The Magnus Effect

We were at this event the other day, and I saw a guy kick a ball. I was distraught over the curved trajectory of the ball. How do they do that? And since you can't get good at soccer using physics, I'd better consult a friend. Kyle! Kyle, I want to be able to bend it.

Okay, so look - I bet you that I can take this ball set it in that corner and bend it into that goal.

So I'm skeptical about this because you can even see the opening up the goal - all you can see is the goal post. And from the trajectory that you chase from the coordinates of the... It's magic!

No, its physics. Do it again. How did you do that?!? Well it's a pretty standard technique in soccer. You use the inside your foot to generate spin on the ball. That spin is actually make the ball curve into the goal.

Let's take a look at how that happens. As Kyle kicks the ball with his right foot on the right side he imparts a spin on the ball. The ball traveling in this direction means air is flowing across it in the opposite direction. Right near the ball there is a thin layer air dragged around. As oncoming streams of air pass the ball side moving in the same direction as a spinning ball is accelerated, follows the curve have the ball, and is deflected off to the right. The side that is moving against the spinning ball meets opposing air and can't continue around the ball. It slows down and goes straight. We end up getting a net flow of air to the right, and the ball must move left.

But wait, that Newton's third law.

Yes. Thank you Kyle. Just like a rocket if you push a gas one direction namely downwards out the rocket, then the rocket have to go up.

So what you're saying is my shot is like a rocket.

It's... thank you, Kyle. Like the rocket as the air moves in one direction the ball has to go the other direction. Also since the air's slowing down the right side you get to build up of air which create the higher pressure on that side and pushes the ball to the left. The airflow is extremely important you could not make this kick in a vacuum. It's not some weird thing the ball does to itself to curve - you need air flowing across the ball to push the ball. This is called the Magnus effect.

So you can use this phenomenon do all sorts of shots in soccer. As we already saw I can use the inside my foot to make the ball curve left. I can also use the outside of my foot to make the ball spin the other way, and then it curves right. If I wanted to, I could even hit the bottom of the ball, give it backspin, and send it up into the air.

With a backspin its actually gonna curve upwards.

Finally, if I want to get real tricky with it, I could put no spin on it all, like a knuckleball. When this happens you get turbulence, unpredictable both sides the ball, and the keeper doesn't know where it's going.

Should we try the knuckleball?

That one's gonna be a little bit tougher.

That's the physics the soccer. Thank you for your help, Kyle. If you wanna see some videos of some incredible soccer players bending the ball, check out these links.