The physics of football

24 June 2014

Interview with

Hugh Hunt, University of Cambridge

We're talking about the science behind the World Cup this week and so far we've A footballheard from a psychologist studying chanting and an economist studying behaviour on the pitch. But football isn't just about the players (however much you might blame Rooney or Gerrard for England's recent disasters). A lot of it comes down to plain old physics.

To find out more Kate Lamble headed down to Coleridge Community College with the Professor of Spin, Cambridge University's Hugh Hunt.

Hugh -   If you spin a ball and it bounces, you can get the ball to spin left, right, all sorts of things.  That's the kind of thing that happens in cricket when a ball bounces and it deviates off the pitch.  In football, that happens too.  But bending the ball in midair is the best part of spin on a football.  I have a ball here and I can just throw it and it pretty much moves in a straight line.  Then I'll put a little bit of spin on it.  Did you see it?

Kate -   Yeah.

Hugh -   Everyone has got to have a go.  I will put some spin on it.  The more spin you put on it, you see that curved around?  You weren't expecting that to come to you because it curved off...

Student - We're just passing the ball, but as we're doing it, we're spinning it so it will go on different directions and so, we don't know where it's going to end up. 

Hugh - Jack, I'm going to spin this ball to you and you don't quite know whether it's going to go to the left or to the right.  you judged that well but imagine if it's coming at you much faster and it spins and you're the goalie and you jump in the wrong direction.  Then you've gone and let the penalty through.

Kate -   If you twist the ball, why does that make it move in the air to the left or to the right?

Hugh -   When the airplane goes through the air, it generates lift because the air is deflected downwards.  The wing pushes air downwards.  If you've got a spinning ball, it pushes air around it as well.  The more it spins, the more the air is pushed around.  Isaac Newton's third law, 'every action has an equal and opposite reaction'.  If you push on the air, the air has got to push on you.

Kate -   So, as the spinning ball drags more air around one side of it, the air pushes back on the ball and forces it to change course.

Hugh -   I've got a couple of paper cups, the sort of cups that you'd normally throw in a bin after you've used them.  I've joined the cups together in the middle so it looks like an hour glass and I've got a rubber band which I've cut in half and I wrapped the rubber band around the cups.  If I pull the rubber band back...

Kate -   That's the reaction.  What happened?

Student -   It spun around in the air and shot forward.

Student -   But it did quite slowly so it kind of - it wasn't like he threw it.  It just like flew through the air.

Hugh -   If I do that same thing without any falls quite quickly.  But as soon as I put a bit of spin on it, back spin in this hovers up.  Now, what would happen if I put some top spin on it?  Well, let's try it... and see, it curved down really, really quickly.

Kate -   Looking at that, do any of you guys see how that would work in football?

Student -   If you kick the football and you gave it down spin then people might think you are kicking it quite far, but actually, you are kicking it quite close so that you could be kicking it to a team member that was closer and all the other players would run in the other direction.

Hugh -   Precisely right.  Now supposing, you guys stand in the middle around here, okay.

Kate -   We're forming a defensive free kick wall here.

Hugh -   Now, if I threw it over the top of you, it's not going to get behind the line.  If I put some top spin on, then I can get the ball to... you see how it went down very quickly.  The spin made it curve down so the ball goes over you and you think, "That's fine.  It's not going to go into the goal" but it curves down and goes into the goal.

Jack -   Yeah, it does help in like situations if you're on a one-on-one so the goalie don't know where you're like going.

Hugh -   Unfortunately, the goalie is pretty good and looking at what you're doing.  The goalie thinks, "He's coming on this side.  I know where it's going to go."  And you want to be unpredictable.  You have to disguise what you're doing with your foot.  The other thing you can do is actually, you can kick the ball dead straight with no spin.  And then you have this really weird thing that happens which is called "knuckling".  Knuckling is a baseball term.  If you have a ball in your hand like this with your knuckles, it's the knuckle ball if you can see your knuckles and you throw it with no spin.  What happens then is, it weaves around rather randomly and it's unpredictable.  And that's what you want, kicking a penalty.  Actually, it's best not to put any spin on it at all.  It's you against the goalie.  It might go left.  It might go right.  It might go left and then right.

Kate -   So spin is mostly following things like free kicks.  When you want to get over the wall directed towards where you want to go, use the spin to make it being unpredictable but you know what it's going to do and knuckling can hit straight on penalties, make it as random as possible so that even you don't where it's going to end up.

Hugh -   There's a critical speed for knuckling which means that if the ball is going at just that right speed, it's going to have maximum amount of deviation.  This year, the new World Cup ball, it's a slower speed than it was in the previous World Cup which means that you might find a few of these penalties, are kicked slower than they were in previous World Cups so that knuckling has its maximum effect.

Kate -   So Jack, you mentioned that you were using spin on penalties when you're one on one in a goalie.  Does that change your opinion on what you're going to do?

Jack -   Sort of in penalties, yeah because if you get like a strong shot, obviously it would go straight.  But say, if you're not in a penalty, you'd use the spin because you'd have more like around the goalie and they won't know where to go.

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