Bend it like Beckham, it's all down to physics

In the Lab
Sports Science
June 19, 2004

There's more to a free kick than belting it, write Justin Kemp and Damian Farrow.

Among the world's premier athletes in any given sport, a select few possess the skills, or a skill, that can turn a game.

In Portugal for Euro 2004, a handful of players have that ability.

England already has witnessed first-hand the magic boot of Frenchman Zinedine Zidane, and when England skipper David Beckham or Pavel Nedved of the Czech Republic stand over the ball at a free kick, players and fans alike will also hold their collective breath. Why? Because these masters can bend a ball with accuracy and speed that leaves even their peers in awe.

At a set piece, like a free kick 30 metres from goal, opposition players will form themselves into a wall to block the ball's direct route to one half of the goal mouth. The goalkeeper will take responsibility for the other half, while also contending with a slower strike that may be lifted over the wall.

Despite these obstacles, Zidane, Beckham and Nedved continually trouble the world's best keepers, and often find the net. This is even more incredible when you consider that the kicker holds no real element of surprise, since the keeper is well aware the ball will eventually head his way.

To bend a soccer ball around a wall of players, keep it under the crossbar, and beat an outstretched keeper is to possess a keen awareness of the physics of ball flight. Whether they know it, these footballers have acquired such a knowledge through thousands of hours of practice.

When a ball is struck with spin, two general forces act upon its flight. The lift force makes it curve, while the drag slows its forward progress. However, their contribution is greatly affected by the air flowing about the ball at any given time.

When travelling slowly, air flows smoothly at the ball's surface (laminar flow), increasing the drag force. But at higher ball velocities, this airflow can become turbulent. This change cuts the drag, meaning a well-struck free kick not only travels at high speed, but does not slow down at the expected rate.

At a given spin rate, increasing a ball's velocity reduces the lift. Therefore, a fast-moving ball will not curl as much. This may appear beneficial for the keeper, but as the ball slows along its goalward path, its sideways deviation is suddenly more prominent.

When taking a 30-metre free kick, Beckham and company strike the 410 to 450-gram ball at around 25-30 metres a second, resulting in an acceleration of eight metres a second squared. The ball would also be spinning at the relatively high rate of eight to 10 revolutions a second. With this motion, the airflow about the ball will be turbulent, providing little drag, thereby better maintaining ball speed.

But at some location during its one-second journey, ball speed slows enough for the passing air to revert to laminar flow, abruptly raising drag. To bend it like Beckham, this point should coincide with the opposition's defensive wall because with drag force increasing and ball speed braking, the lift force can now play its part. As such, the ball will begin to curl around the wall of players. If struck just right, the ball could move by as much as four metres left or right, making any goalkeeper a spectator.

Justin Kemp is the exercise physiologist at Australian Catholic University. Damian Farrow is the skill acquisition specialist at the Australian Institute of Sport. Their sports science book Run Like You Stole Something is out through Allen and Unwin.