Zinedine Zidane's heartbreakingly perfect shot into the bottom left hand corner of the England net may have left goalkeeper David James scratching his head, but it was less of a surprise to those au fait with the Magnus effect. Whether they know it or not, the Magnus effect is what Zidane and England's own David Beckham have an instinctive feel for.
Strike the ball right of centre, as ZZ did, and if you get a good enough contact, the ball will spin anti-clockwise if looked at from above.
According to the Magnus effect, the anti-clockwise spin causes a drop in pressure on the left hand side of the ball and a rise in pressure on the right. The result is a ball that swerves to the left, and from the foot of a seasoned pro, can veer a metre or more from its otherwise straight trajectory.
Tests in windtunnels suggest curling balls may be even harder for goalies to intercept because the curving isn't even. The curl of the ball is related not only to how fast the ball is spinning, but how fast it is moving.
"When you kick the ball, the forward speed drops off because of aerodynamic drag, so as it approaches the goalkeeper, spin becomes more dominant and it starts to curve more," says Peter Bearman, an experimental aerodynamicist at London's Imperial College.
The effect may be more pronounced in Adidas's new "seamless" Roteiro ball, which is supposed to be smoother than balls used before. Whether the effect makes much difference on the pitch, when a free kick is all over in one tenth of a second, is arguable.
After getting your shot on target, curling the ball left or right is the simplest lesson at the school of free kicks. But merely spinning the ball about a vertical axis has its limitations.
Most footballers who curve their free kicks don't hit the ball too hard, because if the ball is still rising when it clears their opponent's defensive wall, it's unlikely to drop quickly enough to land in the back of the net.
This is where the likes of Zidane and Beckham leave others behind. They can use an extra trick - top spin - to let them blast a curving ball without skying it. Just as golfers typically put backspin on a ball to make it hang in the air for longer, footballers can do the opposite. By adding top spin to a football - a tough skill that involves almost rolling the foot over the ball - they can put more power into a shot without sending it over the crossbar.
"It's extremely difficult to kick a ball off the ground with top spin, but if you can do it, you can hit the ball much faster and it will still drop down," says Ken Bray, who studies football aerodynamics at Bath University.
