Beautiful goals are the crowning glory of every football game. And the crowning glory among shooting techniques is probably the flutter ball. With this shot, the ball changes its trajectory – such shots are almost untenable. But how can this be explained physically? French researchers have studied the phenomenon. In doing so, they not only found the explanation they were looking for, but also clues as to why the idiosyncratic flight is typical of foot, volleyball and baseball.
The flutter ball is notorious: the leather sails over the wall and suddenly turns in a different direction. For a goalkeeper who orients himself to the trajectory of the ball for his defensive movement, such shots can hardly be assessed. But how is it possible for a ball to change its trajectory in the air? The physicists Baptiste Darbois Texier and Christophe Clanet from the École Polytechnique in Paris took a closer look at the phenomenon. To do this, the researchers first constructed a “kick machine” consisting of a steel shaft with a plate. The shaft imitates the leg, a motor lets the metal foot – aka the plate – swing through and kicks the ball. To collect measurement data, the researchers chased various types of ball through a wind tunnel and recorded their trajectory with a high-speed camera. They programmed the kick machine in such a way that it imitated the shooting technique of the flutter ball.
Unsteady forces act on the ball
And it works like this: In order for the ball to change its trajectory abruptly, the players kick the leather in such a way that it hardly rotates in the air and flies extremely slowly. Measurements in previous studies have shown that flutterballs in soccer can travel between 20 to 25 meters per second, while the maximum speed is 51 meters per second. In addition, the ball does not complete more than a tenth of a complete revolution until it reaches the goal. The situation is similar with volleyball and baseball – the sports in which the phenomenon of the flutter ball has also been observed so far.
In the wind tunnel, Texier and Clanet measured the behavior of a flutter ball in relation to the air flow and found that so-called unsteady forces act on the ball and deflect it. That is, forces that occur randomly and with different strengths, comparable to the turbulence during a journey in an airplane. “In every sport, however, unsteady forces act on balls that fly through the air,” explains physicist Texier. “So, to complete our work, we wanted to find out why such zigzag shots only exist in a few sports like football and baseball.”
Sufficient distance, correct speed, no rotation
To do this, the scientists calculated how much the ball deviates from its trajectory on average. They also modeled the wavelength of a flutter ball, more precisely the typical oscillation amplitude when the ball goes sideways. Their result: after around 30 meters, a football had shifted to the side by the length of its diameter. However, this phenomenon only occurred at certain, exceptionally slow shooting speeds – and when the ball covered a certain minimum distance. According to the researchers, this could explain why flutter balls are not possible in other sports such as handball, basketball or boccia. “With boccia, for example, a zigzag track would have to appear after a length of 27 meters. But this distance is much longer than the typical throwing distance, ”says Darbois Texier. “That means the flutter ball effect is not complete.”
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© Wissenschaft.de – Karin Schlott