一只足球的空气动力学：任意球破门要看梅西，也要看球皮

    如果今年的世界杯出现了许多直接破门的任意球，球迷们千万别忘了足球自身的功劳。

    根据法国杂志《理工学报》(Les Editions de l’Ecole Polytechnique)本周发布的一篇论文，任意球得分在很大程度上应该归结于足球表面的粗糙程度。今年巴西世界杯官方用球“桑巴荣耀”似乎刚好符合这个需要。这项研究的执笔者、美国麻省理工学院(Massachusetts Institute of Technology)教授约翰•布什估计，这款足球的接缝比饱受诟病、有些滑溜溜的2010年南非世界杯官方用球“普天同庆”长50%以上。

    他总结称，今年世界杯用球【阿迪达斯公司(Adidas)制造】的运行轨迹更容易预测，从而也就更容易破门得分。相比之下，更光滑的足球通常更加难以控制，因为它们更有可能飞向一个意想不到的方向。

    “皮球周围空气的流动细节非常复杂，特别是，它们取决于皮球的粗糙程度，”这位致力于研究足球空气动力学的应用数学教授说。“如果皮球非常光滑，它就会转向错误的轨迹。”

    布什把所谓的马格努斯效应(Magnus Effect)归功于足球的运转。它最早是由英国物理学家艾萨克•牛顿提出的。牛顿发现，网球的上旋导致球下沉，而下旋球往往沿水平轨迹飞行。棒球比赛的弧线球是另一个体育方面的例子：投手使用特别紧密的上旋或侧旋掷出棒球，棒球就会沿旋转方向划出一道弧线。

    足球比赛的任意球、角球，甚至球员之间的传球也会出现类似一幕。以大卫•贝克汉姆为代表的任意球大师踢球时能够施加旋转，导致皮球沿曲线滑行——右脚球员通常会搓皮球的外侧，使它从右向左旋转，而左脚球员的射门通常从左向右旋转。

    布什把这种效应归因于皮球表面在“边界层”（即旋转的皮球和空气之间）的运转方式。皮球越粗糙，它就越容易产生可导致马格努斯效应的教科书式旋转。

    布什在这篇论文中指出，单靠皮球并不是总能确定它的飞行方向。另一个重要因素是踢球的方式，以及球员的脚接触皮球的时间。接触时间的差异可能多达20%，皮球越软，差异越大。此外，球员还可以有策略地寻找最佳击球位置——最好的球员往往青睐相对坚硬的阀侧。

    “在一般情况下，接触时间将决定射手控制皮球的能力，”布什写道。“特别是，接触时间越长，射手就能够更加游刃有余地对球施加旋转，让它沿着曲线滑行。”

    当然，拥有贝克汉姆或C罗这类国际巨星的能力和技术也会有帮助。这些球星能够让对手眼睁睁地看着皮球神奇般地滑过防守人墙，飞入球网。（财富中文网）

    译者：叶寒

    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.