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I was forwarded this article (wsj subscription required) in which author Jason Gay issues a warning: SPORTSWRITER DOING PHYSICS! Well, I've done a lot of sportswriting and I've done a lot of physics. I give Gay enormous credit, because his physics explanation was crystal clear to me.
Gay explains a recent paper in the American Journal of Physics, in which the authors show that gravity by itself is not sufficient to cause a football to spiral in the direction of its motion throughout its flight - conservation of angular momentum suggests that the initial angular momentum can't change without an external torque (which gravity does not supply). The football's path should bend in a parabola... but the point of the ball should always point in the same direction. Then air resistance should cause the ball to rotate end-over-end, "like a duck". Presumably he means a duck in flight who suddenly experiences cardiac arrest, but I won't quibble with sportswriters' analogies.
This paper's authors - Richard Price, William Moss, and TJ Gay - show that contrary to previous models, torque provided by air resistance continually changes direction. This changes the direction of the angular momentum such that the spiral always points tangent to the football's path. Cool. That makes sense.
Now, I'm an experimentalist... my next step would be to hire Patrick Mahomes (the reigning Super Bowl MVP, quarterback for the Kansas City Chiefs) to attempt to throw a spiral in NASA's enormous vacuum building.* If the paper is right, then even Patrick Mahomes should not be able to throw a proper spiral. Rather, though the football should spiral, the nose of the ball should continue to point in whatever sorta-upward direction it was spinning when the ball was released. Without air, there'd be no threat of a dead-duck motion, but the spiral shouldn't gracefully arc throughout the flight as in the NFL Films films.
* Or perhaps on the moon.
Anyone want to write this grant?
It's interesting to note that when the Chiefs travel to Denver at Mile High Stadium, Mahomes is able to throw 15 or 20 yards farther than closer to sea level with his spiral relatively unaffected. It seems that while air resistance is likely responsible for imparting torque on the ball and maintaining its spiral and orientation, it still has some detrimental effects for quarterbacks trying to hit a deep pass. It's likely that the air isn't thin enough (approaching that of a vacuum) for a noticeable change in the ergonomics of the throw. Before we put the reigning Super Bowl MVP in a vacuum, we should consider tracking the changes in the motion of the ball in increasingly thin air. Great read!
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