Discussion in 'Strings' started by mikeler, May 9, 2013.
Edit: they said it better.
The answer to that question is in several papers by several teams of researchers, as I've pointed out previously, with links to some of those papers. For your convenience, this is from "Effect of inter-string friction on tennis ball rebound", Sports Engineering, May 2012:
At low inter-string friction, the mechanics of the impact are the most complex as the strings tend to displace more. At the lowest [inter-string friction], the longitudinal [main] strings are likely to displace a large amount earlier during impact and return while the ball is still in contact with the stringbed. At 40 [degrees] this causes [horizontal impulse/tangential impulse] to increase compared to medium [inter-string friction]. At the start of the impact the movement of the strings reduces the relative velocity between the ball and string bed. Towards the end of the impact the strings move back towards their original positions since there is in increase in string tension (due to their lateral deformation) and the inter-string friction is low. The strings apply a [torque] to the contact zone of the ball causing it to overspin (SR>1, Table 3). The effect is more pro- nounced when the inbound ball has backspin [topspin from the court frame of reference] due to the greater lateral deformation of the strings.
FWIW you said it pretty well too.
The real answer is: Yes, snapback does actually push the ball in a significant extent.
Perhaps you missed this one:
Another interesting point seen here, as well in the various TWU articles, is that Dwell Time Does Increase with more lubricious string-string setups. It follows quite logically that if dwell time is increased, this gives the snapback that extra ms it needs to help impart spin. Meaning to say, snapback is one of many important and real factors that aid spin production, in addition to technique, racquet head speed, string pattern, tension and material, etc etc.
Case closed. Snapback is real.
Thanks for the high five!
But in all honesty I'm convinced that it's a lost cause with NoPoint. His SOP is to use false analogies, anecdotal evidence, false premises, and straw men. The rate at which he uses them (prodigiously) and the frequency with which he posts (obsessively) actually doesn't suggest someone who's terribly rational or open-minded.
Once upon a time he actually tried arguing that Newtonian physics were irrelevant in a racket string to ball collision. As if in evidence, at one point he even tried claiming that because a tennis ball is decelerating before contact, it has zero acceleration and therefore zero force!
At first I thought he was trolling, except that trolls won't allow themselves to look that moronic.
I'm not sure if he even took high school physics or just failed it, but I'm of the clear view that he is singularly incapable of openly accepting contrary evidence. It's far too important to him to try to appear superior to others at all times, and someone that insecure with those kinds of social skills and cognitive problems likely has far bigger issues in their life than logical fallacies.
But in any event, I'm sure we'll hear from him shortly.
Well said Jimbo. You are a gentleman and a scholar.
I thought this was a tennis forum not a nerd forum???
There would be no advances in the sport if the technology didn't advance. It makes perfect sense to wonder how those advances work and why. People are not limited by their knowledge or wisdom, but rather by if they refuse to learn. Very few people here will refuse to learn something if it helps understand how something interesting to them works.
You thought wrong...
LOL, great post, I wonder if BP would ever read
Wow, someone here sounds REALLY desperate!
A force "produced" by the wall should be able to compress the ball regardless of the amount of momentum on the ball, correct?
So does your normal force compress the ball when the momentum of the ball is equal to zero?
No? Then that fails the test of a force that is "produced" (i.e., generated) by the wall. A force that is "produced" by the wall will act upon a body regardless of the momentum of said body.
The compression of the ball is dependent upon and proportional to the magnitude and direction of the momentum of the ball.
That's it, I'm done with this stupid back and forth with you.
Were you able to see the snap back with your naked eye in real time? If so, it's hard to imagine that it happened as fast as 1/1000th of a second. At that high rate of speed, I would assume that all you'd see is the string back in it's original vertical position without ever seeing the actually snap back process.
But the original question was, shouldn't the snap back be significantly slower when there is the force of the ball on the strings pushing the strings in the opposite direction to the snap back than when there is no force from the ball at all (as when you let go freely with your fingers)? If the rate of snap back is the same in both scenarios, it should be safe to conclude that the ball is actually no longer exerting a force on the strings (no longer in contact with the strings) when it snaps back, just like when you let go freely with your fingers. In fact, in many of the slow-motion videos of strings snapping back while there is a ball on the strings, it always looks to me like as the ball rolls across the stringbed, each string snaps back independently of the others as the ball leaves contact with each particular string. This is why the non-believers believe the snap back of each particular string occurs too late to affect the ball since the ball has to leave contact with that string first before it is able to snap back.
You were off by two orders of magnitude in how fast you thought the strings snapped back.
I've shown that unimpeded snapback is less than .001s, well shorter than dwell time.
If dwell time is greater than .002s (commonly accepted to be .003-.005) then the string exerts a force upon the ball from the moment of contact, until the ball leaves the bed and snapback which begins at the time of maximum displacement must produce a torque upon the ball.
Einstein didn't have an opinion on this.
I looked it up.
Um...I'm pretty sure I never said that because a decelerating body has a force in the opposite direction, not zero.
What I probably said is that in a ball-racquet collision, what matters is the conservation of momentum because a decelerating ball still has momentum, which is true. Oh, and conservation of momentum IS Newtonian physics!
Did you actually see the snap back in real time with your naked eyes? I'm curious as to if the human eye can see something that only moves for 1/1000th of a second.
And how about the force the ball exerts on the strings, thereby impeding its snap back? As in all the videos that I've seen, TMU's measurement of 1 ms for the snap back is probably for when the ball rolls across the stringbed and the time it took for a string to snap back once the ball is no longer exerting a force upon it in the opposite direction (no longer in contact with that particular string). This is probably why it closely matches your experiment of an uninhibited string snapping back. This is also why I found it hard to believe that a string could snap back that quickly while the force of the ball is pushing it in the opposite direction to the snap back so that string has to first overcome that force and then produce enough force to reverse the momentum of a heavy ball and doing all of this as quickly (or quicker) than when there is no force of the ball present at all.
If the ball is on the string, impeding its snapback, then the string is exerting a force on the ball, ergo, snapback adds to ball rotation.
I don't understand your question, and likely you don't either. You will find several places in which I have mentioned that TWU prof has already determined that the snap back time can be less that 1 ms. Did you read the posts?
Most likely you think that I "support" BP. I don't, nor do I support any one else. I just state it as it is, or as stated by well-respected people. I am not interested in joining one bandwagon or another, nor do I think everyone is entirely correct or wrong.
Yes it does need some maturity to see multiple viewpoints, and you should try to achieve it.
Of course it does. It is just common sense. It is just the stored potential energy getting released. The question is how important is it compared to RHS, racket angle, tension, and string-to-ball friction. It could be massive like a spring loaded mechanism which fires off an object, or it could be like me compressing a sponge, which isn't going to launch me anywhere.
Did you think that anyone suggested that the snap back exerted no force?
Cheers, since I realised BP is impossible to debate with due to the fact he never admits he is wrong, and just tries to change the criteria of what is being discussed; I must admit I do enjoy seeing him go at it with others, it really can be very entertaining!
True, but the ball is also exerting a force on the string thereby impeding and slowing down its snap back. The fact that it snaps back just as quickly as when no ball is exerting a force on it and impeding its snap back, tells me that the ball is no longer exerting a force on the string and impeding its snap back, i.e., the ball is no longer in contact with the string.
Yes, Breakpoint has been arguing that for years.
Since I first began to talk about snapback, well before TWU was in existence.
Again, everyone seems to be yelling that one piece of the puzzle is the entire thing, instead of just agreeing that stiffness + snapback + technique + etc etc all matter. Sure, there are some setups with no snapback that still produce spin, because that player's technique is superior enough to do so. There are also setups that rely heavily on snapback to add spin, and those also work.
I think everyone is now just arguing to get the last word, which is not going to happen.
Anyway, thanks Mikeler for the thread. It's amazing what one picture and sentence can do to the tt tw forums. LOL
False premise, argument from ignorance, and non sequitur.
Long before that, string companies knew everything about it. What is your point?
I did not say TWU prof mentioned snap back first. You should try to read the posts before responding. I actually said he wrote the article after the 99S came out and someone asked him to comment on snap back. From that how did you infer that I said something about when you began to talk about snap back? It has nothing to do with you. I bet not one guy in the industry will mention your name in a snap back discussion. There are already many experts in the field and money is riding on it.
Let me see. According to your logic,
Minimal (what BP said) = none (what you think he said)
Does it sound right?
Oh, the strings exert a force alright. Just not much on the ball since the ball has to leave each particular string first before each string is able to snap back freely. I just don't believe the very weak snap back force is enough to overcome the tremendous force the ball exerts on the strings pushing them in the opposite direction to the snap back and then snap back just as quickly (or quicker) as when there is no force from a ball at all.
Care to elaborate? How can a string snap back faster when there is a tremendous force acting against it impeding its snap back than when there is none?
Oh, I see. You have no answer. Got it!
I was explaining how long BP had been arguing that snapback had no effect, and that was since I first mentioned it, long before TWU, or before it was a popular topic on TW.
You asked me if I thought anyone actually believed that snapback had no effect. I answered your question.
And I think I already gave the answer. BP said it was minimal, not no effect (at least according to the previous quote). That does not mean I also think it is minimal, just to clarify before someone makes one more tiring post claiming I said so.
Nope. He has repeated constantly the notion that he thinks the strings are snapping back after the ball has left the strings.
That would be "none".
That's ok, you don't have to believe it; the cool thing about science is that is is true regardless of if you believe it.
Then congratulations! You are the first person in history to defy the laws of physics!
I'll gladly elaborate.
You have written a false premise, and then asked me to explain it.
The string doesn't snap back faster when in contact with the ball than when it is unimpeded.
Correct, that is how it works, similar to a bow, catapult, or slingshot.
As stated above, your premise is false, therefor your argument is invalid.
You think you gave the answer?
Let's be sure.
Hey, BP, do you agree that strings snapping back impart a torque on the ball adding to its spin?
What? Does any of what you said has anything to do with the fact that the force that bounce the ball back is applied by the wall? Also the fact the ball cannot bounce itself back?
I like your answer but the arguing is more fun, sorry.
Well played Mikeler, well played.
Whatever. I don't think minimal is none, but whatever.
BTW, Jolly's video shows that a displaced string can snap back fast, in a ms (which was already known and documented). It doesn't show anything else, I hope you realize that. The string snapped back once the finger was removed. BP thinks that this is what is happening with the ball too. Jolly's video does not imply that the strings spun his fingers around. So, there is no new information in his video which sheds light on this issue.
If we could make sureshs and BP fight, it might be the beginnings of a perpetual motion machine.
Unrelatedly, my phone autocorrects sureshs to airways.
The string was "trying" to snap back when the finger was still there, and is constantly applying a force to the finger, the same force when acted upon a ball to produce the spin. The ball isn't attached to a hand to hold it in place like a finger, so the string would push it to make it move and spin.
That's some 1337 auto correct.
You have been here long enough, you should know about the PEDMAS thread about arithmetic expressions. BP and I went at each other for weeks
It was all about what is the value of 6/2*3 (or something similar).
I once went to text my buddy "It's on!" and my phone autocorrected it to "It's PMS!"
Which was awkward.
6/2*3 =9!!! :twisted: BTW was that 2 or 288 thing? I think in the end, TT had more correct vote by % than physics forum...
Wrong about the first assertion, and wrong about the second. :lol:
LOL at the weak attempt at revisionism!! :lol:
Quoted for supreme irony! :lol: :lol: :lol:
Are you sure it is not 1
Yes that was the famous 288 thread.
But do you remember what happened? BP produced a calculator which supported his answer! Then we were left arguing about the calculator.
No one wins against BP, no one. Many have tried and been reduced to TT dust.
Separate names with a comma.