dwelltime

JacekGM wrote: @ mercuur hey bud, you should apologize- I did not steal anything. A brief posting mistake was corrected within 5 minutes or so. Don't go too far in your public opinions, I just don't appreciate your remark.

I was just mocking from general irony.  
Publishing on a forum or a book or speaking contradicts to ownership of words. ideas and even more questions.
And considering the quoted question as - also - a question for you would only please me.

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?

Interesting question. Though I think it reasonable to expect that the Dwelltime would vary, wouldnt be more scientific not to assume that, or so the pedantic side of my mind prefers. "Identical ball, identical stroke, different setups - what changes?" Deserves a separate thread IMO




Edited by pingpongpaddy - 09/14/2013 at 6:37pm

Can you hazard a guess as to the answer?

@mercuur

http://www.hk-phy.org/contextual/mechanics/mom/impul03_e.html





Finding the instant Force to press on the table tennis ball

Dima Ovcharov's racket hand is travelling at speed of 122 km/h (35 m/s)

m=0.0027 kg
v =35 m/s
U=35 m/s
T= 0.001 s

F= 190 N

Sure, this impact force is no way sufficient to crack the ball.. celluloid ball. And how about the plastic???

tt4me wrote: mercuur wrote: Your response is highly inconsistent with the idea that dwelltime is more or less constant.. No,  look at the videos.  You can see the dwell time for the Toxic 5 is one frame but the dwell time for the Firewall Plus with T25 is perhaps 2 or 3 frames.   Some at the office say 2 and some say 3.   However, these are extreme examples.  I just wanted to make it clear that the dwell time is short but could still vary 100s of microseconds either way depending on the conditions.

Ok

tt4me wrote: Yes, ball deformation would be the same if the same force is applied

My mistake, I should have asked for strain instead of force.

The kinetic forces for the video of the toxic are even out of sight for viewers. Must be a robot or invivible player who accellerates the ball to the bat.
From start of contact to end the ball only decellerates. It has pressure then and new momentum distribution from a sum of momentums (in this case only the ball contributes)  but not an accellerating force. Regarding the pressure as causing an instant force instead of the pressure caused by an earlier force or even adding the two up for double force would highly confuse things.

So wrong question, excuse for that.

It was mentioned somewhere (would have to search for it)  for tennisrackets and - ball that dwellperiod wasn,t influenced much from more or less snaretension.
There it was explained  from increased framedeformation with increased snaretension and decreased deformation for the snarebed that comes with it. This puts more strain on the racket and it deforms with a larger amplitude. 
When deformation contributes to longer dwell period also, the snares would contribute less but frame more all from the tenser snares. All in all time of contact doesn,t have to change much then because all these effects compensate for each other more or less.

It seems logic  tough that the ball also deforms more then from a higher snaretension (for same reason as the frame, higher snaretension).
So when more deformation of the ball also means more strain to the ball this can be with more or less same contactsurface and dwellperiod (or accelleration).



Edited by mercuur - 09/15/2013 at 1:48pm

wturber wrote: mmerkel wrote: I have experienced similar lightning speed reactions from myself when I held my beer bottle a little too loosely and it slipped from my hand. Without even realizing my hand shot downwards to catch the falling bottle. Cheers!! Well, the reality is that our reactions times are on the order of 200ms at best.  I do have video of me returning smashes in time intervals slightly shorter, but the reality is that these successes were a combination of good reaction, good anticipation, and good luck - though at the time it felt like I was Neo in The Matrix stopping bullets.  Our minds "lie" to us all the time.  They fill in missing information (or gloss over it so we don't notice) and they back figure events so that the events will seem sensible to us. There's a show called Brain Games from National Geographic that does a pretty good job of showing some of the "shortcomings" of how our brains work.  I say "shortcomings" in quotes, because without some of these tricks, we'd become overwhelmed with information and would have trouble making decisions.  So these "shortcomings" are actually beneficial characteristics overall. The problem is that when we aren't aware of them, it becomes easy to sometimes draw the wrong conclusion about what we've just experienced or remembered. Considering that a block (or a beer bottle catch) is actually a fairly complex motion, I wouldn't be surprised if good scientific investigation might show that we can make small adjustments to our actions in intervals perhaps as short as 100ms.  But that's a guess at best and probably a bit optimistic.  The idea that we can react to things as fast as even 5ms seems to defy pretty much everything we've learned so far about human reaction times.

It takes about 0.2 seconds for the bottle to drop 8 inches. So nothing 'superhuman' to react to that. It just appears a lot faster when you surprise yourself while you are talking to someone else at the time.

http://books.google.nl/books?id=8a5yFzTjZqIC&pg=PA11&lpg=PA11&dq=tennisball+dwelltime&source=bl&ots=roOq2i9SeR&sig=6Sxjg0GqSpY9KqEeN7s24jzh_Cc&hl=nl&sa=X&ei=5L02Us2eB8bd7Qauu4CAAw&ved=0CDMQ6AEwAA#v=onepage&q=tennisball%20dwelltime&f=false

Thinking of it,, dwelltime is the real faktor for forcedistribution over the line of motion when the energy was all applied to bat and ball before the contact.
It can be seen from unpacking momentums P (newton second) with P/T1 during a collision into Newtons and time as if both are dimensions.
T1 as the first part of dwell until where the relative speed becomes zero for a moment.
Delta P =  Ma * T1 or delta V = a .T1. This is from start of contact to where P is zero with highest pressure and  deformation. Degree of deformation is not a rate or degree for deforming.
Force is for the period T1. Other shorter periods of T1 will have less pressure and force unpacked with also more unpacked momentum. It takes the whole period to unpack all momentum to a certain maximum force.

Unpacking completely more gradually during longer T1 or suddenly, also completely with a shorter T1 leads to a force difference in magnitude then while the momentumchange stays indifferent.
This change is also the total momentum involved for the collision.
 
Neglecting this dependancy of force to dwelltime would lead to a different momentum change to time and then change the momentum going into the collision with variable dwell from different equipment.. It would be kinda weird when a thicker sponge could affect the momentum of a ball before the contact with a ball.

The next part of the collision sorta recreates the momentum as F.T2 or FT2 = Ma * T2 ...
It,s a bit more complex because F is not constant through all this offcourse but this is the basic idea (as I see it).

It can be interesting to imagine a planewindow between the racquet and ball in a way that distance * mass for bat and ball to this plane is always equally balanced. It,s the balanceplane then for the system  (so not really imagination, more an abstract plane that needs some mental imagination)
It,s interesting because I think players really use such an imaginary plane as sort of a window for focus and timing of strokes.

Bat and ball will meet somewhere in the near future with this plane still somewhere in between then and both bat and ball at a small distance to it.
The distances never becoms zero but decreases further during the compressionstage.

A racquet/blade is much heavier then a ball and the balance plane will move faster to the ball with more batspeed and with same batspeed faster with more batmass relative to the ballmass (?).
The ball also moves faster to this plane then (relative).
This can increases future pressure for a stroke before the contact.
But it does this from the combination of mass and speed ie momentum not just speed or force from speed.



Edited by mercuur - 09/16/2013 at 10:42am

The book that mercuur posted a link to is a good book.  I have read much of this before and much directly applies to TT too.

Edited by tt4me - 09/16/2013 at 12:50pm

tt4me wrote: Baal, there is translational and rotational momentum.   Obviously the dwell time is longer if you brush the ball since the impact speed normal to the paddle starts going down.

I'm not sure why you continue to propagate this when it's hilariously wrong to assume same deformation for all shots as pointed out at least twice. A v=>0 impact is not going to create the same 2mm dent nor therefore near infinite dwell.

Regardless, you actually answered correctly accidentally that dwell is longer but for the wrong reason: given reasonably similar elasticity both normal and tangential to rubber surface, a "brush" is no different in its ability to impart deformation and thus dwell. The dwell is not longer because it's a loop per se, but because you impact the ball harder (regardless of angle).

I am sure most of us can brush loop and know that ball will seem to "drag on the rubber" as one of my practice partners puts it. At this point it is best to break down the problem into a normal component and a tangential component to the impact.  It is still best to think of the ball moving relative to the paddle and paddle is fixed just to keep things simple.   Physics people would say the paddle is the reference frame. Now consider a simpler example of your counter loop.  Serving.   One must throw the ball straight up so it comes straight down.   Assume one is making a back spin serve and the paddle is horizontal to the floor and moving horizontally.  The normal component of the impact speed is now quite low relative to the tangential impact speed.  Now Baal's question is how does dwell time affect the back spin of the ball and the speed at which the ball goes forward.

It doesn't. Dwell is mostly incidental to desirable behavior (ie elasticity).

tt4me wrote: @tassie52. Instead of making fun of mercuur did you look at the link he posted? The book that mercuur posted a link to is a good book.  I have read much of this before and much directly applies to TT too.

The reasoning there is also wrong. For example: "since the strings get stiffer non-linearly the more they deform; this leads to lower dwell times for harder hits".

For real reason consult this: http://www.physics.umd.edu/lecdem/services/refs_scanned_WIP/3%20-%20Vinit%27s%20LECDEM/D221/8/AJP000482.pdf

"The most surprising thing is that it appears that stringing the racket looser rather than tighter will actually lead to slightly higher ball rebound velocities (more “power”). This is due to the fact that tennis balls have a rather low coefficient of restitution and a dwell time on the strings which is short when compared to half of the natural period of vibration of many tennis rackets."

AgentHEX wrote: tt4me wrote: Baal, there is translational and rotational momentum.   Obviously the dwell time is longer if you brush the ball since the impact speed normal to the paddle starts going down.   I'm not sure why you continue to propagate this when it's hilariously wrong to assume same deformation for all shots as pointed out at least twice. A v=>0 impact is not going to create the same 2mm dent nor therefore near infinite dwell.

You haven't been paying attention to the videos.  The Toxic 5 deflects a lot but the Firewall Plus doesn't appear to deflect at all.   Obviously the deflection of each blade will be differ.
I never said the deformation is the same for all shots.  That is an assumption you have made.  I generated a symbolic formula.  You can assume what ever you want.  I have videos that show two extremes.

Baal wrote: Can you hazard a guess as to the answer?

The answer is that the longer you can apply a tangential force ( torque ) the more the ball will spin.  Duh.  Obviously the ball must be in contact with the rubber to apply a tangential force. Actually I could have provided this answer immediately but there are two variables.   The part I am thinking about is that one can apply a lot of torque over a short time or a lesser amount for a longer time and still achieve the same spin.

I don't think there is a better answer unless one does some modeling like in the Andro video a month or two back.

Still thinking.


 

tt4me wrote: AgentHEX wrote: tt4me wrote: Baal, there is translational and rotational momentum.   Obviously the dwell time is longer if you brush the ball since the impact speed normal to the paddle starts going down.   I'm not sure why you continue to propagate this when it's hilariously wrong to assume same deformation for all shots as pointed out at least twice. A v=>0 impact is not going to create the same 2mm dent nor therefore near infinite dwell. You haven't been paying attention to the videos.  The Toxic 5 deflects a lot but the Firewall Plus doesn't appear to deflect at all.   Obviously the deflection of each blade will be differ.

Relative to the sponge on the blade? If you put pressure on the bat can you actually measure ratio? At least any of the blades I have seem quite stiff compared to sponge.

I never said the deformation is the same for all shots.  That is an assumption you have made.  I generated a symbolic formula.  You can assume what ever you want.  I have videos that show two extremes.

Your symbolic equation necessarily assumes both a given speed and deflection. There's a definitive relationship between the two so you cannot only assume one and claim to know the relationship of the other to the solution (ie dwell). That second unknown has a definite value given the former and isn't a free var.

I suppose it's always empirically possible that an odd inverted relationship exists (not 1/x but no quite flat), but not from what you were claiming about speed in the denominator. Any such claim would be at best accidentally correct.

tt4me wrote: I don't know what you think is wrong.  The book pointed to by mercuur's link says the same thing. Your link and mercuur's link are written by the same person.  They are the same article in fact.

That's odd. They're not same article but written by same person 10 years apart. Those lines are way too clean to be actual data and he shows no work in the book.

tt4me wrote: ??? what angle? See this article. http://www.ittf.com/ittf_science/SSCenter/docs/199408014%20-%20%20Tiefenbacher%20-%20Impact.pdf There is a normal and tangential coefficient of restitution.

That's what I said? The angle determines weighing of each component, but more importantly the amount of spin vs. speed.

BTW, re: "know a lot of physics", it's not an exaggeration that this is literally the simplest possible physics (ie. first few classes on mechanics which is generally the first thing taught in physics due to its simplicity). To be clear, this isn't intended to make anyone feel bad unless it's their own prerogative; it simply highlights the material is as basic as it can get.

Actually, these topics aren't that basic.  Sure the 'napkin' dwell time estimation is basic.  So is the impluse, force, mass, dwell time and momentum relationships.

However Baal's question is not simple.  I actually went to a physics forum and asked about the serving scenario and no one answered.  I did finally find a good article by a PhD in Australia and it looks good.  It doesn't answers Baal's dwell time when hitting at oblique angles though but it provides a good start.
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You can similarly guesstimate in the tangential direction so I'm not sure what the novel difficulty is. 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 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?