R&S: Bend it like Becca
There's no better way to understand the physics of football than by having two glamorous chemists do the explanations. Rebecca and Sarah prove there is SCIENCE in CENTRE CIRCLES.
R - God Sarah, I was down the pub last night being forced to watch the Liverpool - Chelsea game, and all the blokes were like "ooo, ahhh" and I was like, yeah right fwoar Frank's bum.
S - Too right. Over the years I've perfected the art of feigning an interest in the state of play when really I'm looking at their legs from a very different perspective.
R - Men are just awful though, I sat there for about 40 minutes listening to some theory on curving balls, I quote "kick the ball using the inside of your foot to curve the ball from right to left, or the outside for left to right" - Hello!
S - I know. Anyone would think that there was some skill involved. I mean surely it's not possible to curve a ball, more likely those watching have had a few too many pints! Do you think there really is any truth in it? It's got something to do with David whatshisname hasn't it?
R - Well apparently he's a pro, look at that dying seconds goal in the world cup qualifier against Greece. Really it's far more interesting than just football. It all relates back to a theory, now known as the Magnus Effect, discovered in 1852 by a German physicist, Gustav 
Magnus, who was trying to determine why spinning shells and bullets deflect to one side. Consider a ball that is spinning about an axis perpendicular to the flow of air across it, the air travels faster relative to the centre of the ball where the edge of the ball is moving in the same direction as the airflow, reducing the pressure. On the other side of the ball the opposite effect occurs, where the air travels slower relative to the centre of the ball.
S -
Ok, I take it all back. I know Newton once noted in 1672 how a tennis ball's flight was affected by spin. Thinking about it, two forces affect a spinning ball as it flies through the air: a lift and a drag force.
R - From a few simple force assumptions, you can calculate that the ball can deviate around 4m from its normal straight-line course. The lift force can be calculated from the velocity of the ball, say 70mph, and how many spins the ball makes per second, about 10, giving us a lift force of 3.5 Newtons. A football weighs between 410-450g, which means that it accelerates by about 8 metres per second and it is in flight for 1 second over its 30 metre trajectory!
S - Weren't studies undertaken in 1976 by Peter Bearman and colleagues from Imperial College, London on golf balls? They found that increasing the spin on a ball produced a higher lift coefficient and hence a bigger Magnus force. However, increasing the velocity at a given spin reduced the lift coefficient.
R - That's right. For a football this means that aslow-moving ball with lots of spin will have a larger sideways force than a fast-moving ball with the same spin. So as the ball slows down at the end of its trajectory, the curve becomes more pronounced.
S - All very impressive. Although I can't understand why anyone would want to bend the ball when you could kick it in a perfectly straight line? Is it not an excuse for boys with poor directional aim? Surely if everyone on the pitch anticipates you bending the ball, the element of surprise would come from kicking it in a straight line?
R - Precisely! On Mars, due to its low-pressure atmosphere, a football would swerve in the opposite direction to the way it goes on Earth. Goalkeepers beware in outer space, although even Beckham can't kick the ball that far!
