Football physics: The "impossible" free kick - Erez Garty

足球物理學：不可能的自由球 - Erez Garty

譯者: Zongzhen Yang 審譯者: 庭芝 梁

In 1997, in a game between France and Brazil,

在1997年，一場法國與巴西之間的比賽，

a young Brazilian player named Roberto Carlos

一名叫Robert Carlos的年輕巴西球員

set up for a 35 meter free kick.

從35米外發了自由球。

With no direct line to the goal,

他無法將球直線射入球門，

Carlos decided to attempt the seemingly impossible.

Carlos決定嘗試一件看起來不可能的事：

His kick sent the ball flying wide of the players,

他的射門讓球繞過球員，

but just before going out of bounds, it hooked to the left

但是就當球快要出界時，它突然向左勾

and soared into the goal.

然後射進球門。

According to Newton's first law of motion,

根據牛頓的第一運動定律，

an object will move in the same direction and velocity

物體運動時會維持相同的方向和速度

until a force is applied on it.

除非它受到外力的影響。

When Carlos kicked the ball, he gave it direction and velocity,

但Carlos踢球時，他已經給球施加方向與速度了，

but what force made the ball swerve

究竟是什麼力量讓球改變方向

and score one of the most magnificent goals in the history of the sport?

並且造就了運動史上最偉大的進球之一？

The trick was in the spin.

關鍵就在於旋轉。

Carlos placed his kick at the lower right corner of the ball,

Carlos的踢球點在球的右下方，

sending it high and to the right, but also rotating around its axis.

讓球向右側高飛，但也讓它繞著軸心旋轉

The ball started its flight in an apparently direct route,

一開始，球沿著明顯的直線飛行，

with air flowing on both sides and slowing it down.

飛行時兩側的空氣讓球變慢。

On one side, the air moved in the opposite direction to the ball's spin,

在其中一側，空氣與球旋轉的方向相反

causing increased pressure,

導致壓力上升，

while on the other side, the air moved in the same direction as the spin,

然而在另一側，空氣與球的旋轉方向相同，

creating an area of lower pressure.

產生一個低壓力區。

That difference made the ball curve towards the lower pressure zone.

壓力差讓球朝向低壓區轉彎。

This phenomenon is called the Magnus effect.

這個現象叫做馬格納斯效應。

This type of kick, often referred to as a banana kick,

這種踢球法，也被稱為香蕉球，

is attempted regularly,

在足球場上經常被使用，

and it is one of the elements that makes the beautiful game beautiful.

而且這也是讓一場漂亮的比賽更加精彩的元素。

But curving the ball with the precision needed

但是香蕉球需要極高的精準度

to both bend around the wall and back into the goal is difficult.

要讓球繞過人牆，然後轉向球門是非常困難的。

Too high and it soars over the goal.

球飛得太高，會飛過球門，

Too low and it hits the ground before curving.

球飛得太低，在轉彎前就會落地。

Too wide and it never reaches the goal.

轉彎角度太大的話，進不了球門，

Not wide enough and the defenders intercept it.

轉彎角度不夠大，會被對手攔截。

Too slow and it hooks too early, or not at all.

球速太慢的話，球會提早轉彎，甚至不會轉。

Too fast and it hooks too late.

球速太快的話，球會轉彎得太晚。

The same physics make it possible

同樣的物理理論，

to score another apparently impossible goal,

也可以讓一個看來不可能的角球射門成功，

an unassisted corner kick.

那是一個沒有受到任何人幫助的角球。

The Magnus effect was first documented by Sir Isaac Newton

馬格納斯效應最初是由牛頓所提出，

after he noticed it while playing a game of tennis back in 1670.

他在1670年打完一場網球後，發現這個現象。

It also applies to golf balls, frisbees and baseballs.

這個原理同樣可以應用於高爾夫球，飛盤和棒球。

In every case, the same thing happens.

在每個情況下，都會有同樣的現象。

The ball's spin creates a pressure differential in the surrounding air flow

球的旋轉，讓環繞的氣流產生壓力差，

that curves it in the direction of the spin.

導致球朝著旋轉的方向轉彎。

And here's a question.

現在有一個問題，

Could you theoretically kick a ball hard enough

根據理論，你可以使勁踢出一顆球

to make it boomerang all the way around back to you?

然後讓球飛回你的方向嗎？

Sadly, no.

可惜的是，不行。

Even if the ball didn't disintegrate on impact,

即使球沒有在衝撞中解體，

or hit any obstacles,

沒有擊中任何障礙物，

as the air slowed it,

沒有空氣阻力減緩速度，

the angle of its deflection would increase,

偏轉角沒有增加，

causing it to spiral into smaller and smaller circles

使螺旋變成越來越小的圓

until finally stopping.

最後停止。

光是要讓球旋轉，

And just to get that spiral,

you'd have to make the ball spin over 15 times faster

你需要讓球的轉速，

比Carlos不朽的射門快15倍。

than Carlos's immortal kick.

So good luck with that.

所以，祝你好運。