Graphic: Curve it like Ronaldo

Legs wide, four or five yards from the ball, hands by his sides as he plots the trajectory. Then a couple of steps forward, studs springing off the turf. The left foot plants itself inches from the ball, the left arm points sideways, his body tilts and the toes in his right foot connect with the ball, centre right. The missile launches, gathering pace, swerving a little one way then a lot the other, and by the time the goalkeeper has figured out the flight it is milliseconds away from slapping the net cord.

On Wednesday, Craig Gordon, the Sunderland and Scotland goalkeeper, merely gawped as the ball roared into his goal. It was Cristiano Ronaldo’s second spectacular free kick in a month after his bullet against Sporting Lisbon. Goalkeepers are baffled and beaten.

In March, when Manchester United faced a Europe XI, a free kick by Ronaldo from a central position drifted left then right and Santiago Cañizares, the goalkeeper, was duped. In February 2006, when United beat Fulham at Old Trafford, Antti Niemi was in a good position to save the kick but was so deceived by the movement that he went to his right and the ball sailed past his left shoulder.

The combination of being unable to determine the trajectory and the power of the shot reduces the reaction time of goalkeepers, so Ronaldo does not always have to find the top corner to score. Time for a Bend it like Ronaldo film? His style is very different. David Beckham’s technique is traditional while the Portuguese makes the most of modern technology.

Beckham’s run-ups are longer and from an acute angle, which allows him to generate extra spin and control as he massages his instep around the ball, striking it well wide of the centre to send it spinning.

Why is Beckham so effective? Talent, practice and the Magnus force. That is the name for the effect by which spin reduces the air pressure on one side of the ball and increases it on another, making it swerve.

Physicists have estimated that over 30 metres, a ball can deviate upwards or sidewards from its normal straight course by as much as four metres. Dr Ken Bray, a sports scientist at the University of Bath, found that Beckham kicks the ball at 70mph and makes it spin ten times a second. By angling the spin direction, Beckham pulls the ball down as well as across, using top spin, like a tennis player, so the ball slows and dips.

Ronaldo’s shots do not sink, spin or slow as much. They are the football equivalent of baseball’s knuckleball: a pitch that seems to dance randomly in mid-flight because when a ball rotates little, its trajectory is more disrupted by the flow of the air around it. Today’s smooth footballs are aerodynamically closer to baseballs than past designs because they have fewer panels and seams.

The explosive nature of Ronaldo’s free kicks may be partly explained by the Barclays Premier League’s balls, which contain a layer of nitrogen-expanded foam. “It acts like a trampoline, propelling the ball quickly off the foot,” a Nike spokesman said.

The winger’s boots are light and thin, giving him extra feel. Ronaldo’s feet are size nine, but he wears smaller boots because he believes it improves his touch.

Given the difficulty of hitting a ball accurately when it is lashed with the laces, Ronaldo gets it wrong more often than not. In 4½ seasons, he has scored only four goals from free kicks in the top flight; in 2001-02 alone, Beckham scored four. That represented a conversion rate of 15 per cent. In 2005-06 and 2006-07, Ronaldo’s success rate was 5 per cent, or one goal every 20 attempts. Overall in the Premier League, about one in ten free kicks finds the net. So while Ronaldo may be brilliant, he is also below average – although his victims, such as Gordon, would surely disagree.