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Post Options Post Options   Thanks (0) Thanks(0)   Quote Baal Quote  Post ReplyReply Direct Link To This Post Posted: 09/17/2013 at 12:44am
I have this picture in my mind of the rubber shaped like the greek letter omega with the ball fitting into it.  But I don't have enough physical intuition to know if that matters.  Would the ratio of the normal vs. tangential COR matter and would that vary with sponge thickness?  Is there any sense in that idea?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tt4me Quote  Post ReplyReply Direct Link To This Post Posted: 09/17/2013 at 12:50am
Originally posted by AgentHEX AgentHEX wrote:

Frankly I don't really the relevance of dwell when we are already working with factors that directly matter. Dwell doesn't change COR nor incoming speed, etc at all.

I agree that it is the normal and tangential COR that are most important.  I have been saying that for years.  The ratio of tangential to normal COR is a much better evaluation of a rubber than throw which is stroke dependent and very subjective.

My interest in this thread is to debunk some myths that dwell time can be many millisecond under normal conditions but I also know that dwell time can be theoretically infinite under certain conditions.  One must know/understand the extremes to understand what happens in between.



 
 


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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 09/17/2013 at 12:57am
Originally posted by Baal Baal wrote:

I have this picture in my mind of the rubber shaped like the greek letter omega with the ball fitting into it.  But I don't have enough physical intuition to know if that matters.  Would the ratio of the normal vs. tangential COR matter and would that vary with sponge thickness?  Is there any sense in that idea?


Yes, the ratio is what matters. For same blade angle and two diff rubbers, all else being equal the one with more tangential elasticity relative to normal will throw higher on topspin shot pretty much by definition. How much this varies according to this sort of simplified model is also relatively trivial math.

For example, assume one rubber w/ COR 0.5/0.5 for normal/tang, another 0.5/0.7. For straight hit, both repel 100 speed no spin ball back at 50. For 100% brush stroke, the second rubber would be 70 at edge of ball at point of contact instead of 50 (ie, more spin), and for anything in between a matter of trigonometry.

This is a fairly simply model but should be significantly correct.

Softness or thickness would effect either COR, likely normal more since the other is in part a mechanical deflection of pips as much as pips pulling on sponge, but this is a more complicated matter.



Edited by AgentHEX - 09/17/2013 at 12:59am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Baal Quote  Post ReplyReply Direct Link To This Post Posted: 09/17/2013 at 1:10am
Very interesting.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 09/17/2013 at 1:14am
Originally posted by tt4me tt4me wrote:

Originally posted by AgentHEX AgentHEX wrote:

Frankly I don't really the relevance of dwell when we are already working with factors that directly matter. Dwell doesn't change COR nor incoming speed, etc at all.

I agree that it is the normal and tangential COR that are most important.  I have been saying that for years.  The ratio of tangential to normal COR is a much better evaluation of a rubber than throw which is stroke dependent and very subjective.

My interest in this thread is to debunk some myths that dwell time can be many millisecond under normal conditions but I also know that dwell time can be theoretically infinite under certain conditions.  One must know/understand the extremes to understand what happens in between.



The problem with the ratio is that COR for given collision is not only a metric intrinsic to a given rubber, as you probably alluded to. Even if a rubber has very good tangential elasticity, if the rebound in that direction is very slow, a quicker normal component might already have ejected the ball and limited the effect. So I guess dwell does matter in that regard, LOL.

OTOH, assuming rubber designers aren't stupid, they probably realize this which is why pips are harder material and probably rebound faster (or enough) for the relevant shots, so this might not matter much maybe except at the edges.


Edited by AgentHEX - 09/17/2013 at 1:20am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JacekGM Quote  Post ReplyReply Direct Link To This Post Posted: 09/17/2013 at 8:27pm
So, during the execution of a real-life TT shot - say a slow loop, or a fast and powerful long-distance loop - the time of the contact between the ball and the racket is less than 1 millisecond? 
Is this a fact to be kept in mind?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 09/17/2013 at 8:31pm
Maybe it can be a bit more, but the overarching point is that it's orders of magnitude shorter than human responses.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tt4me Quote  Post ReplyReply Direct Link To This Post Posted: 09/17/2013 at 11:39pm
Originally posted by AgentHEX AgentHEX wrote:

Even if a rubber has very good tangential elasticity, if the rebound in that direction is very slow, a quicker normal component might already have ejected the ball and limited the effect. So I guess dwell does matter in that regard, LOL.

It is exactly this issue that has me thinking about how the impact can be modeled simply without going through all the trouble of doing what Andro had to do with their modeling.

The brute force way to do this is to shoot balls at the rubber at different speeds and angles to build a table.    For instance there could be a table of tangential COR as a function of normal impact speed.
In the end the brute force table needs to be compiled so that the model can be fit to the actual data.
I bet the Andro guys have done it all and are laughing at our ignorance.




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Post Options Post Options   Thanks (0) Thanks(0)   Quote mercuur Quote  Post ReplyReply Direct Link To This Post Posted: 09/18/2013 at 5:56am
Picture (or try) a spinning ball (sidespin) on a floor or table.
With a low as possible speed/velocity someone approaches it with a bat that has a grippy/tacky inverted rubber highly resilient,
The linear momentum and energy change for the bat is extremely low then. Touching the ball softly it accellerates rapidly from the bat.

I assume momentum, linear and angular, are all preserved just fine between before and after and bat+ ball as kinetic system.

But are they conserved seperate or as a total combined ?
Considered seperate it must have the same ratio between angular and linear momentums between before and after dwell for bat and ball combined. Otherwise the total momentum would have changed.
During dwell there is no force from outside on the bat + ball. The bat was accellerated long before that (considered how short the dwellperiod is).
So torcque force and reactionforces are balanced and working inside this system.
There is some angular momentum of the pimples but they rebound back during dwell to good extend (resilient). So not much as a resultantchange between before and after dwell from that.

I see this as an obvious increase of linear momentum combined with a decrease of angular momentum.
So a change of ratio between angular and linear, momentum and energy. The sum for nominator and numerator of a ratio can stay the same just fine.

So momentum and energy, angular + linear, can be preserved just fine.

Big part of tabletennis is about spin/speed ratio and how it can change for the ball between strokes offcourse.
This example is just an extreme for that. 



Edited by mercuur - 09/18/2013 at 6:04am

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Post Options Post Options   Thanks (0) Thanks(0)   Quote tt4me Quote  Post ReplyReply Direct Link To This Post Posted: 09/18/2013 at 11:42am
Originally posted by mercuur mercuur wrote:

Picture (or try) a spinning ball (sidespin) on a floor or table.
With a low as possible speed/velocity someone approaches it with a bat that has a grippy/tacky inverted rubber highly resilient,
The linear momentum and energy change for the bat is extremely low then. Touching the ball softly it accellerates rapidly from the bat.

I assume momentum, linear and angular, are all preserved just fine between before and after and bat+ ball as kinetic system.

But are they conserved seperate or as a total combined ? 
Combined.

Quote
Considered seperate it must have the same ratio between angular and linear momentums between before and after dwell for bat and ball combined. Otherwise the total momentum would have changed.
Since the momentum is combined the ratio can change.  It must be this way.  Think of my serve scenario.  The ball has no angular momentum until it hits the ball. Think of breaking down the paddle and the ball into little pieces and compute the angular momentum around the the point of impact.     The angular momentum is computed around the point of impact so the math requires integrating in multiple dimensions.  The math is messy. 

Quote
During dwell there is no force from outside on the bat + ball. The bat was accellerated long before that (considered how short the dwellperiod is).
There may be force but unlike what many think it doesn't have that much effect.


Quote  
So torcque force and reactionforces are balanced and working inside this system.
There is some angular momentum of the pimples but they rebound back during dwell to good extend (resilient). So not much as a resultantchange between before and after dwell from that.
I see this as an obvious increase of linear momentum combined with a decrease of angular momentum. 
In your example where the ball is spinning like a top on the floor and you touch it with a paddle, you are correct.  This also shows the linear or transitional momentum and rotational momentum can be converted back and forth so the total momentum is conserved but not the ratio.

Quote
So a change of ratio between angular and linear, momentum and energy. The sum for nominator and numerator of a ratio can stay the same just fine.
So momentum and energy, angular + linear, can be preserved just fine.
Yes.

Quote
Big part of tabletennis is about spin/speed ratio and how it can change for the ball between strokes offcourse.
This example is just an extreme for that. 
 

What is the spin to speed ratio where the ball will hit the table an neither speed up or slow down after the bounce?  At this point what is the balls linear and rotational momentum?  What is the ratio between the two?  What is the ratio of the linear to rotational kinetic energy?

No one has figured out how to make the dwell time infinite yet.   


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Post Options Post Options   Thanks (0) Thanks(0)   Quote mercuur Quote  Post ReplyReply Direct Link To This Post Posted: 09/18/2013 at 4:22pm
Infinite dwelltime is possible for a collission of two lumbs of clay when they stick, throwing a dart arrow to a dartboard or two magnets collide. These are all examples of a bounce but without a strong enough rebound to brake contactforces.

Throwing a piece of clay to a wall can combine plastic deformation (not elastic) with a re-bounce and two claylumbs can have a re-bounce and deform plastical with it.

Some new rubbers have a slight grainy texture to the surface that avoids a vacuumsealing effect between the ball and an all slick surface for more speed with - what they hope - little loss of spin.

spin speed ratio to avoid accelleration or decelleration to the table
depends on more then speed/spin ratio.
When speed parallel to the table is high it needs more spin to avoid friction. 
It has a gravity component involved also. The ball also falls which gives a normal force between ball and table that depends on the height the ball comes from. 
Spin/speed ratio adapted to height / forward ratio  then  ?
But things as a harder floor or softer floor or different table could also make a difference.

Maybe use angle in and angle out. When this is mirrorlike reflected the loss of kinetic momentum could also be minimal.







 




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Post Options Post Options   Thanks (0) Thanks(0)   Quote tt4me Quote  Post ReplyReply Direct Link To This Post Posted: 09/18/2013 at 4:43pm
Originally posted by mercuur mercuur wrote:

Infinite dwelltime is possible for a collission of two lumbs of clay when they stick, throwing a dart arrow to a dartboard or two magnets collide. These are all examples of a bounce but without a strong enough rebound to brake contactforces.
I mean with a TT ball and paddle.

Quote  
When speed parallel to the table is high it needs more spin to avoid friction. 
Yes, so what is the spin to speed ration to avoid friction.

Quote
It has a gravity component involved also. The ball also falls which gives a normal force between ball and table that depends on the height the ball comes from. 
Why does it depend on height?  The ball will not deform in any significant way from falling.

Quote
Spin/speed ratio adapted to height / forward ratio  then  ?
What??? 

Quote  
But things as a harder floor or softer floor or different table could also make a difference.
Insignificant because the mass of the table is so much more than the ball.  The table could be in free space and it wouldn't make much difference.  None that you would notice. 

Quote
Maybe use angle in and angle out. When this is mirrorlike reflected the loss of kinetic momentum could also be minimal.
Yes, that would affect the horizontal speed of the ball.  Only consider the horizontal speed of the ball to make calculations easier.


Edited by tt4me - 09/18/2013 at 5:08pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote wturber Quote  Post ReplyReply Direct Link To This Post Posted: 09/18/2013 at 4:47pm
Originally posted by mercuur mercuur wrote:


But things as a harder floor or softer floor or different table could also make a difference.


Given the high mass and stiffness of a 1" top that is used in most tournament tables (never mind the attached structure) vs. the 2.7g mass of a TT ball, it seems unlikely to me that floor type would have a meaningful difference in the way the ball bounces.  At a mere half a kilogram, a perfectly elastic collision would cause the TT ball to lose only 1% of its speed (5m/sec collision).  So there's only 1% of loss to be gained for larger masses - best case.  I'd think the location of contact on the table relative to braces and supports would be more influential than the floor type.  And how close the ball bounced to its own seam would likely have an even greater influence.

I know some people swear that they can tell the difference.  But some people think they can guide the ball during contact time as well. The basic physics suggests, I think, that it doesn't matter.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Baal Quote  Post ReplyReply Direct Link To This Post Posted: 09/18/2013 at 6:25pm
Originally posted by wturber wturber wrote:

Originally posted by mercuur mercuur wrote:


But things as a harder floor or softer floor or different table could also make a difference.


Given the high mass and stiffness of a 1" top that is used in most tournament tables (never mind the attached structure) vs. the 2.7g mass of a TT ball, it seems unlikely to me that floor type would have a meaningful difference in the way the ball bounces.  At a mere half a kilogram, a perfectly elastic collision would cause the TT ball to lose only 1% of its speed (5m/sec collision).  So there's only 1% of loss to be gained for larger masses - best case.  I'd think the location of contact on the table relative to braces and supports would be more influential than the floor type.  And how close the ball bounced to its own seam would likely have an even greater influence.

I know some people swear that they can tell the difference.  But some people think they can guide the ball during contact time as well. The basic physics suggests, I think, that it doesn't matter.

I have always hypothesized that the effect of the floor that people think they perceive is really an illusion caused by the different sound in the room.  Different tables matter though (not for blade dwell time, just for how you play).  The ones with very slick glassy tops it seems like the ball tends to slide a bit more, like a fast court in tennis.  In China there are a lot of Double Fish tables with some of the slickest tops I have ever seen.  It takes a bit of getting used to coming from Tibhar Smash 28.  This is all a different off-topic issue, but it did come up, so........
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mercuur Quote  Post ReplyReply Direct Link To This Post Posted: 09/19/2013 at 4:28am
@tt4me,

Doesn,t height affect the eventual angle to the table ? Forward was meant as stroke length.
On the moon it would need a longer table or very little energy to keep the ball on the table for length and height.

With momentum as  linear plus angular.
When the sum needs to be preserved a linear ór angular cor can be higher then 1 but not both higher then 1. For momentum conservation this shouldn,t be a problem. It,s even better because a higher linear cor can compensate for a lower angular cor then (or vice versa).

1,5 and  0,5 would loose no momentum as 1 and 0,5 would.
Spin /spinsensitive rubbers tend to be slower and speed rubbers tend to be less spinny could be explained from this. It makes it impossible to increase the cor for speed as much without having to pay for with less spin or vice versa.
The fastest racquet and shots will always have less spin then and spinniest always less fast.



 






Edited by mercuur - 09/19/2013 at 8:00am

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Post Options Post Options   Thanks (0) Thanks(0)   Quote mercuur Quote  Post ReplyReply Direct Link To This Post Posted: 09/19/2013 at 6:51am
http://en.wikipedia.org/wiki/Coefficient_of_restitution

Under further details this wiki page mentions a (linear) Cor higher then one.

Quote A COR greater than one is theoretically possible, representing a collision that generates kinetic energy. For example, some recent studies have clarified that COR can take a value greater than one in a special case of oblique collisions.[3][4][5] These phenomena are due to the change of rebound trajectory of a ball caused by a soft target wall.
.

http://adsabs.harvard.edu/abs/2002PhRvE..65b1303L

Publicationdate 2002.



Edited by mercuur - 09/19/2013 at 8:20am

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Post Options Post Options   Thanks (0) Thanks(0)   Quote wturber Quote  Post ReplyReply Direct Link To This Post Posted: 09/19/2013 at 1:52pm
Originally posted by Baal Baal wrote:

I have always hypothesized that the effect of the floor that people think they perceive is really an illusion caused by the different sound in the room.  Different tables matter though (not for blade dwell time, just for how you play).  The ones with very slick glassy tops it seems like the ball tends to slide a bit more, like a fast court in tennis.  In China there are a lot of Double Fish tables with some of the slickest tops I have ever seen.  It takes a bit of getting used to coming from Tibhar Smash 28.  This is all a different off-topic issue, but it did come up, so........


Yes.  The friction of the tops does vary and does affect how spinning balls bounce. And I think that different sounds might very well color our perceptions of ball speed etc.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 09/19/2013 at 4:08pm
Originally posted by tt4me tt4me wrote:

Originally posted by AgentHEX AgentHEX wrote:

Even if a rubber has very good tangential elasticity, if the rebound in that direction is very slow, a quicker normal component might already have ejected the ball and limited the effect. So I guess dwell does matter in that regard, LOL.

It is exactly this issue that has me thinking about how the impact can be modeled simply without going through all the trouble of doing what Andro had to do with their modeling.

The brute force way to do this is to shoot balls at the rubber at different speeds and angles to build a table.    For instance there could be a table of tangential COR as a function of normal impact speed.
In the end the brute force table needs to be compiled so that the model can be fit to the actual data.
I bet the Andro guys have done it all and are laughing at our ignorance.



Generally with these sorts of models there are plenty of parameters to tweak and significant empiricism to validate them is necessary anyway, enough that I suspect they're not really fully characterizing the bat (because they don't need for TT purposes). IMO they're using the CAD models more for the production processes than physical modeling.

This is somewhat interesting stuff, but general design/use goals like "hard enough to not bottom out" and feel to the player in aggregate is more important than minor technical differences.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tt4me Quote  Post ReplyReply Direct Link To This Post Posted: 09/19/2013 at 6:25pm
Originally posted by mercuur mercuur wrote:

http://en.wikipedia.org/wiki/Coefficient_of_restitution

Under further details this wiki page mentions a (linear) Cor higher then one.

Quote A COR greater than one is theoretically possible, representing a collision that generates kinetic energy. For example, some recent studies have clarified that COR can take a value greater than one in a special case of oblique collisions.[3][4][5] These phenomena are due to the change of rebound trajectory of a ball caused by a soft target wall.
.

http://adsabs.harvard.edu/abs/2002PhRvE..65b1303L

Publicationdate 2002.

A COR greater than one is not possible without some other means of adding energy to the system.  If a COR greater than 1 were possible without adding other energy then we would have our energy problems solved.

@mercuur,  at the bottom of the coefficient_of _restitution it mentions Rod Cross.  Rod Cross has some interesting articles.  Enjoy.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 09/19/2013 at 6:31pm
COR is just a number to balance the equation. It has no actual physical manifestation. The paper is just about a corner case where given how everything else is measure, the number happens to be >1 not "free energy the gubmint doesn't want you to know about".


Edited by AgentHEX - 09/19/2013 at 6:33pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote zeio Quote  Post ReplyReply Direct Link To This Post Posted: 09/19/2013 at 6:45pm
Originally posted by Baal Baal wrote:

Something I would like to know from people who know a lot of physics.  A ball hit in most table tennis shots spins.  So it has angular momentum.  It also moves away from the blade so it has regular momentum --I'm not sure about the right term in physics, maybe linear momentum?  So, anyway, assume an identical swing delivered against an identical incoming ball.  Assume that the swing is one that would normally give an ordinary topspin counter.  (Or even a modest loop).  Now imagine that this is done with many different setups, and the only thing that varies is the dwell time of the blade and rubber. Imagine you had a robot that precisely delivered the ball and another robot that precisely moved the blade so everything was highly reproducible, also imagine unlimited budget to measure everything. Would one expect that with more dwell time, you would get more angular momentum and less linear momentum or are things more complicated than that?  I guess, what I am trying to get at is why would a difference in actual dwell time matter for what happens on the shot and in regular TT performance -- and now I am not referring at all to what it feels like.  Only what actually happens to the ball.  What is the expectation?

Given those ideal conditions, both the linear and angular momenta(assumed average from here on) should end up with different ratios for different dwell time, but the amount of change for angular momentum should be less than that for linear momentum regardless of dwell time.

Here is why.  During a collision, changes in angular momentum(angular impulse) are fundamentally more difficult to achieve than changes in linear momentum(impulse) because the moment of inertia of a ball that needs to be overcome is much greater than its mass.  This is the primary reason that spin has seen a larger decrease than speed after the change to 40mm ball.

With more dwell time, though, the work done on the ball(as in the amount of effort put in) is greater as it remains in contact with the paddle over a longer distance.  The power expended on the ball also comes out less as that amount of work is spread over a longer duration.  So you as a player would have an easier time spending a larger portion of your energy towards generating more torque to increase the angular momentum of the shot. This may give off the illusion that more dwell time leads to more angular momentum generated, but do not forget that momentum is increased in the process as well.

Now, if a setup with less dwell time turns out taking more effort to play, why would anyone sensible choose that?  Here is the catch.  Remember that dwell time and COR are negatively correlated?  That means setups with less dwell time will incur less energy loss on shots, so less capable players can theoretically get more out of what they put in and for those high-level players with a sufficiently high swing speed it should be possible to achieve a higher level of linear and angular momenta for the same effort.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 09/19/2013 at 7:08pm
Originally posted by zeio zeio wrote:

Originally posted by Baal Baal wrote:

Something I would like to know from people who know a lot of physics.  A ball hit in most table tennis shots spins.  So it has angular momentum.  It also moves away from the blade so it has regular momentum --I'm not sure about the right term in physics, maybe linear momentum?  So, anyway, assume an identical swing delivered against an identical incoming ball.  Assume that the swing is one that would normally give an ordinary topspin counter.  (Or even a modest loop).  Now imagine that this is done with many different setups, and the only thing that varies is the dwell time of the blade and rubber. Imagine you had a robot that precisely delivered the ball and another robot that precisely moved the blade so everything was highly reproducible, also imagine unlimited budget to measure everything. Would one expect that with more dwell time, you would get more angular momentum and less linear momentum or are things more complicated than that?  I guess, what I am trying to get at is why would a difference in actual dwell time matter for what happens on the shot and in regular TT performance -- and now I am not referring at all to what it feels like.  Only what actually happens to the ball.  What is the expectation?

Given those ideal conditions, both the linear and angular momenta(assumed average from here on) should end up with different ratios for different dwell time, but the amount of change for angular momentum should be less than that for linear momentum regardless of dwell time.

Here is why.  During a collision, changes in angular momentum(angular impulse) are fundamentally more difficult to achieve than changes in linear momentum(impulse) because the moment of inertia of a ball that needs to be overcome is much greater than its mass.  This is the primary reason that spin has seen a larger decrease than speed after the change to 40mm ball.

So terrible. The 40mm ball is >5% larger so it's by definition slower to spin for same tangential speed. It also has greater aero resistance so less spin is necessarily placed on the ball by the players for same flight path.

Quote
With more dwell time, though, the work done on the ball(as in the amount of effort put in) is greater as it remains in contact with the paddle over a longer distance. 


Pointless, amount of work varies with speed.

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The power expended on the ball also comes out less as that amount of work is spread over a longer duration.  So you as a player would have an easier time spending a larger portion of your energy towards generating more torque to increase the angular momentum of the shot.


Again pointless. No significant energy is added during dwell.

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This may give off the illusion that more dwell time leads to more angular momentum generated, but do not forget that momentum is increased in the process as well.


Irrelevant.

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Now, if a setup with less dwell time turns out taking more effort to play, why would anyone sensible choose that? 


Why would that ever be the case given dwell is ~1ms?

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Here is the catch.  Remember that dwell time and COR are negatively correlated?  That means setups with less dwell time will incur less energy loss on shots, so less capable players can theoretically get more out of what they put in and for those high-level players with a sufficiently high swing speed it should be possible to achieve a higher level of linear and angular momenta for the same effort.


No, there's a clear limit to the amount of angular momentum possible given low dwell in normal direction, which should be clear enough if you bothered to read the simple posts above.

This whole treatment is so poor overall that even mercuur manages to string together random technical sounding words more coherently.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tt4me Quote  Post ReplyReply Direct Link To This Post Posted: 09/20/2013 at 12:09am
Originally posted by AgentHEX AgentHEX wrote:


Again pointless. No significant energy is added during dwell.

When then is the energy added?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 09/20/2013 at 12:59am
The energy is from the relative speed of the collision, ie present in the system from before any dwelling happens.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tt4me Quote  Post ReplyReply Direct Link To This Post Posted: 09/20/2013 at 11:04am
Originally posted by AgentHEX AgentHEX wrote:

The energy is from the relative speed of the collision, ie present in the system from before any dwelling happens.
I agree that no or little energy is added to the system during the dwell time but I think zeio was just referring to the ball and the ball does get a big boost in energy.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote mercuur Quote  Post ReplyReply Direct Link To This Post Posted: 09/20/2013 at 11:14am
Originally posted by tt4me tt4me wrote:

A COR greater than one is not possible without some other means of adding energy to the system.  If a COR greater than 1 were possible without adding other energy then we would have our energy problems solved.


All momentum must be conserved and all energy right ?
You agreed to that. So keep that in mind.
There is no disproof then from theory for the possibillity that a linear cor can be higher for -elastic energy corresponding - lower angular cor or (with a next stroke maybe) vice versa.
The energy here is elastic energy in between relative V in and - out. What do we really know what happens in this short period ? All (angular and linear) comes together in elastic energy ; bounce/rebounce.

Cor has a period for dwell implied. The first conversion of kinetic mometums into elastic energy and second an opposite conversion into kinetics.  Not a sudden change of kinetics into kinetics (which would neglect dwell as a period because it,s so short while it,s all about what happens during this period).   Even 0,001 sec is still a period T as  t1-t0. (t for clockcoordinates, -moments, T for clock periods).

To picture the logic :

Compare an empty bucket throwing hundred tabletennisballs in it.
Some amount Yellow (Y) other amount white (W).
Y and W can both vary from 0 to 100 with W+Y = 100.
The balls are invisible for a short period so we don,t know what all happens or could happen in the bucket. For one thing they could all shift colour (or partly ?). W coming out as yellow and yellow as white for further the same ball. No problem with Balls out/balls in (no matter what colour) or ((W+Y)out/ (W+Y) in.
One colour could have more problem to escape the bucket kineticly. Back could have more problem but when black-out was former white-in this is  loss for white-in.

Example :

When 100 balls have 40 W + 60 Y and all balls turn colour inside the bucket (during dwell with the bucketbottom) this becomes 60W+40Y.

When 50 W escape of this 60 it still has 50 W out and 40 W in. Of 40 now Yellow 20 could escape as ball (or as sound, themperature raise and other "residue vibration"). So yellow out /yellow in = 20/60 but Wout/Win > 1. When all 60 escape it has 60/40.

This just focusses on balls plus bucket and on their kinetic relation which is somewhat narrow.  The bucket could slide to a floor which can take account for white.  Then some more balls could stay in the bucket but the bucket to the floor takes account for some white and yellow.
Clinear and C angular can very well be both lower then one for ball-bucket bounce.
But with a bat developed for a high resilience with a celluloid ball Cor linear > 1 is not a problem.
Other stroke with same bat having a cor angyular > 1 is also not a problem.
First case Cor angular will be lower for this and second case cor linear.

That explains then how spinsensitivity can become lower with a light quick frontal stroke that increases the cor for linear and same time decreases the cor for angular.

"Dwelltime for spin is too short to workout" (as Agenthex explained this)  exactly says the same. Dwell for spin is the same as for speed (not shorter) for such a bounce/rebounce and cor refers to a bounce/eebounce (in future, present or past). Not to seperate objekts before they bounce (in past, present or future).

So Agent-Hex agreed with me that players can affect how the cor ratios for angular and linear relate. More or less as communicating vessels (instead of all seperate).




Edited by mercuur - 09/20/2013 at 11:36am

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Post Options Post Options   Thanks (0) Thanks(0)   Quote mercuur Quote  Post ReplyReply Direct Link To This Post Posted: 09/20/2013 at 11:36am
Originally posted by AgentHEX AgentHEX wrote:

COR is just a number to balance the equation. It has no actual physical manifestation.


Cor is a rationumber, A rationumber reperesents a ratio and this is a true phyical manvfestation.

One reason for that is that Vin --> V out has time : -->. The topic for this thread.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote jt99sf Quote  Post ReplyReply Direct Link To This Post Posted: 09/20/2013 at 12:02pm
This is like watching paint dry,  you guys need to go play TT.  LOL
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Post Options Post Options   Thanks (0) Thanks(0)   Quote zeio Quote  Post ReplyReply Direct Link To This Post Posted: 09/20/2013 at 12:20pm
Originally posted by AgentHEX AgentHEX wrote:

Originally posted by zeio zeio wrote:

Originally posted by Baal Baal wrote:

Something I would like to know from people who know a lot of physics.  A ball hit in most table tennis shots spins.  So it has angular momentum.  It also moves away from the blade so it has regular momentum --I'm not sure about the right term in physics, maybe linear momentum?  So, anyway, assume an identical swing delivered against an identical incoming ball.  Assume that the swing is one that would normally give an ordinary topspin counter.  (Or even a modest loop).  Now imagine that this is done with many different setups, and the only thing that varies is the dwell time of the blade and rubber. Imagine you had a robot that precisely delivered the ball and another robot that precisely moved the blade so everything was highly reproducible, also imagine unlimited budget to measure everything. Would one expect that with more dwell time, you would get more angular momentum and less linear momentum or are things more complicated than that?  I guess, what I am trying to get at is why would a difference in actual dwell time matter for what happens on the shot and in regular TT performance -- and now I am not referring at all to what it feels like.  Only what actually happens to the ball.  What is the expectation?

Given those ideal conditions, both the linear and angular momenta(assumed average from here on) should end up with different ratios for different dwell time, but the amount of change for angular momentum should be less than that for linear momentum regardless of dwell time.

Here is why.  During a collision, changes in angular momentum(angular impulse) are fundamentally more difficult to achieve than changes in linear momentum(impulse) because the moment of inertia of a ball that needs to be overcome is much greater than its mass.  This is the primary reason that spin has seen a larger decrease than speed after the change to 40mm ball.

So terrible. The 40mm ball is >5% larger so it's by definition slower to spin for same tangential speed. It also has greater aero resistance so less spin is necessarily placed on the ball by the players for same flight path.

Quote
With more dwell time, though, the work done on the ball(as in the amount of effort put in) is greater as it remains in contact with the paddle over a longer distance. 


Pointless, amount of work varies with speed.

Quote
The power expended on the ball also comes out less as that amount of work is spread over a longer duration.  So you as a player would have an easier time spending a larger portion of your energy towards generating more torque to increase the angular momentum of the shot.


Again pointless. No significant energy is added during dwell.

Quote
This may give off the illusion that more dwell time leads to more angular momentum generated, but do not forget that momentum is increased in the process as well.


Irrelevant.

Quote
Now, if a setup with less dwell time turns out taking more effort to play, why would anyone sensible choose that? 


Why would that ever be the case given dwell is ~1ms?

Quote
Here is the catch.  Remember that dwell time and COR are negatively correlated?  That means setups with less dwell time will incur less energy loss on shots, so less capable players can theoretically get more out of what they put in and for those high-level players with a sufficiently high swing speed it should be possible to achieve a higher level of linear and angular momenta for the same effort.


No, there's a clear limit to the amount of angular momentum possible given low dwell in normal direction, which should be clear enough if you bothered to read the simple posts above.

This whole treatment is so poor overall that even mercuur manages to string together random technical sounding words more coherently.

So terrifying.  Seeing that we have another confirmed case of jumping-the-gun.  I've got some bad news for you.  Bet you wish you'd read the paper through after this.  Those were measurements taken moments after the ball had left the paddle, ~67ms to be precise.  There's more.  They also compared the effect of drag for the 38mm and 40mm balls and found that they slowed down at the same rate.  Oh, no!  The drag explanation is flawed.  Don't know why?  Let me fill you in.  The increase in drag gets canceled out by the increase in inertia.  So what's the actual increase in moment of inertia?  ~19.8%, which makes it over 376% higher than the ~5.26% increase in diameter and over 247% higher than the 8% increase in mass.  It's unfortunate but poor physical intuition gets the better of you again.

I could let that one off, but work varies with speed?  Darn, when the crap hits the fan.  You keep mixing distinct concepts up and putting things backwards here.  Do you not even realize you're talking about power the way you use the word work?  This is just as hilariously bizarre as pnatchwey's take on mangling the speed-after-impact formula.

Darn right, dwell does not affect how you play, given you can't differentiate between work and power.  Just like color doesn't make any difference to the totally color blind.  Or just like normal human works in decimal; computer works in binary; you work in HEXadecimal.  Just that simple.

You don't get it, do you?  It's clear enough when I couldn't help but glimpsed through that backtracking post.  Dwell is not the limiting factor here.  Swing is what matters most.  As High-level players are capable of reaching maximum velocity just before impact given their ridiculously high acceleration, they are said to have a powerful shot for they can get the same amount of work done over a much shorter period.  It is only for people with a slower swing that dwell becomes the bottleneck.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tt4me Quote  Post ReplyReply Direct Link To This Post Posted: 09/20/2013 at 12:22pm
Originally posted by mercuur mercuur wrote:

Originally posted by tt4me tt4me wrote:

A COR greater than one is not possible without some other means of adding energy to the system.  If a COR greater than 1 were possible without adding other energy then we would have our energy problems solved.


All momentum must be conserved and all energy right ?
You agreed to that. So keep that in mind. 
No I didn't.  Momentum must be conserved but not energy.  There are no perpetual motion machines yet.

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There is no disproof then from theory for the possibillity that a linear cor can be higher for -elastic energy corresponding - lower angular cor or (with a next stroke maybe) vice versa.
I can see where in a normal COR for a particular impact can be greater than one but only if the ball has some rotational energy that is converted to linear energy but overall the energy will be reduced unless some other source is added like a player hitting the ball.   There is no 'free' energy. 

I shouldn't need to keep repeating this.   Study the pdf files in the links.  Get a copy of wxMaxima or Sage that can do symbolic math and work through the problems.

I didn't understand what point you are trying to make with the orange and white TT balls.


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