Carlos '97 free kick no fluke, say French physicists

Roberto Carlos' free kick goal against France in 1997's Tournoi de France is thought by many to have been the most skilful free kick goal - from 35m with a powerful curling banana trajectory - ever scored; but by others to have been an incredible fluke.

Taken in 1997, a year before the French won the World Cup, Brazilian Carlos's held France to a frustrating draw but, now, a group of French physicists - perhaps with a nostalgic eye to a happier time for French football - have computed the and shown that Carlos' goal was no fluke.

Carlos' free kick

The research published today, Thursday 2 September, in (co-owned by the Institute of Physics and German Physical Society), explains why French Fabien Barthez made no move for the (but why a ball-boy ten metres from the goal did duck to safety) as the ball made a last moment sweep left and landed in the back of the net.

Using tiny plastic (polypropylene and polyacetal) balls and a slingshot, the French research team from the École Polytechnique in Palaiseau varied the velocity and spin of balls travelling through water to trace different trajectories.

While their research quickly confirmed the long-known Magnus effect, which gives a spinning ball a curved trajectory, their research revealed fresh insight for spinning balls that are shot over a distance equivalent to Roberto Carlos' free kick.

The friction exerted on a ball by its surrounding atmosphere slows it down enough for the spin to take on a greater role in directing the ball's trajectory, thereby allowing the last moment change in direction, which in the case of Carlos' kick left Barthez defenceless.

The researchers refer to their discovery as the 'spinning ball spiral', comparing the spiraling effects of Roberto Carlos's kick with the shorter-distance (20-25m) 'circular' free kicks shot by the likes of Beckham and Platini.

As Christophe Clanet and David Quéré, researchers from École Polytechnique, write, "When shot from a large enough distance, and with enough power to keep an appreciable velocity as approaching the goal, the ball can have an unexpected trajectory. Carlos' kick started with a classical circular trajectory but suddenly bent in a spectacular way and came back to the goal, although it looked out of the target a small moment earlier.

"People often noticed that Carlos' free kick had been shot from a remarkably long distance; we show in our paper that this is not a coincidence, but a necessary condition for generating a spiral trajectory."

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Limitations of human visual system hinders goalkeepers from predicting free kicks

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Sep 02, 2010
The spiral effect might also account for the "late break" of a baseball when certain pro baseball pitchers throw a particularly nasty curve ball or slider. The rapidly spinning ball slows down approximately 10mph from the moment it leaves the pitcher's hand to the moment it hits the catcher's glove. This decrease in velocity might be enough to produce a visible spiral effect.

Sep 02, 2010
Pool shooting better explains the spin effects seen in all soccer ball trajectories, but more so at longer distances. More spin and curving can be achieved quite simply by kicking the ball of center with the toe of the boot to create a spin, both down and sideways. The customary kick with the wider contact from the frontal boot surface creates only forward motion with natural downward drift with little spin over longer distances.
Soccer fan

Sep 03, 2010
I have to agree with you jimbo. In my days of bowling, I would pratice combining the release velocity of my ball while trying to add the proper ammount of spin to it so that it would travel in a shalow arc on the right hand side of the lane, taking it extremely close to the gutter and then slowly come away from it. Once it got closer to the pins it would then make a notably sharper curve towards the "pocket" while apearing to speed up in that direction as the spin took over. Since this is basiccly a 2d representation, I can't see why it can't be applied to a 3d aspect as in baseball or soccer(footbal).

Sep 03, 2010
Actually, neither the example of balls on a pool table or the path of a bowling ball are good examples. Both travel along a solid surface and the trajectories are in a 2d plane. On top of that the forces that are applied to, and afterward act on, one of these balls is not the same as for a football in flight. Friction from the surface(the felt covered table or a bowling lane alternately covered in oil and then dry) is only applied at the points of contact with the ball. The magnus effect occurs due to forces surrounding the entire football in addition to those initially applied to it. On another note, stiking a football with the toe, while it may provide more spin, does things that make it impractical. Mainly, it creates a distortion in the shape of the ball causing unpredictability of flight and thus control. Ball control is what it is about.

Sep 03, 2010
And ball control is precisely what it is about in pool as well as in bowling.

The pool / bowling analogies may not be perfect, but a model need not be to prove a point. You will see a very similar shot path when using a masse shot in pool, where the ball cue ball will slowly curve toward one direction, and the spin will take over as the dominant force and drastically change the trajectory of the ball.

Sep 03, 2010
Yes, from a visual stand point they can be used as an example but not if you are trying to explain the mechanics. They are not the same. But I see your point. The increased ball curve over distance as the ball slows can be shown in those examples. But Like I said the reasons for these visuals are not the same.

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