Theory of the American Twist Kick Serve

[Hmgraphite1]

Of course there are many forces acting in all directions during the impact. But any forces acting outside of the 2D plane are balanced out by equal forces on the opposite side of the plane. We only care about the net resultant vector, which can be reduced to two components.

If there were unbalanced forces the resultant would be some odd spin components/ forces on the ball, like when you impact the frame. The strings nearer to the frame are tighter and add side components dynamically as the are stretched and unstretched. Not really getting to molecular level but I do want to stay at the zoomitis level and say the rest is approximated though I can't proove to what degree

 

[mucat]

When your opponent hop to the wrong side to receive the serve and it almost hit them....You know then your twist/kick serve is working. I found string affect this a lot, I got way less twist effect with softer poly. So for those who has problem hitting this serve, it could be a string issue.

 

[IowaGuy]

Of course there are many forces acting in all directions during the impact. But any forces acting outside of the 2D plane are balanced out by equal forces on the opposite side of the plane. We only care about the net resultant vector, which can be reduced to two components.

Just to clarify,

In real 3d space (a real serve on a tennis court) you think the net resultant vector is 2d?

 

[Deleted member 23235]

Of course there are many forces acting in all directions during the impact. But any forces acting outside of the 2D plane are balanced out by equal forces on the opposite side of the plane. We only care about the net resultant vector, which can be reduced to two components.

you mean the forces acting on the ball?
i count 3
some force going into ball
some force going up
some force going sideways

nm, i see what you're doing, you're counting up and sideways, as some single diagonal vector...

 

[travlerajm]

Just to clarify,

In real 3d space (a real serve on a tennis court) you think the net resultant vector is 2d?

No. You are thinking in the court frame of reference. In the racquet (or ball) frame of reference at the moment of impact, the net resultant vector only has 2 dimensions ( the normal force component and the tangential force component).

 

[Chas Tennis]

No. You are thinking in the court frame of reference. In the racquet (or ball) frame of reference at the moment of impact, the net resultant vector only has 2 dimensions ( the normal force component and the tangential force component).

Do you have a reference for the ball or racket frames of reference?

It seems as if the coordinate systems that you consider are accelerating. "In the racquet (or ball) frame of reference at the moment of impact..." But at the moment of impact hardly any momentum is transferred. The ball and racket are accelerating in a very complex way.......

I think that coordinate systems of the ball or racket during impact and after must be too complicated to work with. Can you show a picture of the frame of reference for the spinning ball? What happens with changing accelerations in various directions?

"An inertial frame of reference may also be called an inertial reference frame, inertial frame, Galilean reference frame, or inertial space. All inertial frames are in a state of constant, rectilinear motion with respect to one another; an accelerometer moving with any of them would detect zero acceleration."

"A non-inertial reference frame is a frame of reference that is undergoing acceleration with respect to an inertial frame. An accelerometer at rest in a non-inertial frame will in general detect a non-zero acceleration. In a curved spacetime all frames are non-inertial. "

The court reference system or "global system" (quoting from the ball spin diagram authors) would be an inertial frame of reference.

 

[Kevo]

I think travelerajm is just trying to simplify the forces involved in the contact. I use the same simplification quite often with students. I ask them to "look through the strings" to see where the ball is going to go. I also tell them to brush up the back of the ball for topspin. Those are the two forces that traverlajm is talking about. It doesn't matter how the strings brush in determining the ball direction. The ball comes off the strings in the direction the strings are looking, and the spin is produced by the direction the strings are brushing. I suppose there are some caveats to that, but they don't apply on serve since the ball has negligible force coming into the collision.

Of course if you view this collision from a world view rather than a racquet/ball view you have to keep up with more vectors.

BTW, it's been a long time since multivariate calculus in college, so please don't ask me to work any equations. I'd have to spend weeks or months on Kahn Academy to get back up to speed.

 

[travlerajm]

Do you have a reference for the ball or racket frames of reference?

It seems as if the coordinate systems that you consider are accelerating. "In the racquet (or ball) frame of reference at the moment of impact..." But at the moment of impact hardly any momentum is transferred. The ball and racket are accelerating in a very complex way.......

I think that coordinate systems of the ball or racket during impact and after must be too complicated to work with. Can you show a picture of the frame of reference for the spinning ball? What happens with changing accelerations in various directions?

"An inertial frame of reference may also be called an inertial reference frame, inertial frame, Galilean reference frame, or inertial space. All inertial frames are in a state of constant, rectilinear motion with respect to one another; an accelerometer moving with any of them would detect zero acceleration."

The court reference system or "global system" (quoting from the ball spin diagram authors) would be an inertial frame of reference.

There are dozens of published academic papers on oblique impacts between a strung tennis racquet and a ball, and I have read most of them. Nearly all of them use the same approach of using the racquet frame of reference for clamped racquets (or in the example reference below, the "lab" frame of reference when the racquet is unclamped) due to the increased simplicity of 2D vs 3D.

http://twu.tennis-warehouse.com/learning_center/spinexperiment.php

 

[Hmgraphite1]

The ball isn't round "on the serve"

 

[Chas Tennis]

There dozens of published academic papers on oblique impacts between a strung tennis racquet and a ball, and I have read most of them. Nearly all of them use the same approach of using the racquet frame of reference (or in the example reference below, calling it the "lab" frame of reference) due to the increased simplicity of 2D vs 3D.

http://twu.tennis-warehouse.com/learning_center/spinexperiment.php

This is where I diverge from Cross for the serve because ISR adds a second rotation that I don't believe can be duplicated in a 2D simulation of the serve with a stationary racket.

I've posted this many times with these comments. Impact lasts about 4 milliseconds. ISR may rotated to, say, 3000 d/sec or 3 degrees per millisecond. The ISR axis is through the upper arm bone. This redirects the racket face to the right moving forward. Also the racket is closing around several axes: wrist, shoulder, spine, hips. The two rotations show here and in all videos of the head on a high level. Maybe the rotation rate is 1 -2 degrees per millisecond. ?

But impact is too complicated. I start with the ball as measured by S. Sakurai in the ball spin publication. Sakurai also discusses how little measurements are available on the serve ball spin after impact. I don't think that we will find spins after racket impact and then also just before bounce. That would show if Cross's description of the direction of the spin axix remaining about the same in flight.

If we had a poster with a strong kick serve we could easily measure these two times with two cameras. I'm waiting for a third camera today...........

 

[Hmgraphite1]

 

[Hmgraphite1]

 

[Hmgraphite1]

 

[mucat]

 

[Hmgraphite1]

\_\\//

 

[Chas Tennis]

"A 12-camera 500 Hz, Vicon MX motion analysis system (Oxford Metrics, Inc., Oxford, UK) recorded the 3D marker trajectories (Figure 2) for 3 m following impact. "

Expand the picture - Hold CONTROL and hold SHIFT press "+". Hold CONTROL and press "-" to make smaller.

The ball with 3 markers is travelling out toward the reader. Here are the first 3 meters out from impact. The wavy lines are trajectories of markers stuck on the ball. It is hard to judge whether the spin axis is stable in direction over the first 3 meters or if the axis is changing direction as travlerajm claimed. This might possibly be the best and most complete measurement of ball spin in flight that can be found. ?

Rod Cross must also have observed similar measurements as he said "The spin axis remains fixed while the ball travels toward the net". That could be in agreement with the the above pictures. ?

Measurements of this type are what is needed to answer whether the ball spin axis changes in flight or whether it stays about the same. If it stays about the same, my post # 58 describes how the kick serve gets its bounce to the right.

This report is very similar and may have been the measurements in the 2013 report.
https://isbweb.org/images/conf/2007/ISB/0790.pdf

 

[travlerajm]

Occam's razor, anyone?

 

[Chas Tennis]

deja vu - interesting 2008 thread

The Twist serve and its TWO Axis of spin. Debate Please

I read some posts about the third spin component of the Twist serve. I disagree with how the concept of longitudinal spin has been presented so I decided to start a new thread to discuss my theory on this.


I support the two spin idea and here is why: From a Right Hander


First lets identify the two main axis the ball spins on. One, a horizontal axis (topspin component), and Two, the angled axis (45 degree?) the sidespin component. I assume this is relatively agreed upon by us all.
With a Twist serve the ball is struck and it has two spins on it. The horizontal axis spin makes it curve downward and the sidespin makes it curve to the left like a slice spin does. What is important to realize is that the axis of spin in relation to the court change as the ball flies and curves due to its spin. What was initially a horizontal axis becomes somewhat angled and the angled side spin axis becomes near vertical or somewhat close at or near the point in which the ball hits the court. This is what gives the ball such an abrupt "kick" to the side (right).

Initially one spin (topspin) was working parallel to the side fence. The second spin (angled sidespin) was working at an angle toward the side fence. After the ball curves (later in flight) these two angles of axis change and become more angled toward the fence than before.
It is the curvature of the balls flight that changes the axis of spin on the ball that causes the sideways signature kick of the American Twist Serve.

A ball can be hit with a pure 7 to 3 angle of attack and have only 7 to 3 spin on it. This ball will curve to the left and to a degree have an enhanced downward flight. And it will bounce to the right some. But, it will not have that abrupt right hand turn that the Twist does. It is the combination of the 6 to 12 spin and 7 to 3 spin that causes the change in the flight pattern that produces that signature Twist action.

For what its worth, if you can throw very well, you can throw a tennis ball pretty easily with that 7 to 3 spin an it will follow what I described above.

Based on what I have said here, I do not believe that there is a third dimensional spin associated with the Twist. The only longitudinal turn is slight, maybe 1/8 of a turn.

Ok, hammer time. Blast my idea if you will......

Another thread , 2016
Basic serve spin direction question

 

[Chas Tennis]

Is that fixed wrt the Earth (or court lines)? Or is it fixed wrt to the trajectory (flight path) of the ball?

I have very high regard for Rod Cross but he might not be infallible. Or he might make some simplification or generalization that might be true for the most part. I am thinking about rifle bullet ballistics. For the most part, the rifle bullet travels in a fairly straight line with only spiralspin (aka bullet spin). But after some time time the bullet deviates from this. It can even experience a tumble -- an obvious spin axis deviation. Forces acting on the bullet are gravity, air drag and wind. There might also be a torque associated with the bullet's spin than might eventually alter its path of spin axis. As the bullet deviates from pure spiral spin, Magnus force can come into play.

I'm wondering if there might be a small change in a ball's spin axis orientation that Cross considered so slight that he states the the spin axis remains fixed. Note that the spiralspin component, even for a twist serve, is considerably less than the topspin and sidespin components. So, for the most part, the resultant spin axis is "pretty much" perpendicular to the ball flight direction after it leaves the racket and just before it bounce -- even tho there might be a small amount of change. Just some food for thought.
.
.

I have realized that the court is on the earth's surface and the earth is rotating once a day and moving in an elliptical orbit around the sun. The tennis researchers called their reference system a "global reference" system. Usually the earth's motion is ignored in most experiments except, I believe, for long range artillery so I guess the effects are small for a 1 second flight of a tennis ball. For rifle bullets see Long Range Shooting: External Ballistics – The Coriolis Effect

In our physics building at the University of Maryland, there was a large Foucault pendulum. It was made of a long cable, 30 ft?, and a very heavy metal ball. It would swing from the ceiling. There was a clock on the floor I believe. The pendulum would maintain swinging in the same direction but the earth would slowly turn under it. The change in direction was very slow from the earth's rotation. 360 degrees in 24 hours or 0.004 degrees per second.

The earth/court would probably rotate under the tennis ball in the 1 second of ball flight. But movement too small to see - 0.004 degrees in one second There is also rotation around the sun. ?

https://en.wikipedia.org/wiki/Foucault_pendulum

 

[IowaGuy]

Occam's razor, anyone?

That's not as much fun

 

[mucat]

Occam's razor, anyone?

off topic...
https://www.classicshaving.com/products/occams-razor

 

[travlerajm]

off topic...
https://www.classicshaving.com/products/occams-razor

The irony is that all available evidence suggests the Occam's razor principle does not apply to the number of blades on your shaving implement.

 

[sureshs]

The irony is that all available evidence suggests the Occam's razor principle does not apply to the number of blades on your shaving implement.

There is already:

OCCAM’S RAZOR
simplify your shaving

https://www.classicshaving.com/products/occams-razor

 

[travlerajm]

off topic...
https://www.classicshaving.com/products/occams-razor

There is already:

OCCAM’S RAZOR
simplify your shaving

https://www.classicshaving.com/products/occams-razor

You guys are violating the principle.

 

[sureshs]

off topic...
https://www.classicshaving.com/products/occams-razor

LOL did not see your post before posting mine

 

[Chas Tennis]

I spent some time on Google Scholar looking for measurements of tennis ball spin in flight. There were few and the 2013 work with the ball spin diagram looked the most advanced for the first pages of Google finds when looking for tennis ball spin measurements.

David Whiteside et al have a publication proposing the use of 3D multi camera systems (motion capture) to measure inflight tennis ball spin. I did not see later publications, but there may be some.

 

[Hmgraphite1]

Came across this pick, seems this is what I always thought, but as you develop a method to blend the tosses for disguise may seem less relevant

 

[Hmgraphite1]

As the ball leaves the racquet, I see the ball since its not round traveling with a "yaw" which will settle down at some point. The top pick is side view the middle traces the tip showing the settling, and bottom pick would be tip looking down its trajectory. Since the ball's walls are flexible and dynamic the yaw" would be readjusted at some rate creating dynamically more interesting effects

 

[travlerajm]

I spent some time on Google Scholar looking for measurements of tennis ball spin. There were few and the 2013 work with the ball spin diagram looked the most advance for the first pages of Google finds when looking for tennis ball spin measurements.

David Whiteside et al have a publication proposing the use of 3D multi camera systems to measure inflight tennis ball spin. I did not see later publications, but there may be some.

I have more 20 papers on this topic on my hard drive from when I did some of my own research 15 years ago.

 

[Hmgraphite1]

I have more 20 papers on this topic on my hard drive from when I did some of my own research 15 years ago.

Your hardrive stores in paper format?j/k , do tell of some interesting ideas

 

[SystemicAnomaly]

@nytennisaddict

Sorry could only find Bill Tilden: (~7:00)

The only time I've heard people try and draw a differentiation between the twist and kick serve is on here. They have always just been interchangeable terms for mine.

Holcombe Ward & Dwight Davis are, indeed, credited with introducing the American Twist serve in the late 19th century.

While some/many sources use kick and twist interchangeably, many do not. The distinction has been made on other tennis forums (MTF, TP, TF) as well. Kathy Krajco made the distinction on her often-quoted website, OperationDoubles (2001-2008). Sadly, Kathy passed away in her 50s in 2008 and her family took down her outstanding tennis web site by 2009. Her site had been a popular resource on TT for more than a decade (can still link to some of her archived pages).

Jeff Cooper also makes a distinction on his web pages. Chris Lewitt & TennisPlayer.net also make a distinction.

https://www.tennisplayer.net/public.../keys_to_the_kick/keys_to_the_kick_page1.html
https://www.tennisplayer.net/public.../keys_to_the_kick/keys_to_the_kick_page2.html
https://www.thoughtco.com/how-to-hit-the-twist-serve-3208250.

 

[SystemicAnomaly]

Your example of the sidespin dropshot by Federer is the perfect example a tennis shot that has negligible spiralspin at impact, but then develops a lot of spiralspin in the air before it bounces. The spiralspin develops during flight because the velocity vector changes direction as the ball arcs through the air under the acceleration of gravity, but the spin axis stays virtually constant...

Ok, so we disagree about the magnitude of spiralspin spin component present at impact. I say small and you say negligible. Let's put that aside. We both agree that a twist serve (and some other serves/shots) will develop (more) spiralspin during flight (prior to the bounce). If we accept that, how can the spin axis possibly stay constant? Does not compute in my head.

With little or not spiralspin present at impact, the resultant spin axis would be perpendicular (or very close to perpendicular to the flight path of the ball. However, with a signficant spiralspin component present just prior to boucne, the spin axis would need to be something other than perpendicular to the flight path. Maybe not a huge offset, but something significant.
.

 

[travlerajm]

Ok, so we disagree about the magnitude of spiralspin spin component present at impact. I say small and you say negligible. Let's put that aside. We both agree that a twist serve (and some other serves/shots) will develop (more) spiralspin during flight (prior to the bounce). If we accept that, how can the spin axis possibly stay constant? Does not compute in my head.

The spin axis stays virtually constant in the court frame of reference during flight due to Newton’s third law (while the direction of ball's velocity vector is changing relative to the court frame of reference but staying constant in the 'ball" frame of reference). There are only two forces acting on it: gravity and air drag. The gravity acceleration vector does not change the spin axis because it always acts on the ball’s center of mass and the ball is symmetrical. Air drag can affect the orientation of the spin axis a little bit, but only as a tiny secondary effect (and only after gravity has introduced a significant enough spiral spin component for the drag vector to act in a different direction than perpendicular to the spin axis and in a different plane than through the center of the ball). In other words, drag is just like a tennis racquet in that drag cannot create spiral spin unless gravity is involved. If there were no gravity (and you were to hit a twist serve inside the space shuttle in zero g), the spin would curve the ball in the air but there would be no spiral spin at any time during the shot.
Gravity is by far the main driver of the development of spiral spin during the ball’s flight.

With little or not spiralspin present at impact, the resultant spin axis would be perpendicular (or very close to perpendicular to the flight path of the ball. However, with a signficant spiralspin component present just prior to boucne, the spin axis would need to be something other than perpendicular to the flight path. Maybe not a huge offset, but something significant.

If the ball is hit with an aggressive twist serve using a 7:30 to 1:30 path on the back of the ball, then the spin axis would be 45 degrees relative to the ground through the entire flight. So if the ball hits the ground at roughly 45 degrees from vertical, indeed, at the bounce, there would be a huge offset angle between the velocity vector of the ball's downward flight path and the spin axis (in other words, there would be a very large spiral spin component, but not nearly as much spiral spin as if you had hit the same serve with a pure slice swingpath).