Dwell Time .....

[NextLevel]

If that's directed at me, I never claimed my (poor, idealised and basic) understanding of (a very small part of) the physics had any practical application. It isn't even at the stage of any possible practical application, I just figured out what the forces at play would look like if we made a lot of simplifying assumptions a little better than a previous post had done. There could be practical applications to this, but they would be many steps away from application. If anything in that analogy, I'd be a low-level physicist who just started doing their modelling of an element of, say, building strain, being told it's self-serving because it hasn't resulted in any practical change to civil engineering yet.


Yes. I never claimed otherwise, in fact I explicitly have said more than once that you can improve play better by just doing what works instead of worrying about the physics. Why so combative? I've never claimed to be helping people improve, I haven't even said anything that someone could even try to use to improve based on the physics.

The combativeness is largely because by reopening the thread, you wittingly or unwittingly are encouraging a guy who repeatedly enjoys claiming his supposed understanding of the physics enables him to rudely tell others how they should play or select equipment etc and that such expertise supersedes what better and experienced players do in helping players. All while never seriously proposing a mechanism for what TT players are doing when they claim to be increasing dwell time etc.

The reality is also that even if people may not be able to feel this based one dimension of sensation (tactile), there is too much evidence that they are doing things that work A lot of this comes down to interpretation. Very often table tennis players know where the ball is going before the opponent hits it. Thwrefore it would be misleading to focus exclusively on their physical reaction speeds if trying to understand why they tend to react to the ball so well.

 

[dingyibvs]

The perception of dwell time is real whether actual dwell time is real or not. If you struggle to explain what it is and how it affects play in scientific terms then it's your failure as a scientist, not that it doesn't exist. This is not a criticism BTW, scientists had for millennia failed to explain why the sun appears to rise in the east and set in the west. We now know why it appears the sun does what it does, and it doesn't make scientists of the past fools. They would've been fools if they had argued that the perception of the sun rising and falling is not true or that its occurrence is not important to our lives.

 

[Atas Newton]

I don't do equipment reviews. I know they are just opinions. Not scientific measurements. I just know what a myth, fraud or marketing hype is.. I have a high speed camera. I have the test equipment. I have one of the best hydraulic labs in the US. Hydraulics are good for doing stress vs strain tests. Oil companies use our controller for these tests on rock samples.

Nextlevel, have you ever done a simulation for the department of energy? I did one a few years back for the simulating the fuel injections system for a diesel generator. No one had done one of these before. I am sure the department of energy has PhD that can do this but it would take time, but this is what I do. It took me 15 hours. After my simulation I was able to show the department of energy how to make their system run. It took them a few days. Before that they were unsuccessful for two years. You have no idea of what I have done. I am an expert at control theory and motion control. I have documented proof. I have plenty of magazine articles. In fact a magazine "Power and Motion" just reprinted a couple of articles of mine about 5 years after the original prints.
The whole world can take pot shots at what I have written but no one has. The articles I wrote are gold.
Have you ever gained enough credibility write technical articles for any magazine. Have you ever had your articles translated a reprinted in another language? I bet not. Some of my articles are real applications articles. Others are about theory. I can do both.

Check this out.

National Airport Pavement Test Facility

National Airport Pavement Test Facility

www.airporttech.tc.faa.gov

I have driven that machine, and I developed some of the code for testing runways. The FAA is using our controllers that control position and force to simulate a plane one a runway. There aren't people that specialize in simulations and testing of runways since that is a one-of-a-kind machine. The US government is kind enough to share the runway testing data with all countries.

When it comes to testing and simulation, I get called.

One thing I have a lot of respect for is musicians that are so good they can play just about anything in any key and fit right in with what ever music is being played. Long ago I had a guitar teacher that showed me how to do this but I needed a lot more practice. I could be good but NEVER great. I know what it takes to be a good musician.
Well, I am the equivalent in engineering, simulations and testing. Basically, I just rely on physics, math. and programming.

I am getting tired of yours and Zeio's BS. In a relative sense, you know nothing.
What have either of you done?


So why do these so called "better players" not debunk the myths?

How do you increase dwell time? Tell us. Show us a formula or similar! I have actually tried many times in the past to explain dwell time but there are too many distractions. If I say someone is wrong I get banned for hurting someone's feeling.


What evidence? Dwell time is shorter when impact speeds are high. The pros play at higher speeds so the average contact time is shorter. The "touchy feely time" takes so long to be felt that the ball is long gone by the time one feels anything. Are you going to dispute that?

Why is dwell time so important? Do you know what an impulse is?


No!


Yes.


This is due to experience and skill. Not dwell/contact time.
What does this have to do with dwell/contract time?

you're taking this way too personally, people don't question your qualification, the question is how much value is there in the maths and physics for an actual table tennis player. One might give a definitive proof that human can't possibly feel the dwell time, but it won't make the metaphor go away if it still gives coaches and players some helpful, or at least common vocabulary.

 

[zeio]

Dwell time is shorter when impact speeds are high. The pros play at higher speeds so the average contact time is shorter.
...
This is due to experience and skill. Not dwell/contact time.
What does this have to do with dwell/contract time?

More myth. Dwell time actually increases when the ball buckles after a certain threshold. (see FIG. 6.)

Now you're saying it has to do with experience and skill. Look what you said last week.
https://www.tabletennisdaily.com/fo...ey-tenergy-05-or-dignics-05.32332/post-436320

https://hal.science/hal-03718994v1/

 

[zeio]

you're taking this way too personally, people don't question your qualification, the question is how much value is there in the maths and physics for an actual table tennis player. One might give a definitive proof that human can't possibly feel the dwell time, but it won't make the metaphor go away if it still gives coaches and players some helpful, or at least common vocabulary.

Oh, I do question hardheaded engineer's qualification.

In late 2010, he posted an excel graph of the speeds-after-impact formula from Wikipedia, that consisted of basically 2 straight lines, parallel to each other, going from the bottom left corner to the top right corner, to "dispel the myth" that mass had any effect on velocity.

I don't remember the smaller value he used for mass, but the bigger value he used was 300g. He assumed the COR went from 0 to 1 and so 2 straight lines from corner to corner were plotted.

http://mytabletennis.net/forum/paddle-mass-and-speed_topic37585_page1.html

 

[Atas Newton]

Spherical cows in vacuum are a physicist's best friends

 

[Loop Lob Smash]

Its quite amusing that pretty much the same conversation is being held in another ball sport (snooker). A well known coach's claim that good players can prolong the contact time between cue tip and ball and that that constitutes 'timing' is analyzed (debunked) by the channel author, with the aid of a high-speed camera.

I used to play snooker competitively before table tennis.
Something I know is that if you're an experienced player coming back to the sport after a layoff, one way of finding your form again is to search for the impact feeling that you used to experience. There is some kind of relationship between a feeling you experience and the quality of the shot (e.g. being able to generate spin effortlessly).
I'm not making any claim here about impact duration (what is referred to as 'dwell' in table tennis terms).

I know that whether selecting a snooker cue or a table tennis blade, I am interested in the feeling of using the tool in addition to the performance and characteristics of the tool.

 

[Trevize84]

I read the entire thing... 5 years of comments. Question was about the blade with most dwell time. Then discussion went south. I read a lot of talking about existence of dwell time, quantification of it and whether that helps. My brain stopped at "blade with dwell time". I mean we play with ~2mm of rubber and ~2mm of sponge on top of 6-7mm of wood (sometimes also carbon) and we are worrying about the contribution of the blade to the ball dwelling?! It's like worrying about elasticity of the wall of a building behind the guardrail during the impact of a car. You focus on the rubber+sponge system and on the guardrail to manage the impacts. If you guys can really get some advantage from the dwell time provided by wood flexibility, you're pro or god level, but don't expect to survive high speed impact into a wall.

 

[Loop Lob Smash]

I read the entire thing... 5 years of comments. Question was about the blade with most dwell time. Then discussion went south. I read a lot of talking about existence of dwell time, quantification of it and whether that helps. My brain stopped at "blade with dwell time". I mean we play with ~2mm of rubber and ~2mm of sponge on top of 6-7mm of wood (sometimes also carbon) and we are worrying about the contribution of the blade to the ball dwelling?! It's like worrying about elasticity of the wall of a building behind the guardrail during the impact of a car. You focus on the rubber+sponge system and on the guardrail to manage the impacts. If you guys can really get some advantage from the dwell time provided by wood flexibility, you're pro or god level, but don't expect to survive high speed impact into a wall.

That lovely feeling of dwell just as you hit the wall though

 

[Trevize84]

That lovely feeling of dwell just as you hit the wall though

I cannot find that wall on the ITTF LARC...

 

[Tony's Table Tennis]

I'm still waiting for a scientist to turn pro and get into the world top 100.
Then we can see some proper theory and practicality put together.

 

[JustANoob]

I'm still waiting for a scientist to turn pro and get into the world top 100.
Then we can see some proper theory and practicality put together.

Idk, I heard being a moderator of the north korean oompa loompa forum automatically makes you correct for all things physics, so no need for top 100.

 

[latej]

Well, the guy is on my ignore list, and I have accidentally seen this last post. And it is clear, the same BS on and on and on... What can I say, ignore-list is a good feature

 

[Atas Newton]

All of this started over a decade ago when people were talking about dwell time and I called BS.

So after all of this, can we agree that dwell/contact time is short. It is normally around 1 millisecond. Sometimes much less.
Can we agree the time we feel the impact that the ball is long gone?

However, for years I have asked a few questions for those that like to talk like they know about dwell time.
1. What is the necessary condition to maintain contact between the ball and the rubber?
2. What is the peak force? This will differ depending on the speed of impact and how hard the rubber is.

tell you what, after watching the snooker video someone posted today I'm really skeptical of the 1ms figure. A cue tip and a snooker ball are both a lot harded than a table tennis racket and ball, and the dwell time is still above 1ms.

 

[Lazer]

tell you what, after watching the snooker video someone posted today I'm really skeptical of the 1ms figure. A cue tip and a snooker ball are both a lot harded than a table tennis racket and ball, and the dwell time is still above 1ms.

But… it takes a lot more momentum to get the cue ball moving….

Cheers
L-zr

 

[zeio]

An engineer or physicist would wonder what is absorbing all the energy.
In zeio's example the ball absorbs almost all of it or all of it since the glass does not deform.
With excessive force the ball will dimple or deform but is I really doubt that would happen at the speeds shown in Zeio's document. Zeio's document does not show how much force is applied to the ball.

So how do you know the ball buckles when it hits rubber? The rubber will absob much of the energy. This research is flawed. Zeio posts a lot of bogus research. I can believe a celluloid ball can buckle but not a plastic ball. Squeeze a plastic ball and then a celluloid ball. Celluloid balls will buckle or dimple with less force than the new plastic balls.

How much force is required to dimple a ball? A few months back I posted a video of stress vs strain for a TT ball. No one cared I squeezed a TT ball about 2mm.

No, a reasonable engineer or physicist would wonder if the same thing happens with other types of hollow balls. Rod Cross did just that with table tennis and squash balls on a force plate (there you have it, knock yourself out) and he found out the latter did not exhibit that behavior. No one cares about how much force is on the ball or even the velocity at which the ball buckles. The key here is when a table tennis ball buckles and then pops back (referred to as buckle and snap-through), it makes a high-frequency sound. That is the natural frequency of the ball (9-11kHz), which doesn't change whether the impact is against a piece of glass, wood, or rubber. Therefore, as long as the ball buckles and snaps through, dwell time increases.

Did you even read the whole study or did you stop when it didn't measure force? The study above uses the Cornilleau P-Ball ABS 3-star. It doesn't matter whether the ball is made of celluloid or ABS, they buckle at relatively low velocities against a piece of glass, above ~5-6m/s for 40mm celluloid and above ~5.5m/s for 40+ ABS. Also, the ball does buckle against rubber above similar velocities.

https://www.researchgate.net/publication/262990295_Impact_behavior_of_hollow_balls

It was concluded that the oscillations in the impact force on a ping pong ball arise from
vibrations of the cap when it buckles. The vibration amplitude is too small to be observed
with a high speed video camera, but it has a large effect on the impact force since the
vibration frequency is relatively high, typically about 10 kHz. A secondary effect is that
the impact duration increased with ball speed. For most ball types, the impact duration
decreases as the ball speed increases.

https://www.researchgate.net/public..._rackets_and_balls_The_acoustics_of_ping_pong

3.5. Ball buckling and snap through

An interesting feature of the frequency spectrum in Figure 8(a) is
the group of broad peaks between 8.5 kHz and 11 kHz, well above
the theoretical upper frequency limit for ball modes. When Cross
(2014) dropped a ball-bearing onto a ping-pong ball, he was trying
to identify the source of a 10 kHz oscillation which he had
observed when a 40 mm ball impacted a rigid surface at speeds
in excess of 7.0 m/s. Zhang et al. (2014) reported a similar oscilla-
tion at 10.5 kHz in their contact force versus time measurements
for impact speeds of 25 m/s. Cross surmised that this high fre-
quency oscillation might be due to the buckling of the ball as it
rebounds from a rigid surface at speeds in excess of 5–7 m/s.
Hubbard and Stronge (2001) and Zhang et al. (2014) used high
speed video to confirm that a ping-pong ball undergoes signifi-
cant buckling upon impact with a rigid surface at speeds of 20 m/s.
This buckling also exists when the ball impacts the rubber surface
of a racket (Rinaldi et al., 2016). Karagiozova Zhang and Yu (2012)
have explored the theory behind the snap-through that occurs in
a thin-walled elastic spherical shell after buckling upon impact
with a rigid surface, as the deformation pops back out and the
sphere returns to its original shape.

...

4. Conclusions and implications for the table tennis
player

...

While the vibrational modes of the ball
do not contribute significantly to the impact sound of the ball
hitting the racket, it might be interesting to investigate how the
high frequency content (due to buckling and snap-through pop)
might change for different types of strokes by the player.

https://youtu.be/ekalYkdrwI8?t=3244

https://www.researchgate.net/public...pact_in_Table_Tennis_Experiments_and_Modeling

 

[Trevize84]

All of this started over a decade ago when people were talking about dwell time and I called BS.

So after all of this, can we agree that dwell/contact time is short. It is normally around 1 millisecond. Sometimes much less.
Can we agree the time we feel the impact that the ball is long gone?

However, for years I have asked a few questions for those that like to talk like they know about dwell time.
1. What is the necessary condition to maintain contact between the ball and the rubber?
2. What is the peak force? This will differ depending on the speed of impact and how hard the rubber is.

I'm not super-good in mechanics but I want to make a guess here and see if after 15 years I still remember something.

1. I would name D the thickness of the rubber (at rest) and assume for the sake of simplicity the rubber surface doesn't overextend after impact (max elongation is exactly D), also blade and ball are inflexible (their shape don't change under a force). So as long as distance between ball and blade surface is less than D, contact is maintained.

2. Assuming one-dimension, assuming no momentums, assuming constant accelerations during impact, assuming friction is negligible, assuming no deformation, etc. Only forces acting here are player+bat mass*acceleration and elastic force that is max when compression is max. We can find max elastic force from energy conservation assuming rubber is an ideal spring following Hooke's law. Basically the compression "x" is function of bat speed, ball speed and elastic coefficient "k" of the rubber.

From this analysis dwell time seems irrelevant, in the sense that outgoing speed/acceleration of the ball don't depend on dwell time but rather rubber characteristics (elasticity and friction) and bat/ball relative speed and acceleration. Dwell time is function of same variables, but it's more like an output. This conclusion perhaps might prove my analysis wrong. However calculation of dwell time is easy applying cinematics (assuming spring is linear).

In conclusion speed isn't impacted by dwell time. It can be 1ms or 1hour, I believe this is true regardless my assumptions about friction. However if we introduce momentum, I expect spin will be impacted by dwell time however modelling should take into account geometry. Another variable that I expect is impacted by dwell time is outgoing direction of the ball, this is also though to model because takes into account geometry.

 

[dingyibvs]

All of this started over a decade ago when people were talking about dwell time and I called BS.

So after all of this, can we agree that dwell/contact time is short. It is normally around 1 millisecond. Sometimes much less.
Can we agree the time we feel the impact that the ball is long gone?

However, for years I have asked a few questions for those that like to talk like they know about dwell time.
1. What is the necessary condition to maintain contact between the ball and the rubber?
2. What is the peak force? This will differ depending on the speed of impact and how hard the rubber is.

I think that's reasonable, whether it's 1ms or 10ms or even 100ms, it's gone before we can feel it.

Your video has some major flaws though. People usually refer to dwell time with loops, which is a brushing stroke producing a fairly fast ball, and emphasis is placed on accelerating through contact. Your video shows a straight on impact producing a very slow ball, and relative deceleration through contact. Just doing some very rough math, a FH drive can be well over 70 kph, or about 2cm/ms. Assuming your video is shot at 2000 fps (0.5ms/frame) and played back at 30 fps (15 ms/s), the ball should travel 30cm/s in your video if it's going at 70 kph, and it's obviously nowhere near that.

The thing holding the racket should also contribute to dwell time, and a rigid vise would probably decrease it compared to a less rigid hand/wrist.

These changes won't change what you said in your post, but it may offer us better understanding of dwell time.

 

[igorponger]

Some fresh meat for discussions.

Ball-to-rubber interactions were recorded in a Japan engineering lab by means of a photometric apparatus. Butterfly Sriver 1.9 mm taken as a testing sample.

Impact time at speed 5 m/s is 1.6 ms
------------------ at speed 25 m/s is 1.4 ms

It is to say that you cant tell any difference in duration of the impacts. Dwell time is a delusion of human biological sensors.

 

[dingyibvs]

Some fresh meat for discussions.

Ball-to-rubber interactions were recorded in a Japan engineering lab by means of a photometric apparatus. Butterfly Sriver 1.9 mm taken as a testing sample.

Impact time at speed 5 m/s is 1.6 ms
------------------ at speed 25 m/s is 1.4 ms

It is to say that you cant tell any difference in duration of the impacts. Dwell time is a delusion of human biological sensors.

While true, it may also be the wrong way to look at it. I think someone mentioned before that it may be the blade's vibration that we feel as dwell time, rather than the actual dwell time. Or perhaps it's a visual sensation, perhaps the ball follows the trajectory of our follow through more for a high "dwell" set up, giving the illusion of higher "dwell".
I don't really know the answer to this, but I do know that it affects our play.

It's like how we know now that our perception of the sun rising and setting is due to the earth's rotation rather than a heavenly body rotating around us, but it doesn't change the fact that its perceived motion affects our every day life. The right way to look at it IMO is to use science to explain why we feel dwell, what exactly is dwell, and how and why exactly does it affect play.