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# How to Curve a Football

• Kicking the ball at an angle produces spin
• Smooth balls can go the other way
• Blame Isaac Newton

The title of the 2002 British romantic comedy, “Bend it like Beckham”, comes from the football players skill. In particular his ability to curl the ball past a wall of defenders from a free kick. But what’s the actual science behind curving the football into the upper corner of the net? Let’s have a deep dive into how to curve a football.

## Missed By a Mile

Every four years, at some point in the World Cup, there will be a free kick or sudden death penalty shoot out. And for some unknown reason, some players decide to shoot one kilometer over the goal. It’s not deliberate, it’s just that in trying to go over a wall of players and sink the ball into the upper corner of the net, past a lunging goalkeeper, things go pear shaped. Before calling for an optician, maybe we should have a look at their technique. As sports book news from the UK tells it, at this point, many punters will blame the new tournament ball as being somehow at fault.

## Curve a Football: Science Bitches!

Before betting on any match outcome with Betsson, let’s even the odds by having a look at the science. There are many people who believe that subtle variations in the balls surface structure effect the way it “flies”. For example, players claimed that it was much more difficult to control the smooth surfaced “Jabulani” ball used in the 2010 World Cup. This is when compared to the “Brazuca,” which had seams that are over 50 percent longer, thus making the ball less smooth and therefore easier to control. John Bush is a professor of applied mathematics at MIT and also the author of a recent article on the aerodynamics of soccer balls. He said, “The details of the flow of air around the ball are complicated, and in particular they depend on how rough the ball is. If the ball is perfectly smooth, it bends the wrong way.” By “wrong way’ he’s referring to the problem whereby a kick to curve a  football, struck in a similar manner, can actually curve in the opposite direction depending on the balls surface.

## Newton to the Rescue

This leads us to the “Magnus Effect”. It’s defined as “the force exerted on a rapidly spinning cylinder or sphere moving through air or another fluid in a direction at an angle to the axis of spin.” This was first described by Sir Isaac Newton after observing the flight of tennis balls. He noticed that topspin caused the ball to dip whereas backspin flattened it’s flight trajectory. You can witness this effect when football players take important kicks. They apply spin to the ball, thus creating rotation that then causes the ball to curve. A right footed player would normally brush the outside of the ball, thereby creating a right to left curve. A left footed player would do the opposite. depending on the desired direction, a player can use either side of his foot to strike the ball and thus impart spin in the required direction.

## Sports Physics is a Thing

We can see this in action whenever wee see Messi or, going back some years, Beckham taking a free kick. But for the Magnus Effect to be at it’s most dramatic, there needs to be some roughness on the ball. In fact, using a smooth ball will produce the opposite effect. As John Bush points out in his “The Aerodynamics of the Beautiful Game,” from the volume “Sports Physics,” published in France. (Yes people really do research these things!). “The fact is that the Magnus Effect can change sign,” Bush says. “People don’t generally appreciate that fact.” A smooth ball will, in fact, go to the opposite direction with the same kick. Pay attention cause here comes yet more science: between the spinning ball and the air there is what’s called a “boundary” layer. The rougher the ball, the more positive it’s curve direction. This boundary layer of air can be smooth or turbulent.

## Your Balls are so Smooth

Now depending on the rate of ball spin, there can occur a change from smooth to turbulent flow. And it’s this change that is influenced by the surface texture of the ball itself. Simply by changing the stitching pattern on the ball, can drastically change the direction of the intended kick. Over time, footballs have been getting smoother. The same way that football boots are evolving. And this probably accounts for those grossly wild shots that can occur during penalties or free kicks. And keep in mind, that all sports book sites in the UK are aware that competition balls are always slightly different in design from match to match.

## Curve a Football like a Dove without Wings

There is another interesting effect from trying to curve a football, but this time with a direct kick, head on and with zero spin. It’s called “pombo sem asa,” or “dove without wings.” by the Brazilians. The ball appears to “flutter” from side to side as it flies. John Bush again: “The peculiar motion of a fluttering free kick arises because the points of boundary-layer transition are different on opposite sides of the ball.” He goes on, “A ball that’s knuckling … is moving in response to the pressure distribution, which is constantly changing.” Well, now you know. Remember to bet on UK football with Betsson!

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