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The Jabulani will it take for a banana?

The balloon of the World Cup - the Jabulani -seems to be a nightmare for the guardians of purpose, as its trajectories are unpredictable. On strikes far, he sometimes "float" in the air, wavering in its path before taking the goalkeeper by surprise as this goal of China against France.


Most explanations of these shots floating (eg here or at Tom Roud ) involve "Magnus effect", known to be originally very famous shots "banana"


admit that the parallel between the two types of shots did not seem obvious: the trajectory of the ball floating curve is much less than that of "shooting banana "and the ball seems to be much less" brushed ". Is this really the same effect? The Webinet is investigating this crucial issue which at least has the merit not to mention the antics of the France team. And this week, I promise: no equation!

Shooting banana
To peel the banana, nothing beats the legendary Roberto Carlos free-kick against the team of France in 1998:

Watch as the ball spins very fast on itself. The effect gave the ball causes air on contact, speeding the flow on one side and slowing the other. But the speed of a gas is related to its pressure (remember Bernoulli's theorem and its ): if its speed increases its pressure decreases and vice versa. The pressure differential induced on both sides of the ball creates a lateral pressure that bends its trajectory. That's the famous Magnus effect:

The Magnus effect (the illustration is adapted from this site )

This effect is accentuated by the deviation of the drag of air behind the ball. Under the effect of rotation, the turbulent wake is offset from the axis of movement of the ball:

(source: here )

In response, the ball undergoes a second lateral thrust which s 'added to the Magnus force:

(illustration drawn and adapted from this site ):

Effect will be even stronger than the bullet "Grip" the air around her. That's why the new tennis balls are more effective than bullets or wet waste and bullets baseball have fun sewing.
The speed of the ball itself also enhances the effect: it must "brush" the ball with his foot or his racket, especially in tennis where the ball is very smooth. In football the effect is more readily available in dry weather, this boot gives greater effect to the ball and better ball causes the air around her.
To these effects in the air there are others even more evil when the rebound when the ball is light. This allows for services formidable ping-pong

The second effect kiss-cool banana
In football the true magic touch of banana shot from Roberto Carlos, does not rebound, but the sudden change of the trajectory of the ball, once it has passed the wall of players. The ball seemed misplaced sharply angled towards the goal, taking everyone by surprise.

What happens there? When the ball slows down the flow of air around her suddenly changes of regime, and a password mode "turbulent" is chaotic to say to a much more stable ("laminar" ) where the effects of air viscosity on the ball (and thus the lateral pressure it undergoes) are much greater. If the ball has kept enough of rotation when it enters this regime the curvature of its trajectory is sharply intensified and took everyone by surprise.

source here

short, to make a beautiful shooting bananas should be kicked very powerful and brushed. The ball then follows a trajectory which rounded the curve is accentuated when the ball suddenly changes from turbulent to laminar fashion. This is obtained more easily in dry weather and with a ball "grip" good air.

The ball floating
Let us loose on penalties. Volleyball is that it comes most readily to "float serve." This is obtained when it hits hard in a beach ball very lightly. Even without wind, it looks like the ball did anything. Without a nice video on a good float serve, here's the animation that gives Wikipedia :

The explanation is more subtle than that of 'Magnus effect. Here is the lack of rotation of the ball which causes instability of the trajectory. And yes! In aerodynamics, the rotation plays the same a stabilizing role in mechanical gyroscope , although the mechanisms are very different. It is no coincidence that all the barrels of guns are rifled. Whatever its nature torpedo, or bullet-arrow a shot that turns into a spin during the race still sees its steady path, because the turbulence of the air is channeled behind him.

If instead the ball goes very quickly without turning, the turbulent flow of air in contact creates mini-whirlpools on its surface. These small perturbations are randomly distributed around it are micro-depressions attract or repel the ball from one side or the other. Its trajectory becomes unpredictable and it is precisely this effect which is sought in ball games. The effect is easier to obtain with a bullet llisse and lightweight which is why he is best known ping-pong and volleyball than football. To pass a floating shot must hit the ball sharply to the center (to avoid rotation) and strong (she goes to steady "turbulent").

Verdict ...
recap: Magnus and floating effects have much in common (powerful shot, struck from afar and unpredictable path), but these similarities hide very different mechanisms: the Magnus effect recquiert a ball that spins very fast on itself and becomes dangerous if it is prolonged when the ball slows down. Unlike a floating shot when the ball is in full speed and it does not turn on itself. The Jabulani, smoother and lighter than other models of balloons (especially when played at high altitude) is an ideal candidate for firing "floating", especially when it rains. However, if my explanation is correct it is less susceptible to the effects much worse Magnus. Brief does not float a banana and a shot floating banana does not ...

Source:
Excellent work of students from Strasbourg on the topic
The science of soccer on the site of the University of Hong Kong
The conference "Balls and Ball" Etienne Guyon
Article on Wikipedia the volleyball float serve

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