Dear All, Gue suka baca Guardian Knowledge base yang biasa memberikan jawaban atas pertanyaan2 yang umumnya jarang terjadi di sepak bola. Salah satu yang dibahas panjang lebar belakang adalah ungkapan Jens Lehmann terhadap tehnik free kick C. Ronaldo yang mana dia klaim bahwa bola tersebut bisa menambah akselerasi setelah di tendang (saat di udara).
Berikut beberapa teori yang menjawabnya (sorry masih dalam bahasa Inggris) : Semoga bermanfaat. regards, Billy Tan Can a football really speed up in mid-air? (2) Plus: Champions without awards; Relegations after winning the title (2); and Ole Gunnar Solskjaer: greatest ever supersub? Send your questions and answers to [email protected] <http://www.guardian.co.uk/football/2009/may/20/[email protected]> Last week <http://www.guardian.co.uk/football/2009/may/13/the-knowledge-football-speed-champions-relegation> we asked three eminent physicians at Oxford University to discuss the veracity of Jens Lehmann's claim that footballs can speed up in the air. Suffice to say it has spurred a furious debate in the Knowledge email inbox. Last week we dealt with general outlines, this week we're looking at specifics. To that end, the most convincing argument, and, to give them due credit, one mentioned by our experts last week, is that Lehmann's case is all about perception. "It is noteworthy that the first conjecture comes from a player," writes Robin Marshall, Professor of Physics at the University of Manchester, who has had a long interest in the physics of football and free-kicks. "It reminds me of claims by footballers and football writers that centre forward "X" could defy gravity by hanging in the air while lesser mortals were pulled inexorably back to earth. The answer was that they jumped higher and/or their timing was better. Panic driven defenders jump too soon and thud back to Earth earlier. All the same, I was always convinced that my cat could hover at the top of her leap. "Ronaldo hits the ball very hard and without spin, so the ball travels in a mode of turbulent flow for much more of its trajectory than a ball hit with less power or with spin. This means that it does not slow down as much, because it is travelling above the Reynolds number <http://en.wikipedia.org/wiki/Reynolds_number> (Manchester's Reynolds) for much more of its trajectory. What Lehmann might be noticing subconsciously, is based on his years of experience, and is a consequence of him having become used to balls slowing down in a certain accustomed way. His perceived profile for typical shots would be: not much slowing down at the start and then much more slowing down as the ball nears the goalkeeper. So Lehmann has built up a professional 'speed profile' of a well-struck ball. Therefore he could quite easily form the opinion that when a ball that does not slow down like he expects, it appears to speed up when matched against his profile. "The spin argument is dodgy and irrelevant because time and again, the slo-mo shows that Cristiano Ronaldo puts virtually no spin on the ball, and this is his special 'trick'." Prof Marshall's view is one shared by many, with Martin Burke making the point that "if modern balls have less friction then they would slow down less, and hence would appear to have accelerated to the goalkeeper" somewhat offset by the fact that "modern balls are also lighter and have less kinetic energy to counter-act the drag and so should slow down more". Dr João Fonseca, also of the University of Manchester, concurs with his colleague. "Ronaldo's strike is tricky because it is hit with no spin," he writes. "Spin stabilises the ball (think about a gyroscope or a spinning ball in American football) and hitting the ball with nor spin makes it unstable, susceptible to any variations in pressure and also likely to stall due to turbulent flow behind the ball. When the ball stalls or encounters instabilities it dips and swerves, specially when the ball is lightweight. "Now in fairness to Lehmann, a dipping ball is accelerating (speeding up) towards the ground and a swerving ball is accelerating left or right, even if it is slowing down in the direction of the goal. And I would have guessed that it's these accelerations that are difficult for a keeper and not the accelerations in the direction of the keeper." Dr Matthew Harding has an alternative view: "Lehmann was comparing the new balls to the old," he writes. "What he was describing (though he didn't realise it) was merely a change in the deceleration of the new balls compared to the old. If the old balls slowed down to a greater extent as they travelled through the air, then to an observer used to this behaviour, the new balls may appear to accelerate." And another theory comes following on from Richard Duff: "Could the Oxbridge boffins be missing two vital elements? Firstly, the ball is misshapen when first starting to travel having been squashed by the foot, upon returning to a spherical shape there could be a kinetic energy release that accelerates the ball. Secondly, the ball is not a completely even shape as it contains a valve. Often you see Ronaldo rotate the ball before taking a free kick so that his foot connects with the valve, could a correct connection with the valve cause it to act as a ball bearing does in a wheel and promote free movement through the air at greater speed?" That theory found some support with our academical correspondents. "The impact between the ball and foot is an 'elastic collison'," writes Dr Martyn Gadsdon, "and this leads us to consider the 'centre of mass of the ball' — something that James Richardson was close to hitting upon in a recent Football Weekly. Loosely speaking, the centre of mass of an object is the point (often within the body - but not exclusively so for, say, a very thin moon shape) at which the body could be described as behaving as if all its mass was concentrated. In an elastic collision with a foot, the ball clearly deforms and therefore, its centre of mass moves. When the ball returns to it's original shape, its centre of mass returns to its original location. "Thus, while the Oxford professors are correct in stating that the ball will not accelerate, what they actually mean is that the ball's centre of mass will not accelerate. If, when the ball is kicked, the centre of mass (due to ball deformation) is moved forwards (nearer the front of the ball), such that, as the ball returns to its normal shape the centre of mass moves backwards (or put another way - the front of the ball moves forwards relative to the the centre of mass), then if you were to define the front of the ball as the point at which the goalkeeper was focussing, this section of the ball would indeed appear to accelerate towards the keeper - purely by the deformation of the ball, even though the centre of mass does not accelerate. "Footballs are continually being made more responsive and this is achieved by making them light and easily deformable, but crucially making them extremely efficient/quick at regaining their shape - unlike a beach ball - so it is expected that this effect is more prominent now than in the past. Although this may be a very small effect, plus it is likely to happen fairly quickly (otherwise the ball would be like a beach ball!) so may be minimal by the time the ball has passed the wall, and be undetectable to the keeper - but it is not impossible for the keeper to experience this in principle." And finally, two links that may help to shed some light on the subject. Firstly, Dr Paul Coe, also of Oxford University's department of physics, directs us to this article at Physics World <http://physicsworld.com/cws/article/print/1533> "which gives extensive scientific credentials to a phenomenon observed repeatedly by goalkeepers and football fans alike". Secondly, Nina Barneih suggests a neat optical illusion: "Don't know if you watch much baseball, but a well-thrown pitch appears to accelerate (to me at least) as it bends in. Same with late (reverse) swung cricket deliveries ... "There's a rather interesting optical illusion that just won the '5th Annual Visual Illusion of the Year' that demonstrates this principle rather compellingly <http://illusioncontest.neuralcorrelate.com/2009/the-break-of-the-curveball/>. To my eyes at least, the ball appears to accelerate as it falls down the screen. The explanation is that this is due to the switch from 'foveal to peripheral vision', which I'm not totally sure I can rationalize for a batter or goalie who's tracking the ball directly, but if you're Jens Lehmann and attempting both to watch the ball and feverishly scan your penalty area for opponents to thow a strop on, it seems quite feasible that this effect might apply." ------------------------------------ HAPUS BAGIAN EMAIL YG TIDAK PERLU SEBELUM ME-REPLY. ========================================================== Milis Tabloid BOLA Untuk KELUAR DARI MILIS INI. 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