Spin decay

Atas Newton said:

In terms of % of the initial spin, how much of the spin do you think wears off during the ball flight?

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There actually has been some research on this issue.

https://www.google.com/url?sa=t&rct...load/846/757&usg=AOvVaw0DUAImicYKBBbDZIbhvlot

and

https://www.google.com/url?sa=t&rct..._English.pdf&usg=AOvVaw2UrPgUtOeh530e4LWlD0NG

I have not read them deeply yet, just having found them but they appear interesting and well supported. Not sure if your specific question is answered in the papers but if I see what I think is an answer I will get back here and post it.

My wild a$$ guess is about a 20 percent drop when striking the table and another 40% drop by time the ball is below table height when returned from a player standing away from the table. Of course this would vary depending on whether we are talking fast loop (or forward drive) vs the slow loops (more like the original loops that went more up than forward and relied just on the spin of the ball to kick it off the table once it landed. It would also depend on which direction of spin you are focused on because I believe side spin is reduced less by the impact with the table - but that might just be because I personally have more problems with side spin. LOL

I suspect that is about right but I could be wrong.

This will depend to some extent on the type of ball you are using, including its weight, diameter and surface texture. Balls that are worn (or Butterfly G40+ balls when new) interact with the air quite differently and have a noticeably straighter trajectory. I suspect they retain their spin longer but it just doesn't affect the ball behavior the same way.

What happens next then depends on the table (after it bounces).

Experiments have been made with celluloid balls in Japan. Don't have the numbers off the back of my head. Need to look that up.

Plastic balls have also been experimented with by JTTA, but it appears they have not released it.

I think Baal is correct in identifying some of the other factors, air isn't the only one. The different behavior of different, or even the same balls during a match has really lead to more errors than we realize.

The further the ball travels, air will reduce the amount of spin... we can call it around 20%, but we have to read the other aspects right to make a good shot vs even a reduced spin ball. Again, these factors can vary for even the same ball in the same match.

Those who get closer to the bounce have to deal with more spin, but less wild bounces.

sderyke2002 said:

There actually has been some research on this issue.

https://www.google.com/url?sa=t&rct...load/846/757&usg=AOvVaw0DUAImicYKBBbDZIbhvlot

and

https://www.google.com/url?sa=t&rct..._English.pdf&usg=AOvVaw2UrPgUtOeh530e4LWlD0NG

I have not read them deeply yet, just having found them but they appear interesting and well supported. Not sure if your specific question is answered in the papers but if I see what I think is an answer I will get back here and post it.

My wild a$$ guess is about a 20 percent drop when striking the table and another 40% drop by time the ball is below table height when returned from a player standing away from the table. Of course this would vary depending on whether we are talking fast loop (or forward drive) vs the slow loops (more like the original loops that went more up than forward and relied just on the spin of the ball to kick it off the table once it landed. It would also depend on which direction of spin you are focused on because I believe side spin is reduced less by the impact with the table - but that might just be because I personally have more problems with side spin. LOL

Click to expand...

Figure 3 of the Zhang and Wu paper shows only a 3-4% loss in spin from initial contact (drive loop) to just before the ball hits the table, whereas speed is reduced by almost 30%. Makes sense to me. Air resistance has a much bigger impact on speed, except maybe at very low velocities. Spin magnitude (and direction) changes mostly from table impact.

None of those articles cover how spin decays.
I agree with the comments above balls with a rough plastic surface will react differently than smooth celluloid balls.
The ball spin will not be reduced by 20% if the surface of the ball is moving slower than the motion of the ball. The ball will speed up. Serve a dead ball. After the ball hits the table it will start to gain a slow top spin. If you hit it light enough it will bounce many times and gain top spin after each bounce.

If the ball is hit with a lot of spin relative to the speed the ball will jump off the table low and fast. In this case the ball will slow down because the rotational energy is converted to speed.

I made an estimate years ago. I spun a ball on a hard surfaced table and estimated a time constant for slowing down. There was some friction with the table but the ball would spin for quite a while, much longer than a ball would take to land after a loop or even a chop. I do have a suspicion that the speed of the ball would also affect how the ball slows down. For instance, if you have ever chopped a ball you know that the ball will rise. The same happens to golf balls. Where does the energy that makes the ball rise come from? It must be from the rotational energy of the ball. If the energy is coming from the rotation of the ball then the spin will slow down. Now the question is the rate of energy loss due to the Magnus effect.

Questions, questions, questions. I know the questions to ask. I know how to do the tests and I have the equipment to do the test. I don't have the time or desire to do the test. There is no money in it. I would rather spend my time playing.

This question will probably arise again in a few years.

Note I am very skeptical of documents like those above.
A person I would trust is Professor Rodney Cross of the University of Sydney, AU. He has done a lot of research in this area but I don't think he has answered the question of how spin decays.

brokenball said:

None of those articles cover how spin decays.

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If you are talking just about spin decay due to friction of the air (before contact with table or opponent paddle then you are indeed getting into complex territory.

Before I start let me tell you a joke - it is said that a physicist can immediately tell you the stability of a one legged table or that of a table with an infinite number of legs - but make a career out of analyzing the stability of a four legged table.

Taking the lower limit (motion in a vacuum) the ball does not stop spinning - no decay in spin. That's where it stops being easy. Now we add a medium to travel through and we end up with a host of variable to consider. Density, composition, humid, turbulence and we find ourselves faced with the same calculations in chaos theory which make predicting the weather fundamentally impossible. In other words the Butterfly Effect. No, not the equipment company - the idea that the flap of a butterfly's wings in Brazil add enough energy to the atmosphere that it can impact the variables of weather prediction in far off places like the United States. This identification of stranger attractor and solving the associated mathematics is simply not possible to the point of making answering such questions from a theoretical perspective impossible. And so we are reduced to doing experiments as closely modeled to the real world as we can make them and using the results conservatively. That is what those papers try to do.

Andy44 said:

Figure 3 of the Zhang and Wu paper shows only a 3-4% loss in spin from initial contact (drive loop) to just before the ball hits the table, whereas speed is reduced by almost 30%. Makes sense to me. Air resistance has a much bigger impact on speed, except maybe at very low velocities. Spin magnitude (and direction) changes mostly from table impact.

Click to expand...

Yeah I saw that and was surprised myself. I don't doubt them, but it was surprising. I would have expected more reduction based on my observations during play. Just goes to show you, you can't trust your own eyes sometimes.