DONāT blame the goalkeeper for failing to block a soccer ball with a spin on it. Likewise in baseball, itās not fair to trash the batter if he misses a curve ball. The human visual system just isnāt equipped to track the curved course of a fast-spinning ball, says Cathy Craig, a psychologist at Queenās University Belfast, UK.
Craig was inspired to investigate after seeing Roberto Carlos score a mesmerising goal for Brazil in 1997. āEverybody seemed to think it was going wide,ā she says. āThen it curved in at the last minute.ā
She decided to test if experienced players could follow the trajectories of balls with side spin. She asked players to say whether balls would end up in the goal when they watched a virtual reality display that simulated shots with a spin of 600 revolutions per minute. Even professionals couldnāt predict how the spin would influence the ballās trajectory (Naturwissenschaften, vol 93, p 97).
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The side spin on the ball produces something called a Magnus force, which accelerates the ball in a direction that we simply are unable to process, says Craig. We can anticipate the effect of gravity on moving objects, as that has been important in evolution. āBut spinning balls donāt occur naturally. Why would nature bother having a visual system thatās adapted to them?ā says Craig.
