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This Scientist Reveals the Physics Behind the Spiral Pass After 20 Years

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Football on the ground, held by player wearing gloves on playfield. (Bernhard Lang/Getty Images)

If you’ve ever watched part of a professional football game, you’ve probably seen a tight spiral pass. They’re those perfect throws where the football leaves the player’s hand and neatly spins as it arcs through the air.

But those passes seem to defy fundamental physics.

And for a long time, scientists couldn’t figure out exactly why — until experimental atomic physicist Tim Gay cracked the case just a few years ago. His answer comes after two decades of hobby research and more than a couple of late-night shouting matches with two other physicists over Zoom.

Accidentally kicking off a mystery

Gay has always loved both football and physics. As a high schooler, he couldn’t help thinking about the sport through a scientific lens, asking questions about the shape of the football and how players were able to execute passes seamlessly.

He loved the sport so much that, as a physics professor at the University of Nebraska-Lincoln, Gay gave one-minute lectures for packed stadiums in the middle of football games.

These lectures caught the attention of Nobel Laureate Bill Phillips, who invited Gay to give a lecture on physics and football at the National Institute of Standards and Technology in 2000.

After Gay finished his talk, Phillips stood up to ask a question.

“I have been to enough meetings with Bill that I knew that if he stood up and asked a question, the speaker had probably screwed something up,” Gay says. “So I was a little petrified.”

Phillips wanted to know why, in a tight spiral pass, the front nose of the ballpoints up when it leaves the quarterback’s hand and then tilts down when it lands in the hands of a receiver. It was puzzling because fundamental physics suggests that the ball should either rotate in the air or just stay mostly upright.

But it doesn’t.

So when Gay heard the question, he racked his brain for an answer until he finally looked at Bill and said, “I have no idea!”

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Journeying through a few false starts

As Gay began searching, he hit roadblock after roadblock. There were papers on the subject, but none of them told the whole story.

Some researchers thought the football might act like a perfectly upright spinning top. In the case of the top, its axis is along an invisible vertical line. The top will return to that vertical even if you tap the top so the axis momentarily moves.

But that’s not what happens when a football flies through the air.

“When you throw a football, it starts out vertical,” Gay says. “But it’s not like you perturb it with a tap. It’s like there’s an increasing force that’s continuing to try to push it either up or down.”

Instead of the front of the football remaining upward at the end of the pass, it points to the receiver.

Other papers tried to explain this inconsistency away with air resistance or air drag. But, among other things, this theory relies on the football being asymmetrical like a weathervane, which it is not. So, Gay found that even air resistance couldn’t completely account for what was happening on the field.

So, while he knew that air resistance was likely another piece in the problem’s solution, Gay also knew that he had to press onward with his research.

Twenty years later, the end zone is in sight

As he continued searching for the answer, Gay enlisted the help of two other physicists, Richard Price at MIT and William Moss at Lawrence Livermore National Laboratory.

“We spent the next three years yelling at each other over Zoom about the problem,” Gay says.

Until one day, Gay started to wonder about another important concept: torque, or how much a force makes an object rotate.

For example, if you throw a pencil across the room, the torque from the thrower makes it flip over itself in the air as it flies. But in a forward pass, “it seems to be causing the ball to tilt down,” Gay says.

So Gay, Price and Moss knew this was only a partial explanation and wondered if there might be another kind of rotation involved: gyroscopic precession.

The concept describes the way the axis of something — like a spinning top or a football — makes a cone shape as it spirals. In the case of a spinning top, it circles around an invisible vertical line that runs through its point of support due to gravity.

But for the football, that line is defined by the airflow around the ball as it travels.

Gay, Price and Moss did theoretical calculations and computer simulations to put the theory to the test.

“It all clicked,” Gay says.

After twenty years of working late nights around his full-time teaching and research job, Gay could finally close the case of the tight spiral pass.

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This episode was produced by Rachel Carlson. It was edited by Rebecca Ramirez. Brit Hanson checked the facts. Gilly Moon was the audio engineer.

Copyright 2024 NPR. To see more, visit https://www.npr.org.

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