Football: The Physics of Curving Free-kicks - Roberto Carlos' 1997 Phenomenal Free-Kick Explained
Football is a fascinating sport; it has been loved by fans, scrutinised by the media, entered into the history books and is now being studied in a science lab. Well, free-kicks are anyway; a couple of scientists wanted to find out how it was possible that a ball when kicked would change trajectory in the air and travel in completely the opposite direction. It would not have seemed that something like a free-kick would be so interesting to scientists but apparently it is. There was one particular kick that had them stumped; it is called the greatest free-kick ever to be taken in the game. It was a shot taken by Brazilian Roberto Carlos against France during a tournament in 1997. This one kick brought out the scientists in droves to try and explain it.
In 1997, playing in a match against France, Brazilian Roberto Carlos scored a goal with the most amazing free-kick that has ever been witnessed in the game of football. Even though David Beckham and many others are very good free-kick takers, this one particular shot was so incredible that physicists have been trying to explain it for years. Carlos took the free-kick from a good distance away from goal and hit it very hard. The ball looked to everyone as if it was going to go into the crowd and become a goal kick. But then suddenly the ball swung in, cleared the wall of defenders and as if guided by a divine wind went into the back of the net. The goalkeeper, French team, spectators and the commentators were all stunned into silence by the shot. It was such an incredible kick that two scientists, Professor Christophe Clanet and Professor David Quere sought to explain how it happened.
The two French scientists explained that they felt the kick was not a fluke - as some people had thought - but it was possible to be recreated. The key was in the distance of the ball to the goal. The free-kick was taken at about 35 metres distance from the French goal and that large distance was one of the factors that allowed the ball to curve as it did. Kicks from short distances do not curve; that is why penalty kicks go in a straight line. The other factor in the curving kick was the fact that Carlos hit the ball with a lot of power. If it had been kicked with less force, it would not have spun and curved the way it did. The two physicists determined that the perfect velocity with which the ball had to be hit was 130 mph; anything slower and the ball would not have ended up in the back of the net. The pair of scientists even gave their discovery of the physics behind the kick a name; they called it the ‘spinning ball spiral’.
Apparently Roberto Carlos had practised this shot many times before during training and he knew exactly where to hit the ball and how hard and how to give it the right amount of spin and curvature. The curving effect that Carlos employed so well in that immaculate free-kick followed a principle laid out by a German physicist named Gustav Magnus. In 1852, Magnus discovered the principles behind why an object in motion suddenly seems to swerve to one side or the other. This was named the Magnus Effect in his honour and even though he had been trying to understand why bullets move in the air, the same effect can be applied to sports and the balls that are used in them. This principle explains why cricket balls swing in the air and why Pakistan cricketer Wasim Akram was able to make the ball turn in the air at will. It also explains why golf balls curve to one side after being hit. The science behind the Magnus Effect is complicated but simply put, because a ball spins when kicked or hit, the air flow on one side is different to the air flow on the other, and whichever side has the stronger pull of air the ball will travel in that direction. So it will curve in the direction which has the most air pressure on it.
With the science behind it explained, maybe we will see more bright young footballers trying to curve the ball as erratically as they can in order to fool defenders and the goalie. If they took some time and studied the physics behind it, it might help them achieve world class free-kicks like Roberto Carlos or David Beckham in the future.
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