Science explains inexplicable goal

When Brazil’s Roberto Carlos struck a soccer ball from 35 meters against France and it appeared to change direction in mid-air on its way into the goal, the phrase “defied physics” was uttered by numerous pundits.

Now physicists say it did nothing of the sorts.

Four French scientists (Guillaume Dupeux, Anne Le Goff, David Quéré and Christophe Clanet) have just published a paper that shows it’s actually perfectly normal for a ball to travel in such a manner when it is struck with enough force. It’s just that normally the ball is stopped in its progress before that pattern becomes visible.

Clanet and Quere were originally researching bullet trajectories but began looking into the movement of spheres. They carried out a series of tests with plastic balls in water, making sure the balls had the same density as the water to create conditions as near to perfect as possible.

It turned out that, because it is a spinning sphere, a ball seemingly moving in one direction will eventually move in an increasingly tighter spiral. The reason we don’t normally see this with balls in the air is that gravity takes effect and limits the curve. However, if the ball is initially propelled with enough force, it takes longer for the gravity to make a noticeable difference.

What made Carlos’ shot so spectacular is that shooting from such a distance meant it traveled far enough without being blocked (either by a player, the goalpost, the goal net or another obstacle) that far more of the spiraling movement could be seen.

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