Getting a Kick out of Physics

I’ve written before about the physics of baseball, and in particular how curve balls curve. But, this year’s World Cup Futbol (that’s Eurospeak for soccer you know) got me thinking about the physics of soccer and scoring goals. And, since it is that season for some of you parents, youth soccer season, that is, here you go…

The famous goal that recently got headlines again, in terms of the underlying science, was actually scored 13 years ago. In the June 3, 1997, match between France and Brazil. In the last seconds of the game, Brazilian Roberto Carlos scored what was later named the impossible goal. He kicked a ball towards one end of the goal, apparently way off target, that banked at the last second and dropped into the net. It tied the game and changed the team’s fate that year. It has the stuff of the mysterious dropping fastball in baseball.

The secret was finally revealed this year by a team of French (of course) scientists.

Soccer balls tend to curve or arc when kicked for much the same reason that curve balls curve in baseball. When they are kicked (or thrown) they tend to rotate. So, one side of the ball is rotating in the same direction as the ball itself, while the other side of the ball is rotating against the direction of motion. The side of the ball that is rotating against direction of motion gets slowed down, just a little, by the resulting friction, and the path of the ball starts to curve to that side. This is called the Magnus effect, which explains the gently curving motion we typically see in tennis balls, golf balls, etc. What about the radical change in direction, like with old-fashioned spit-balls and this famous soccer goal?

Spit-balls change drastically because the shape of the ball is altered. This causes the spin to be asymmetrical, and things start to wobble. Just think of your washing machine on the spin cycle with the load out of balance. It just takes one wobble to throw things really out of whack. If your washing machine drum were not attached to the machine, it would take one strong turn, and crash through your laundry room wall.

Something quite different apparently happened with this soccer ball. No alteration of the ball was required. It was simply that the ball was kicked from so far away that the Magnus effect went into overdrive, so to speak. The forces on the right and left side of the ball got so out of balance that it started to wobble on its own, and the result was just like a dropping curve-ball.

The key was that the kick was from really far away, 35 meters to be exact, and only a player like Roberto Carlos could deliver that kick with enough speed for it to actually make it to the goal, estimated at 130 kilometers per hour. Hence, you almost never see this happen. You never see it in baseball or tennis, the ball does simply not get to travel far enough. And, it is obviously incredibly rare even in soccer, kickers rarely deliver 100+ km/hr kicks.

In fact, scientists did not know this was the outcome of the Magnus effect until seeing that goal, and spending many years since then studying it. It has now been replicated in the lab. We have yet to see if improvements in player training technology will yield more Carlos-style performances.