In American football, some passes are caught and some are dropped, but all of them wobble as they fly.
Coiled pig skins tend to visibly wobble at a slow or fast ratedepending on how the ball is thrown, researchers report online June 23 in the ASME Open Journal of Engineering. That wobble, and to a lesser extent the rotation of the Earth and the spin of the ball, cause passes to go sideways, so don’t blame the quarterback entirely this coming season.
“The fact that [a football] it wobbles and the fact that it doesn’t go straight — those are the two big effects you see in a pass,” says mechanical engineer John Dzielski of the Stevens Institute of Technology in Hoboken, New Jersey.
Using physical equations from ballistic studies and data from wind tunnel experiments with soccer ballsDzielski and software engineer Mark Blackburn, also of the Stevens Institute, ran computer simulations of a spiraling football pass.
They found that a pigskin flying at about 27 meters per second with about 600 rotations per minute wobbled visibly one to five times per second. Wobble occurs when the spinning momentum of the ball interacts with a torsional force that acts to deflect the tip of the ball from the direction of flight. The faster wobble dominates when additional power is applied during throws by twisting the wrist or moving the arm laterally.
That wobble creates lift that pushes the ball sideways, which could shift the landing point by several meters, the team reports. The Earth’s rotation could also cause a pass to be displaced by several centimeters. And the Magnus effect, whereby a rotating projectile gets trapped by pockets of high- and low-pressure air, bending its trajectory, had twice that impact.
Dzielski and Blackburn are now interested in refining their simulations by developing an instrument to collect data from soccer balls in flight.