dwelltime

zeio wrote: AgentHEX wrote: I'm curious whether zeio has figured out what's going on and just posting garbage to cover it up, or actually believes that's a coherent reply to tt4me. 97.7% of the kinetic energy of the paddle has been transferred to the ball during the collision. Quoted again for posterity. I say it's the other way around. That number is valid because it's during the compression phase of the collision where the ball is under acceleration in the opposite direction by the paddle.  Without realizing that, tt4me made the "not even wrong" comment that the ball "went back at only 3.7m/s."  Still in ignorance, he even went on to show how could "turn the crank" with the speed-after-impact formula, again without realizing that equation applies to the situation after the collision. As an aside, remember your "not even wrong" comment about dwell time?  It is unfortunately the lower the harder you hit(Figure 15). p.s. added link to the "not even wrong" comment

I guess we've established you really do still believe 97.7% of the kinetic energy of the paddle has been transferred to the ball during the collision. No further comment is necessary.

Same here.  It can't be helped if you truly think what happens after the collision must also be what's happened during the collision.

OTOH, don't try to cover up your dwell time opinion with that.

are there any useful conclusions to be drawn by the ordinary player from this discussion?

pingpongpaddy wrote: are there any useful conclusions to be drawn by the ordinary player from this discussion?

1. I think most of us agree the dwell time is much shorter that 4 ms that was suggested before.
2. Baal seems to be pretty sure we can't tell the difference 1ms dwell time and a 1.1ms dwell time.   I think this is significant to most people think they want more dwell time for more spin but the question that remains is does the extra micro seconds of dwell time result in more spin.
2. The dwell time can be longer than 1 ms under certain conditions but no one has decided to tackle that problem.  I think this is important because so many obsess about dwell time.
3.  Lower CORs result in longer dwell time.  I think this is intuitively obvious.
4. Tangential force or torque spins the ball.  One can get the same amount of spin by using a low force over longer dwell time or a higher force over a shorter dwell time as along as the Force*DwellTime is constant. 
5. I provided some high speed videos so that people can see for themselves and stop arguing but we can see it hasn't made any difference.

@pingpongpaddy,  I said at the beginning I knew what the dwell times are ( high speed videos plus I can do the math to make pretty good estimations ) and it really didn't affect how I play or what I buy.  I don't see why some are so obsessed about this.  I look at this more from and engineering curiosity point of view.  I have no idea what mercuur is going to do with all of this information.

 

Tassie52 wrote: wturber wrote: pingpongpaddy wrote: are there any useful conclusions to be drawn by the ordinary player from this discussion? No. 1. wturber is usually correct - except in this instance. There are useful conclusions to be drawn, such as: 2. Some people believe that more posts equals useful posts, and 3. Point 1 trumps point 2.

Yeah.  It was supposed to be a humorous exaggeration.  :^) 

bluebucket wrote: Well it's blatantly obvious slower more flexible blades make more spin, why? does it really matter?.

No, and I made it very clear earlier.

As you get older and see more things you realise most experts know very little about the field they are in and most explanations are nothing more than guesswork.

Did you look at the high speed videos?  The high speed videos makes it easy to verify assumptions are in a valid range.  So who is guessing?

Tassie52 wrote: wturber wrote: pingpongpaddy wrote: are there any useful conclusions to be drawn by the ordinary player from this discussion? No. 1. wturber is usually correct - except in this instance. There are useful conclusions to be drawn, such as: 2. Some people believe that more posts equals useful posts, and 3. Point 1 trumps point 2.

To expand slightly on why I expounded on various types of students to explain this situation, generally in industry where being right matters the former group who grasp the concepts are best sorted into creative/design type roles, while the latter are fit for menial work assigned by the former. This is an important and valuable distinction to make even outside a professional capacity in places of limited context like the internet, because there's a vast difference in reliability of info presented by the two.

mercuur wrote: Different camera rates and/or slow motion and/or display rates will give various values.

AgentHEX wrote: Tassie52 wrote: wturber wrote: pingpongpaddy wrote: are there any useful conclusions to be drawn by the ordinary player from this discussion? No. 1. wturber is usually correct - except in this instance. There are useful conclusions to be drawn, such as: 2. Some people believe that more posts equals useful posts, and 3. Point 1 trumps point 2. To expand slightly on why I expounded on various types of students to explain this situation, generally in industry where being right matters the former group who grasp the concepts are best sorted into creative/design type roles, while the latter are fit for menial work assigned by the former. This is an important and valuable distinction to make even outside a professional capacity in places of limited context like the internet, because there's a vast difference in reliability of info presented by the two.

Thank you for confirming my second point.

wturber wrote: pingpongpaddy wrote: are there any useful conclusions to be drawn by the ordinary player from this discussion? No.

AgentHEX wrote: Just because something isn't useful to you doesn't meaning it's not useful. The world is likely full of such things.

Of course this is true. I am absolutely convinced that the research being done with the Large Hadron Collider at CERN is of immense usefulness to the world in general and the members of the LHC Computing Grid in particular. But that's not the point.

IIRC, the question I am responding to is:

pingpongpaddy wrote: are there any useful conclusions to be drawn by the ordinary player from this discussion?

And to this point I have yet to find anything in this loooooong discussion which is remotely useful for the ordinary player.

There's lots of interesting discussion, some thinly veiled insults, and a huge amount of mathematical argument, but not one jot of it makes any difference when anyone (the calculating genii included) picks up a bat and steps up to the table.

And before you want to know why I bother to read the thread, I'm only here to help boost the number of posts towards 500.   

Tassie52 wrote: IIRC, the question I am responding to is: pingpongpaddy wrote: are there any useful conclusions to be drawn by the ordinary player from this discussion? And to this point I have yet to find anything in this loooooong discussion which is remotely useful for the ordinary player.

I was criticized for dismissing this thread early on.
None of this makes any difference to the average player yet many ordinary players seem to be obsessed with dwell time.  This can't be denied.

There's lots of interesting discussion, some thinly veiled insults, and a huge amount of mathematical argument, but not one jot of it makes any difference when anyone (the calculating genii included) picks up a bat and steps up to the table.

I agree that it shouldn't make much difference but I say again, you can't deny there are many obsessed by a few extra microseconds of dwell time.

There are all many that think that accelerating through the ball is good.   While I agree it does increase dwell time for the most part it is only micro seconds.
 

And before you want to know why I bother to read the thread, I'm only here to help boost the number of posts towards 500.

Actually, back when this thread began I made a post to Fatt saying I would participate if he would screen out all the 'noise'.   I got no reply, I think he must have been 'burned out' by then. So I decided to take a chance.  I am very disappointed that this thread will be out of sight and out of mind once it scrolls down the bottom of the post list and that there is so much 'noise' that no one will bother to read it all.

I think the high speed videos are useful because NOW YOU KNOW.  There will be no more nonsense about long dwell times under normal conditions although I am still trying to get somebody to tell us under what conditions the dwell time can be extend.

 




Edited by tt4me - 09/24/2013 at 3:02am

tt4me wrote: When something moves x pixels per frame you have to multiply that by mm per pixel and seconds per frame to get the mm per second.

Thats mm /sec. A mm/ sec change needs at least three fremes.

http://www.mijnbestand.nl/Bestand-WUP7FSWMBORN.bmp.

Change of aerodynamic decelleration  (higher velocity higher decelleration) would need a minimum of even four frames for aerodynamics study of a celluloid ball because the decelleration decreases with a decreasing velocity.

So maybe that explains the out of space resemblance for the breaking lamp video ? When someone knows my thinking is fault or comes short (I,m certain for that) I.m all open for a better explanation to that resemblance.

Has it to do with the more uniform accelleration from gravity compared to aerodynamic decelleration for instance ? 

The resistance of a thick soft "aery" spongerubber (for as far as the ball penetrates the rubber/sponge) resembles the resistance in a viscous medium somewhat.

That compares with aero dynamics. So an approach from that angle is another possible approach.

Difference is that it has a medium with increasing density.  
With a constant density for a medium decelleration decreases with lesser velocity (as it increases with higher velocity).  Increasing density does the opposite.
That way a spongerubber has a more gradual decelleration as ox rubber.
Sponge thickness, - density, impact,, bladeflex, blade softness/hardness,, topsheet soupplesse all affect this.

So  (to the "know it alls" here) give a few dwelltime related tips to readers what sponge thickness, density, bladeflex variation all do for this then.
Like what does the additional "give" from a more flexible blade (larger amplitude) do for this ?
Or what does it different then the give of a soft sponge for this ?

When this thread means nothing for most forum readers you, instantly knowing how to correct others, should be able to make it more meaningfull instead of almost blaming me for starting it.




Edited by mercuur - 09/24/2013 at 7:02am

tt4me wrote: bluebucket wrote: wturber wrote: pingpongpaddy wrote: are there any useful conclusions to be drawn by the ordinary player from this discussion? No. +1 Then why are you reading this thread?

Maybe he's EXTRAordinary.  :^)

The last part is more relevant for tt then first long part.  (not meant to shout )

tt4me wrote: mercuur wrote: With a constant density for a medium decelleration decreases with lesser velocity (as it increases with higher velocity).  Increasing density does the opposite. This isn't clear.

The faster you drive on a bike the higher the decellerating force from driving wind. 
The velocity kept as a constant uniform speed has a balance of accellerating and decellerating forces. Thus also for decelleration and accelleration in geometry.

Say this is a bike at constant speed on a Dutch flat road. :

           <---------- x ----------->

Vectors for accelleration and decelleration in opposite direction are even for geometry so it has a uniform speed.

Same as in an elevator going up with gravity and liftforce. F= Ma hence A=F/M (up)
and F=Mg :   g=Fz/M  (down). Going upward g is a decelleration to that for geometry to the velocity for lift with or without people in it.

The weight force the people feel as normal weight on their feet works inside the system (lift + people) and has no additional influence on accelleration when the passagers weight is allready taken in account as part of the total weight for lift plus people.
For the lift engine to pull up this weight force (as experience) this experience has no meaning in technical sense. Not for uniform velocity and not for accelleration or decelleration with experiencing more or less weight,

It makes more sense in notation (for yourself) when consekwently using a D for A when it,s from a decellerating force to the velocity (with vector) and an a when it,s from an accellerating force. Or A(a) and A(d) can do or (A-) and A(+).

The wind is to a bike what gravity is for an elevator.

It can have four types of vector then for geometry of accelleration on one workingline  : 

  ----Accelleration----> , <---Decelleration.-----  ,   <--- Acc------- ,   -----Dec--->..

A child can distinct between these four when driving a bike and doesn,t need Physics for that.

Even against a decellerating wind it,s often possible to accellerate offcourse.
Then it has ----Aa----> + <--Ad-   = --A-> as resultant accelleration for instance. A shorter accelleration for kinetic geometry as resultant of a decelleration and accelleration in opposite direction.

A push from mothers or on the pedals (accelleration),  a frontal wind (decelleration) and driving from home and back home for direction with both of these in both direktions,
Turning the neg and pos for direction while the kid's at school would not have it come home again.

Can,t adopt the mathrule that all numbers are positive for decelleration and accelleration (or use a lot of minusses to compensate).

tt4me wrote: mercuur wrote: What does the additional "give" from a more flexible blade (larger amplitude) do for this ? Or what does it different then the give of a soft sponge for this ? Softer, slower and more flexible result in longer dwell times.  Take this to an extreme and you have an inelastic collision.

Sponge compression compares to a viscosity increase for something moving into/through a medium (gas. fluid or solid).
For decelleration a brakesystem works constant (W=constant) when it has a constant force. With decreasing speed it keeps doing same work per second but with much less energy,
Brakes can even melt at high speed or burn but don,t do more work then per second as at lower speed.
It has decreasing distance/sec (velocity now not speed) for the brakeforce to work the longer it wirks.
That,s why the energy per second becomes lower with still doing same work per second.

When energy per second becomes lower decelleration to the velocity per meter decreases with lower speed.

Likewise as experiencing a much stronger force from wind driving at higher velocity on a bike.
Work per second that this wind does to the bike (or opposite) (FS/sec) is the same but not work per meter and also not FS/T..

When T > sec  FS/T < FS/sec and T<sec FS/T > FS/sec.
This is the reason why energy and work must be distincted. 
So work on the ball (bat outside the system) and energy for the bounce and decelleration, rel speed (bat plus ball) is totally different from that.
You can,t use part of the energy in the system for the bat to do work as on another system.
That,s like trying to speed up a sailboat with a ventilator in the boat forgetting that the ventilator is on board of the ship and investing energy can not do work then.

That,s also why unitnotation always belongs between brackets for physics.
Not E=10 Joule as if 10 joule work but W=10 (J)  E = ? or vice versa. As if Joule is more then a unit.

The paddle/clamp floor can be seen to do work to the ball while energy adressed to and for the collision.  That,s a choice of system to have a reactionforce do work then. But the actionforce from the ball can,t do the same work also then. That would be double work from same energy ; both actionforce and reactionforce doing work.
When that would work (action and reactionforce both doing work for accelleration) throwing pingpongballs balls to the frontwindow of a car while driving to a gym would also accellerate the car. You won,t do that because you kbow it won,t work so why do it here ?

With a low cor it has more work to decellerate a ball more as with a high cor while it has same energy for the collision. Not more energy and same work (or irrelevant). The ball does no work then to the paddle as it stays in the hand or vice at same position. The paddles reactionforce does  work on the ball during the tensioningphase. Lost energy for kinetics maybe but not at all for playing tt.

The more or most interesting part for tabletennis.

In-elastic collision does not say the parts are less or more elastic  (even a piece of paper recovers to original shape after bending so fully elastic) or even degree of  resilience for the parts (plus ball) alltough that plays a role. Dwelltime matters for this in a different way.

As on the toxic five video. It shows the blade has not rebounced full the moment the ball jumps off. It rebounces mostly with the ball allready on it's way. Clearly visible and trusthworth also.
So the blade is all resilient but not the bounce. The natural resilience of the ball is just quicker then the paddle.
The rendition of resilience is low from lack of resonation. But same blade in your hands with enough skill will have much better resonation for a flathit. It,s in your hands to make it a faster or slower paddle. It,s the idea of gears for a racquet to what extend this can be varied. Fastest blades don,t allow much variation, Slower blades lack a top gear because to achieve resonation is more difficult. To some extend skill can compensate this for a higher gear but only to some extend.

Likewise on a swimmingboard for diving. Jumping off the moment the board has a large downward amplitude leaves the plank and energy behind in the board with heavy all resilient residue vibration as result. Just not effectively resilient for the bounce,

Players influence the effectiv resilience for the bounce this way making the racquet slower or faster. (that,s why these speed numbers for blades and rubbers are idiot just as cor numbers would be).

One can interrupt/break the resonance to slow down and control a ball or aim for maximum resonance for maximum accelleration. Tt players do this probably unconscious in the sense of not aware of the physics but they do it and, in tt sense, all conscious of what they do.