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Concave vs. Convex Loop (Which is which/better?) |
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V-Griper
Silver Member Joined: 09/19/2011 Location: United States Status: Offline Points: 879 |
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Here is WLQ counter hitting. He is clearly guiding his paddle in a modified circular path. I say modified because the take back is like a semi circle but the forward part is flattened out.
Again. This time the well referenced ML slow mo FH. Similar motion. Starting from the ready position. Paddle goes down, then back beside the right knee. Then slightly up and then flattens out as it moves forward into the ball. So the question is why do a circular motion at all? Why not do more of a straight back and strait forward motion? Like this. Or this. Iwill grant you that Timo and BP are still making a slightly squashed oval, albeit slight, but it looks nothing like what WLQ and ML are doing. Some related questions- Why is ML's elbow so close to the body on the take back? why does the paddle usually stop beside the knee? Why does ML's knee seem to collapse inward slightly?(So does RSM's btw https://www.youtube.com/watch?v=RMVzq-OnBcM&feature=player_embedded)
I am interested in peoples opinions. P.S. Please refrain from playing the "Chinese vs Euro" card. It gets old. I want you guys to think not recycle arguments. Edited by V-Griper - 06/24/2012 at 1:39am |
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power7
Silver Member Joined: 01/25/2012 Location: USA Status: Offline Points: 745 |
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Because ML elbows are at a fixed angle when he is loop driving. So the forward rotation is done with the shoulders. They are taught to use the shoulder as major pivot point of their swing, not the elbow.
TB flexes his elbow more during the swing for forward motion. That's why his stroke seems more linear in the video following the same path hitting and retracting. |
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V-Griper
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So your answer to my question is, that is what they were taught. Maybe I need to clarify my question. I understand how they got their stroke mechanics. What I want to know is why. Why use the shoulder as a pivot point instead of the elbow? If you are a coach, and you are teaching young kids how to hit the ball, why would yoou have them do it one way over another? Are they both equally valid ways of hitting the ball? What set of advantages/disadvantages are there? Edited by V-Griper - 06/24/2012 at 2:25am |
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power7
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I've had a few different coaches that span many different points in my life. PRC coaches teaches the basic FH that way. Mostly, shoulder movement. They don't say lock the elbow, but it just happens, to give more control over the ball, if you pivot with mostly shoulder movement.
Other coaches I've had never really emphasize the stroke Mechanic like that. Mostly emphasize stiff wrist for control. As for why? More powerful muscle group. Less chance of crossing body with that swing. Maybe carried over from taichi principles (guess?). But that's what PRC system teaches. Or maybe kids start so young there they need to use more shoulder to get blade over the table? Edited by power7 - 06/24/2012 at 2:59am |
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popperlocker
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If you've played with both a tacky chinese rubber and Tenergy 05, you will understand.
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hobbes203
Member Joined: 04/17/2012 Location: United States Status: Offline Points: 17 |
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@Zeio
Couldn't resist answering that question, both objects are of the same mass and has the same qualities (same kinetic and static friction, etc), and the ramps are also equivalent, then both would hit the bottom at the same time whether they slide or roll. Easiest way to do this is energy, but I'll leave it at that. The important question is what are the accelerations of the objects at the bottom? (Answer should be a vector quantity, magnitude and direction). Given a very, very general scenario, the comparison of the balls at the bottom of the ramp can be applied to the moment a table tennis ball at a fixed point is contacted by the concave and convex strokes. At this point, I won't try to get into springs, torque, friction, and the magnus effect. Generally, the ballistics of the ball is determined by the magnitude and acceleration at different contacts of a controlled ball. I'm no expert but really just try the strokes for yourself, have a robot hit one spot and hit the ball at the same spot with controlled similar-accelerated but different swings. Also
Edited by hobbes203 - 06/24/2012 at 6:03am |
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zeio
Premier Member Joined: 03/25/2010 Status: Offline Points: 10833 |
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It is obvious, isn't it? I bet he must both be kicking in his coffin and having a good laugh, to see that human nature has not changed all that much since his time. |
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Viscaria FL - 91g
+ Neo H3 2.15 Blk - 44.5g(55.3g uncut bare) + Hexer HD 2.1 Red - 49.3g(68.5g 〃 〃) = 184.8g |
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zeio
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Doesn't matter whether there is friction or not. The end result is the same. You are correct in picking the green, but you may be surprised to know that the acceleration of the ball down the path is actually constant. The mass and thus inertia of the ball also does not have any effect on its speed, as evidenced by the conservation of energy. It just so happens that the curve produces the best speed from the acceleration due to gravity. The curve seen in the image is referred as the Brachistochrone curve, which is Greek for "the shortest time." The brachistochrone curve happens to be a segment of an inverted cycloid. Before the brachistochrone was solved, the closely related Tautochrone curve, meaning "same time," was found to be part of a cycloid also. For those who are still reading: The brachistochrone problem was first posed by Johann Bernoulli, in which a point is to start from rest at point a and, solely under the effect of gravity, follow down to point b along a curve that is to be covered in the least time. Follow the second quoted passage to see the maths behind it. Equations (16) and beyond consider the condition where friction is present. The curve is slightly different as a result of that. Edited by zeio - 06/24/2012 at 7:01am |
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Viscaria FL - 91g
+ Neo H3 2.15 Blk - 44.5g(55.3g uncut bare) + Hexer HD 2.1 Red - 49.3g(68.5g 〃 〃) = 184.8g |
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power7
Silver Member Joined: 01/25/2012 Location: USA Status: Offline Points: 745 |
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Not really following this argument.
What aspect of a TT forehand swing is powered by gravity? So the fastest swing follows the path of a cycloid? You think any of the top TT players in the CNT can do a simple math proof... These models are not intuitive at all.
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pnachtwey
Platinum Member Joined: 03/09/2010 Location: Vancouver, WA Status: Offline Points: 2035 |
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Nice, the problem can be solved symbolically and there are only 16 steps! If I had to solve that problem I would have written the differential equations and solved iteratively using RK4. That would have been a lot of work. So many problems I run into don't have a symbolic solution and the problem must be solved using brute force iteration. With a symbolic solution you have a proof. With a iterative one you don't because that works with one set if numbers. |
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mikepong
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can someone correlate these math equations or whatever it is to TT? sorry not really good at math
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Viscaria
FH: Tenergy 05 black BH: Tenergy 05 red |
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pnachtwey
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You can't don't sweat it.
Those that figured this problem out were the best of their time. The Timo Bolls and Ma Longs of math and physics. There I related this to TT. |
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zeio
Premier Member Joined: 03/25/2010 Status: Offline Points: 10833 |
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I am getting there.
Forget the maths and the gravity. The keyword here is time. It is time, not distance, that really matters in producing the best swing. And for that to happen the best path to follow during a swing is a curve that most closely resembles the brachistochrone. Concave or convex, positive or negative, however you name it. It may sound counter-intuitive, but the longer path does produce the least time and we see signs of it in videos of the pros. And one thing is for sure, that path has never been and probably never will be a straight line as those who are in denial of any curvature in swing mechanics may have imagined. They could have been doing it all along without even realizing. The answer is out there, that the way we live, the tools we use, and the many things we come across during our lives are largely inspired by Nature. As it turns out, we humans tend to ignore it and overcomplicate things. As Johann Bernoulli himself has put it - "Nature always tends to act in the simplest way, and so it here lets one curve serve two different functions, while under any other hypothesis we should need two curves..." Edited by zeio - 06/24/2012 at 1:55pm |
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Viscaria FL - 91g
+ Neo H3 2.15 Blk - 44.5g(55.3g uncut bare) + Hexer HD 2.1 Red - 49.3g(68.5g 〃 〃) = 184.8g |
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pnachtwey
Platinum Member Joined: 03/09/2010 Location: Vancouver, WA Status: Offline Points: 2035 |
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The brachistochrone works because the brachistochrone allows the ball to drop or accelerate faster initially. This isn't the case with a swing that opposes gravity. I know where you are going with this. You are thinking of the brachistochone with a upwards swing where the swing starts out shallow to get moving and then starts in the upwards direction. Good one, but if the paddle attitude is always changing the resulting timing errors will result in poor control. However, if the paddle attitude can be kept constant during the up swing it may work. The big question is will this make any practical difference? Most of us can swing far faster than what is necessary anyway. |
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Baal
Forum Moderator Joined: 01/21/2010 Location: unknown Status: Offline Points: 14336 |
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There is more than one way to skin a cat. That is why several different players who have been in the top five in the world appear to have a different swing, and they all have great forehands. Also, it is very very hard to remodel a stroke once it is formed -- and in adults in general. It can be done, but it takes a long time. That may not be apparent from an engineering perpective, but it is true from a physiological perspective. Those motor patterns get really engrained. Tape yourself and find out. It may feel like you are making big changes in your stroke and when you actually see it, you just look like you. (Especially once you play a match). Feel and reality do not always coincide. The most important things are (1) does your swing produce reasonable spin, pace, and accuracy, and (2) can you maintain your balance so as to be able to hit the next ball? If you forehand does this with consistency than it doesn't matter if you look like WLQ, ML, or the Dalai Llama. You will be good. And Bernoulli, Gauss, and Laplace be damned.
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Baal
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The reason why a follow through matters is because a swing that results in a proper follow throw begins before you hit the ball, and therefore affects the way you hit the ball.
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zeio
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Both objects are identical in size, shape, and weight etc. They are under uniform gravity. Friction or frictionless does not affect the outcome. They reach maximum velocities at the bottom. Assuming no friction, the vertical accelerations of both objects are constant but of different values as they run down the paths. This difference in vertical acceleration is what separates the two. |
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Viscaria FL - 91g
+ Neo H3 2.15 Blk - 44.5g(55.3g uncut bare) + Hexer HD 2.1 Red - 49.3g(68.5g 〃 〃) = 184.8g |
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power7
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This isn't non-Euclidean geometry, so the linear path takes the least amount of time. Nor is this a gravity power problem with no friction. The modelling is incorrect. The only thing applicable is Bernoulli's principle on the ball spinning creating low pressure systems thus make the ball "curve" in flight... |
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pnachtwey
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You assume that the velocity profiles are the same. If there is no friction then the speeds will be the same at the bottom of the ramps but the cycloid ramp will allow the ball to accelerate to faster. The area under the velocity curve, the distance, traveled will be greater when comparing with the constant acceleration ramps at the same time.
So far yes, lets wait for zeio to make his case. |
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Krantz
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Nice observation about brachistochrone curve. For now it tells me that we cannot outright exclude the possibility that a curved motion may be “faster” then a motion along a straight line. Here are few other possible explanations why curved motion (with a long follow-through) can be a good option: 1. It may be easier to aim with that kind of swing. While it’s true that all it really matter is an angle (direction, speed) of a racket at point of contact it may be easier to adjust these factors during longer swing. Compare this to shooting with a rifle and with a pistol. If you adjust your sights perfectly then it doesn’t matter which gun you shoot with, but it is easier to adjust sights of a rifle because of longer distance between a notch and a bead - and this results in overall better accuracy of a rifle. Consecutively, a longer curved swing with a long follow through may just help you in placing the ball at the exact point you aim. 2. Generally, the harder you hit, the more closed the angle of a racket should be. A positive arc of a movement assures just that. At the beginning of a swing the racket’s speed is low, so its angle is open. While you accelerating, you are continuously closing the angle and during the whole swing their combination is just about right to place the ball on the table. Moreover, I would say that this option of closing the angle is on the safe side of an error margin - closed angle is good. If the angle is to low then in order to prevent the ball from hitting the net you have to just hit harder - and hitting harder in order to not miss the table is a win-win scenario. 3. Natural ready position between shots is with a hand relatively low. Sufficient reason that we don’t wait for a ball with a hand raised high is that it would be to tiring. But quite often we have to hit the top part of the ball – the only way we can do this (from this low hand starting position) is by performing this curved, positive arc motion. |
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V-Griper
Silver Member Joined: 09/19/2011 Location: United States Status: Offline Points: 879 |
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There is also conservation of momentum and energy.
If you use a more straight forward/straight back motion you have to expend a lot more effort accelerating and decelerating the paddle. By following a more circular stroke path momentum and energy are conserved. There is also adjustment to incoming ball to consider. As an example if you look at ML slow mo vid again, when ML begins the forward part of the stroke, you can see there is more of an virtical component to his stroke. To me this is where the height adjustment is being made for the height of the ball. However there is still a forward component to the stroke. It's just more efficient. BTW this is the best thread in awhile. This is where we challenge our assumptions and refine our paradigm. Time to get your geek on
Edited by V-Griper - 06/25/2012 at 9:32am |
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V-Griper
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I love things that challenge my intuitive assumptions about how things work. Even if this is not substantially relevant to TT it is still cool.
Thanks Zeio. Sorry won't embed. Edited by V-Griper - 06/25/2012 at 9:47am |
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V-Griper
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I would say that the paddle angle can and is kept constant by the better players who seem to be using a more circular motion. ML seems to hold blade angle more constant throughout his swing cycle than Timo. At least that I can perceive. BP seems to hold paddle angle more constant than ML or TB but I am not sure he could generate the paddle acceleration that ML or TB can. Imo minimizing timing errors is mainly a nervous system issue. I see a lot of players with very inefficient strokes who have incredible timing because they do exactly what you say, they hold the paddle angle relatively constant. I was practicing with a beginner Yesterday and he could do 150 ball rallies. I noticed, however, that in order to keep his paddle condition constant he made all kinds of contortions with his wrist elbow and shoulder. Then there is the 1800 level player at our club who has to quit training for a couple of days if he does a lot of looping. His loop mechanics are so bad(FH & BH) I worry about him tearing his rotator cuff. He has already had minor tearing in his shoulder muscles. The ball may not care what you do prior to and after contact but your body sure does. Edited by V-Griper - 06/25/2012 at 11:51am |
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zeio
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To keep things straight, the energy source behind the two balls is actually the potential energy. Potential energy can exist in quite a few forms, and each of them comes from a particular type of force, namely the conservative force. In the brachistochrone problem, the force of gravity used to initiate a change of state from rest to motion is a type of conservative force and it gives rise to gravitational potential energy. As the balls trace down the paths, the potential energy within them is converted into kinetic energy. By the conservation of energy, in a closed system where friction is neglected, both potential and kinetic energy are conserved. In layman's terms, the balls are powered by an engine that is 100% efficient with no loss. Theoretically speaking, they start off at the same level of potential energy and should end up with the same final velocities at the end of the paths. In an experiment run in Germany where kinetic energy is not conserved due to friction, the final velocity of the ball along the brachistochrone curve has been found to be greater than that of the ball along the straight line, as cited by this source. So it looks like the brachistochrone is also more efficient in aspects other than time.
For table tennis, we draw on a number of potential energy. When one winds up during a backswing, elastic potential energy is stored in parts of the body, waiting to be released. Chemical potential energy from food is transformed into kinetic energy when we do work to swing the paddle back and forth. When players lower their stances, they also gain gravitational potential energy. Instead of using the gravitational potential energy gained from the force of gravity alone to start motion as in the brachistochrone problem, the goal here is to make the most out of the additional potential energy available to us to overcome gravity in a three dimensional swing. The timing problem is not really a problem as we have all witnessed. This is what training is for. One may be a fluke, two could be coincidence, but three is a trend. Every pro may approach it differently, but the basic rule still holds. Just because the human nature within most of us tends to think only the pros can do it well should not deter the rest from pondering, discussing, and mimicking, for this is how one gains knowledge. Of all western sources I've managed to dig up, only those in skiing and surfing is the brachistochrone curve ever directly mentioned and applied for performance gain. Not to worry, here is a thesis just about its application in table tennis, titled "Kinetics Analysis of the Kinetic Chain of the Execution of Strokes in Succession while in Motion in Table Tennis" by 賈鵬(Jia Peng) and published in 2007. I do not have access to the full paper, but the abstract will suffice. Below is an excerpt about the benefits of integrating the brachistochrone curve into stroke mechanics. (3)人体在移动后动力再链接包含了“引拍、挥拍、还原”的周期过程,尤其在连续击球时,“引拍、挥拍、还原”的线路轨迹、周期长短、频率高低以及击球前的准备充分程度都直接影响击球的效果。在研究过程中把“引拍、挥拍、还原”的轨迹、周期、频率和单摆、锥摆的轨迹、周期、频率运用力学的方法做了详尽的比较分析,也利用“最速降线”对“还原、引拍”进行了力学上的比较分析,得出人体在持拍连续击球的时候,利用近似椭圆的轨迹特点,利于发挥速度、频率优势,在节奏快的激烈对抗中获取时空上的利益,以期能够获得技战术上的优势。 (3) The kinetic chain of the human body after some displacement involves a cycle of "backswing, swing, and recovery". The path, period, and frequency of the cycle, as well as the level of readiness before each stroke all have a direct effect on its result. During research, the path, period, and frequency of the "backswing, swing, and recovery" and of the simple pendulum and the conical pendulum are given extensive comparative analysis using kinetics. The same analysis is also performed using the "brachistochrone curve" for the "recovery and backswing." The results derived suggest that when the human body executes shots in succession, use of the ellipse-like path facilitates the play of speed and frequency to get the upper hand in space-time during a fast-paced and fierce confrontation in hopes of gaining further technical and tactical advantages. At this stage, we see the potential advantage of applying the brachistochrone curve for the recovery and backswing before and after a swing. Other than the thesis above, there is another user by the name "jlw" posting about the brachistochrone curve(1), (2). The poor dude got practically zero constructive feedback. Edited by zeio - 06/25/2012 at 12:02pm |
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Viscaria FL - 91g
+ Neo H3 2.15 Blk - 44.5g(55.3g uncut bare) + Hexer HD 2.1 Red - 49.3g(68.5g 〃 〃) = 184.8g |
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pnachtwey
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I find it interesting but we don't swing using gravity alone. It is would very nice to put a 3D accelerometer on the back of a blade to see what is really happening. It doesn't surprise me that some of the authors didn't get any attention without hard data.
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V-Griper
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Alright, you've got the know how, let's get some hard data then. How cost prohibitive is it?
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power7
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DHS PG-7, H3 Neo, 729-5
Butterfly Power-7, Red TG2 Neo 39degree, Black Donic Bluefire M1 |
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V-Griper
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This issue came up in another post and we kicked around the idea of collecting hard data. pnatwey works with that kind of equipment for a living. He knows exactly what it would take and how to do it. It is a matter whether or not he wants to put in the time. I would be willing to put money in the pot for equipment depending on the cost estimate.
Edited by V-Griper - 06/25/2012 at 3:44pm |
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king_pong
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I'm enjoying this thread as well. I was away for the weekend, and see that I've missed a lot.
The posting below by V-Griper is the closest I've gotten to anyone informing me which are concave and which are convex loops. Would love to know what people think/believe
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zeio
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The brachistochrone problem was solved and proven in the Euclidean space in the 17th century. The classical mechanics was developed during this period as well. Non-Euclidean space did not emerge until the early 19th century. What matters though is that the brachistochrone was first experimentally observed under the hypothesis of Galileo in the physical space, which, by Albert Einstein's theory of general relativity, is curved. As the balls are traveling at far below the speed of light, the classical mechanics used to solve the problem is extremely accurate. Unless you are hallucinating or something, I don't see how this model can be labeled wrong. Yes, the reason a table tennis ball curves can be explained by the Bernoulli's principle. But the Bernoulli's principle alone is insufficient in describing the sequence of events leading up to the pressure difference. The boundary layer separation is necessary to fill the void. |
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Viscaria FL - 91g
+ Neo H3 2.15 Blk - 44.5g(55.3g uncut bare) + Hexer HD 2.1 Red - 49.3g(68.5g 〃 〃) = 184.8g |
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