Watch me explain the science of soccer on CBC Newsworld, July 22, 2007:

Hundreds of millions of soccer fans are now tuning in to the World Cup, where they’ll see, not just exciting games, but a fascinating display of scientific principles.

Let’s start with the ball. The basic physics haven’t changed: when a ball is kicked, the side that’s kicked flattens out, then snaps back into shape, hurling the ball away. But this year’s ball, the Fevernova, is new.

The Fevernova is covered with polyurethane instead of leather. Beneath that is syntactic foam, consisting of equal-sized, highly elastic gas-filled microcells, designed to give the ball more bounce, so it travels faster, and ensure the same amount of energy is returned wherever the ball is kicked, making the ball’s flight more accurate and predictable. A woven mesh surrounds the central rubber bladder, designed to keep the ball from deforming when kicked, so it won’t veer in flight. Adidas says the ball is 25 percent more precise and 10 percent faster than the ball used in the 1998 World Cup…but some players find it too light, springy, and hard to control.

How the ball travels depends on the position of the kicker’s foot relative to the center of the ball. If the foot is above the center of the ball, the ball will roll along the ground. If the foot hits in the center, the ball will skid across the ground until friction slows it to a roll. If the foot hits below the center of the ground, the ball becomes airborne.

A kicker can put spin on the ball to make it curve by kicking it off-center with the instep or outside of his foot. A right-footed kick will drift right to left when kicked with the instep and left to right when kicked with the outside of the foot.

A spinning ball curves primarily because of the Magnus Effect. Spinning the ball on an axis perpendicular to the ground causes the flow of air to be faster on one side than the other. This results in a different amount of push on one side than the other, and thus the ball curves in flight. Spinning the ball on the axis parallel to the ground actually causes the ball to fly straighter, because irregularities in the surface of the ball are averaged out. (A soccer ball kicked with no spin at all tends to bobble and weave.)

Soccer balls will often slow dramatically in flight. This is because in early flight, the ball is moving very quickly and is surrounded by a turbulent air flow. As the ball slows down, the turbulence vanishes and the airflow becomes smooth, which more than doubles drag, suddenly slowing the ball even more. Exactly when in the flight this transition from turbulent to smooth airflow happens is affected by the ball’s velocity, the spinning rate, and the surface seam pattern of the ball. Good players take this effect into account instinctively, using it to make seemingly impossible shots that dip at the last minute into the goal.

Even the best player’s best kick can go to waste if an assistant referee claims the player was offside–that is, that there were fewer than two players between himself and the goal (one of whom is typically the goalie) when the ball was passed to him. Research in the Netherlands found that linesmen in the Dutch professional league incorrectly judged 40 of 200 potential offside scenarios. That’s because the player furthest away appears closer to the goal to the assistant referee than the closer player. Unless the assistant referee is lined up with the second last defender as the attacker approaches, its easy to call someone on the far side of the defender offside when he really isn’t–or let someone who is closer get away with being offside when he really should be called for it.

The defense’s last hope is the skill of the goalie. In Liverpool, Mark Williams of John Moore’s University had both expert and amateur goalkeepers watch goalie’s-eye videos of players taking penalty shots, and move a joystick in the direction they expected the ball to go before the ball crossed the line. The expert goalies “saved” one-third of the penalties, while the amateurs saved only one-quarter. Further analysis showed that the amateur’s eyes tended to flick all over the kicker’s body, while experts focused on the kicker’s legs, picking up unconscious cues of which way the kicker would kick.

Like everything else in soccer, it all comes down to training and practice.

And, of course, science.

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