose the club face's motion. By Newton's Third Law, the club face exerts and equal but [b]opposite[/b] force on the ball. That's the frictional force arrow in my diagram (below) that you apparently incorrectly believe is pointing in the wrong direction. Because the vertical component of the frictional force on the ball is less than the vertical component of the normal force (contact force) the club face exerts on the ball, the fall moves upward immediately, never downward (provided the coefficient of friction is less than 0.6). You WERE the poster who said he took "a lot more physics classes" than others, weren't you, or did I mis-identify that poster as well? Do you agree with my description of what happens, and why, or do you not care, because as a golfer, as long as you know the ball is never compressed against the ground, that's all you need to know? [/quote] Your model might be more complete than @saevel25 's, but even yours doesn't show everything such as ball compression nor the fact that it changes over the roughly 400 microsecond time interval that this entire collision takes place. The bottom line is that no actual pinching of the ball against the ground takes place, and both diagram's his and yours show that. Before you tell me I don't know any physics, I should let you know that both my parents are PhDs in physics and I'm an engineer who grew up with physics and uses physics every day, and have done numerous FEM models of many systems ranging from stepper motors to fluid mechanics. Do I really care to model this collision in more detail? Not really. I would probably prefer to model how our bodies move using 3D imaging so we can improve our swings, because that might be more useful.