Bend it like Beckham: why "easy" balls are so hard to catch

Why "easy" balls are so hard to catch...
13 April 2008


Crowds at cricket and baseball games often stare increduously as fielders fumble what was, at first glance, a dead easy catch: the ball was chipped straight up in the air off the bat, and it should just be a case of putting yourself beneath it surely? 

Not so, say US researchers Alan Nathan and Terry Bahill, who have modelled the trajectories of balls like these and published, in this month's American Journal of Physics, why they're so hard to catch.

The reason is that most "pop-up balls" as they're known have enormous amounts of backspin, and this causes the ball to move in a difficult-to-predict manner, because as it spins it drags a sheath of air with it, which causes the ball to follow a curved course. 

That doesn't sound too bad, but the effect, known as the Magnus Force (after the German scientist Heinrich Gustav Magnus who studied the aerodynamics of spinning cylinders and spheres in the 1850s) is not constant throughout the ball's flight. 

When the ball travels fast, the air finds it hard to stick to the surface so the Magnus effect is minimal. 

But as the ball slows down, and a layer of air sticks to it, the bending effect becomes highly pronounced, and the ball can change direction quite suddenly.

To find out whether they were on the right track the researchers ran computer simulations of how a player might react to balls displaying these behaviours. 

They found that their results were a close match with the indecisive dance exhibited by players attempting to catch balls like these.

The findings agree with previous studies of footballers facing corner kicks from the likes of David Beckham and Wayne Rooney. 

Spinning balls behave identically under these circumstances and are equally hard for goalkeepers to save. 

The reason, scientists think, is that although the human brain has evolved very well to anticipate the effects of gravity, because spinning objects are unusual in nature, we have limited cognitive abilities to compensate for them. 

And who'd have though David Beckham or Wayne Rooney were physics geniuses to boot...?


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