????如果今年的世界杯出現了許多直接破門的任意球，球迷們千萬別忘了足球自身的功勞。

????根據法國雜志《理工學報》(Les Editions de l’Ecole Polytechnique)本周發布的一篇論文，任意球得分在很大程度上應該歸結于足球表面的粗糙程度。今年巴西世界杯官方用球“桑巴榮耀”似乎剛好符合這個需要。這項研究的執筆者、美國麻省理工學院(Massachusetts Institute of Technology)教授約翰?布什估計，這款足球的接縫比飽受詬病、有些滑溜溜的2010年南非世界杯官方用球“普天同慶”長50%以上。

????他總結稱，今年世界杯用球【阿迪達斯公司(Adidas)制造】的運行軌跡更容易預測，從而也就更容易破門得分。相比之下，更光滑的足球通常更加難以控制，因為它們更有可能飛向一個意想不到的方向。

????“皮球周圍空氣的流動細節非常復雜，特別是，它們取決于皮球的粗糙程度，”這位致力于研究足球空氣動力學的應用數學教授說。“如果皮球非常光滑，它就會轉向錯誤的軌跡。”

????布什把所謂的馬格努斯效應(Magnus Effect)歸功于足球的運轉。它最早是由英國物理學家艾薩克?牛頓提出的。牛頓發現，網球的上旋導致球下沉，而下旋球往往沿水平軌跡飛行。棒球比賽的弧線球是另一個體育方面的例子：投手使用特別緊密的上旋或側旋擲出棒球，棒球就會沿旋轉方向劃出一道弧線。

????足球比賽的任意球、角球，甚至球員之間的傳球也會出現類似一幕。以大衛?貝克漢姆為代表的任意球大師踢球時能夠施加旋轉，導致皮球沿曲線滑行——右腳球員通常會搓皮球的外側，使它從右向左旋轉，而左腳球員的射門通常從左向右旋轉。

????布什把這種效應歸因于皮球表面在“邊界層”（即旋轉的皮球和空氣之間）的運轉方式。皮球越粗糙，它就越容易產生可導致馬格努斯效應的教科書式旋轉。

????布什在這篇論文中指出，單靠皮球并不是總能確定它的飛行方向。另一個重要因素是踢球的方式，以及球員的腳接觸皮球的時間。接觸時間的差異可能多達20%，皮球越軟，差異越大。此外，球員還可以有策略地尋找最佳擊球位置——最好的球員往往青睞相對堅硬的閥側。

????“在一般情況下，接觸時間將決定射手控制皮球的能力，”布什寫道。“特別是，接觸時間越長，射手就能夠更加游刃有余地對球施加旋轉，讓它沿著曲線滑行。”

????當然，擁有貝克漢姆或C羅這類國際巨星的能力和技術也會有幫助。這些球星能夠讓對手眼睜睜地看著皮球神奇般地滑過防守人墻，飛入球網。（財富中文網）

????譯者：葉寒

????If this year’s World Cup ends up with its share of winning free kicks, give a little credit to the ball.

????A big part of scoring with a free kick comes down to the roughness of the ball, according to a paper published this week in the French journal Les Editions de l’Ecole Polytechnique. This year’s “Brazuca”, the official ball of the World Cup currently underway in Brazil, appears to fit the bill. John Bush, the Massachusetts Institute of Technology professor who authored the study, estimated that the ball’s seams are over 50% longer than the much-maligned and almost slippery “Jabulani” ball from 2010 World Cup in South Africa.

????He concluded that this year’s ball, manufactured by Adidas, are more predictable in flight, which makes them easier to score with. Smoother balls, in contrast, are more difficult to control because they are more likely to take off in an unexpected direction.

????“The details of the flow of air around the ball are complicated, and in particular they depend on how rough the ball is,” said Bush, a professor of applied mathematics whose study examined the aerodynamics of soccer balls. “If the ball is perfectly smooth, it bends the wrong way.”

????Bush credits what is called the Magnus Effect for a ball’s behavior. It was first described by Isaac Newton, the English physicist, who saw how topspin in tennis causes the ball to dip while backspin flattens out the trajectory. A curveball in baseball is another example from sports: A pitcher throws the ball with especially tight topspin, or side spin, and the ball curves in the direction of the spin.

????A similar thing happens in soccer with free kicks, corner kicks and even other kinds of passing between players. A player like David Beckham, the retired free kick master, applies spin when kicking the ball, creating rotation that causes it to curve – a right-footed player will brush toward the outside of the ball causing it to curl right-to-left while a left-footed player’s shot will curl left to right.

????Bush attributes this effect to the way the surface of the ball creates motion at the “boundary layer” between the spinning ball and the air. The rougher the ball, the easier it is to create the textbook spin that causes the Magnus Effect.

????In his paper, Bush said the ball alone doesn’t always determine its direction. Another key component, he said, is the how the ball is struck and the amount of time a players foot makes contact with the ball. Contact times can vary by as much as 20%, being larger for the softer ball. Players also strategically search for the sweet spot, with the best favoring the valve side of the ball where the ball is relatively stiff.

????“The contact time will, in general, determine the ability of the shooter to control the ball,” Bush writes. “In particular, the longer the contact time, the more readily the shooter can impart spin to the ball when trying to bend it.”

????Of course, it helps to have the power and technique of international superstars like Beckham or Portugal’s Ronaldo, who can leave an opponent starstruck with balls that seem to magically bend their way around a defensive wall and into the net.