How does a soccer ball swerve? | Hacker News Reader

How does a soccer ball swerve?

newsoffice.mit.edu

84 pointssrikar21 comments

The paper referred to is here: The aerodynamics of the beautiful game by J.W.M. Bush

http://math.mit.edu/~bush/wordpress/wp-content/uploads/2013/...

  one can only be puzzled to note that striking a smooth   
  beach ball or an old, worn volleyball with spin has just 
  the opposite effect

I'm having great difficulty believing this. I've spun many beach balls and struck man old, worn volleyballs and have never seen a ball curve the wrong way. I've also dealt with many smooth ping-pong balls, and have even sanded off the dimples on golf balls to see the aerodynamic effect.

I'm not saying it's mathematically impossible, just that I didn't think it would be possible to find this effect in the real world. Maybe I've just never tried something smooth enough. Is there some reasonable home demonstration of this that I can try to prove it to myself? They say they did it with a latex band around a soccer ball?

It sounds like the effect depends on whether the surrounding airflow is turbulent or laminar. I believe that making an object smaller biases the airflow more to turbulence, so ping pong balls and golf balls may be too small to show the negative effect at their respective game-relevant velocities.

Also, just having a rough surface texture can be enough to cause turbulence, since it causes the air to stick more to the surface. In my experience, ping pong balls have a definite texture to them, and unless you polished that sanded-down golf ball to a mirror finish, it probably has a non-smooth texture too.

You should try it with a lightweight plastic football. They are smooth and really easy to spin so the effect is quite obvious. The more harder you strike it the more it will curl the wrong way.

This video (https://www.youtube.com/watch?v=W4tGaoSz14g) at 3:20 has a good summary of how the concept works in cricket. With a ball that is equally worn on both sides it is the seam (like the latex band) breaking up the air flow that causes the ball to curve the wrong way ('reverse swing' in cricket).

edit: and the position of the seam is obviously important in directing the force, which is why you will see this more often in football free kicks when the player can position the ball before taking the shot.

This effect is also seen in cricket where the new ball and the old ball do different things, and also the ball is actively manipulated (within rules).

Same thing happens with a baseball. Most professionals prefer "fresh" balls so they can have better control.

I put fresh in quotes because all baseballs are rubbed with what's colloquially called Mississippi Mud before every game.

Pirlo's free kick mentioned in the article: http://gfycat.com/CourteousHatefulBat

Roberto Carlos' free kick: http://www.youtube.com/watch?v=Pl0LHM-33Io

Don't forget this classic Carlos goal - https://www.youtube.com/watch?v=ZnXA0PoEE6Y

(Nitpick: Carlos is not a family name. Roberto Carlos is the full given name. His surname is da Silva Rocha).

Nitpickier: Carlos is not his family name, but there are people with Carlos as family name.

Notice how the ball in Pirlo's free kick is barely spinning, goes very fast and the path is very unpredictable. It's called a "Folha seca" and it exploits the turbulences to make the ball drop at the last moment. Very spectacular, but hard to score consistently.

It's spinning, and the amount of curve matches the amount of speed. The unpredictability comes from the fact that the axis is also spinning, but very slowly. It's hard to get the topspin and rotation on the ball, and still hit the target. But these guys train shots for a living.

If you have a few minutes have a look at Juninho, one of the best ever. https://www.youtube.com/watch?v=NesfdRNtwsE

Also don't forget the boots are designed to work the ball, for example http://en.wikipedia.org/wiki/Adidas_Predator

This is the same principle that makes a knuckleball drop unpredictably and very, very difficult to control. The way that a great pitcher can control a baseball is amazing to watch.

https://www.youtube.com/watch?v=iiSucVCuc8g

Hakan Calhanoglu's absurd free-kick from near the half-field line has to rank up there with Roberto Carlos's: http://www.highlighthub.com/hakan-calhanoglu-scores-amazing-...

Here's a slightly better quality version of Pirlo's free kick:

http://gfycat.com/CompetentDefensiveGosling

Football*

Friend of mine recently did a stand up about this, it's a little NSFW (c-bomb): https://www.youtube.com/watch?v=82mMuSZPohM#t=00m38s

https://www.youtube.com/watch?v=dF1NUposXVQ

I thought Hacker News were the only place of the world where soccer weren't a conversation theme.

Run to the hills!

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