90th minute. Beckham sets the ball down. If he scores, England goes to the world cup. He takes a look at the wall and shoots. The ball bends in mid-air, tricks the keeper and goes in the top corner.

Ok, you don’t need to be Beckham to bend it like he does. But how did he do it?

As you probably heard, physicists like leave air friction out of their calculations. We do it because it makes things easier (like now-we-can-actually-calculate-stuff easier) and it’s usually not too wrong.

experiment

The curveball is one of those times when it really is wrong: you need air friction to explain it, and it’s not at all easy to calculate.

The key is the spin. For example, I’m right-footed (just like Beckham, btw…), so if I kick the ball with the inside of my foot, I make it spin counterclockwise as it moves forward.

So air passing left of the ball is moving in the same direction of the ball surface and gets dragged along, ending up a little behind it. The one on the right faces resistance from the ball first, then hits the air that was dragged behind the ball from the left. All these obstacles slow it down a lot, and it travels less space.

All in all, the spinning ball gives a net left-to-right push to the air. And, since (repeat with me) to every action corresponds an equal and opposite reaction, the air pushes the ball right to left, making it curve.

Clockwise spin is similar, but pushes the ball to the right. The real cool ones are the backspin that lifts the ball and the forward spin that makes it drop faster than it should (for which Andrea Pirlo is famous).

This phenomenon is called Magnus effect, in honor of Heinrich Gustav Magnus, the second scientist to describe it.

Great free-kick shooters know how to combine several rotations to make the ball curve just right, go around walls, and leave the keepers helpless. If you know the physics behind it, it’s even more impressive.

PS: Between the first posting of this and the translation, Physics Girl made another very cool video about this. As it turns out, the roughness of the ball is very important too: using a very smooth one reverses the effect!

 

Foto: Bellamy Free Kick, CC-BY-ND Simon Williams, via Flickr. Some rights reserved.

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