学进去

The highlights of every World Cup are the impossible goals, especially the arc (弧线) ball goals. It’s breathtaking to watch. Every spin (旋转) of the ball moves air across the surface, pushing it into a bend.

When a soccer ball flies, the air forms a layer around the surface of the ball. As the balls pins, it changes the direction of the air to one side, says John Bush, an applied mathematician at MIT. This air pushes the ball in the opposite direction. The player starts with a strike on the outside of his right foot, which hits the left side of the ball, starting a clockwise spin. It throws the air off to the left, and the force created by the air leaving the ball pushes it to the right, explains Bush. Thus, a ball spinning to the right (that’s clockwise) will also are towards the right. This force is called the Magnus Effect.

It helps the goalkeepers, because they see uniform curvature (曲度) when players are taking bending shots at them. If they can pick up the spin right, it’s going to be the same amount of curvature, and they know where to put their hands. This is partly why players are much more likely to take bending shots during free kicks when goalkeepers can’t see their kicks quite as well because of the wall of defenders. If a ball isn’t spinning, it does something called knuckling, where the air turns the ball to one side in random directions, causing it to bounce in the air unpredictably. “It’s usually achieved when a player manages a sharp, fast touch of the ball, typically right on the air valve where the ball is most firm.” says Bush. Its lack of spin kept the goalkeeper from being able to predict where it was going until it was too late. All the goalkeepers tremble before the kicker that can shoot this kind of soccer ball.