A (Monday Night) Football Mystery

Monday night football is back! What better way to celebrate than a close look at some of the physics powering the sport? Specifically, the spiral pass. 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. Those passes can seem to defy fundamental physics — and for a long time, scientists couldn't figure out exactly why. That is, until experimental atomic physicist Tim Gay cracked the case within the last few years. His answer comes after two decades of hobby research and more than a couple late night shouting matches with two other physicists over Zoom. (encore) Want more stories on sports science? Email us at [email protected] mailto:[email protected]. Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave http://plus.npr.org/shortwave. See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

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• 0:16Speaker 01

  You're listening to Short Wave from NPR.

• 0:21Regina Barber?

  Football season is in full swing, shortwavers. It's probably the physicist in me, but when I think of football, I can't help but think of air resistance, all the different forces and laws of physics happening as the game plays out, not unlike Tim Gay.

• 0:36Speaker 03

  As a physicist, I tend to look at everything I observe through a physics lens, and that held true for football as well.

• 0:44Regina Barber?

  Tim's an experimental atomic physicist with a passion for football. And even as a high schooler, Tim thought about the sport through a scientific lens. When he wondered-

• 0:53Speaker 03

  Why do they make the helmet that way? Why is the ball shaped that way?

• 0:58Regina Barber?

  But more than helmets or footballs, there was one elegant move to the game that he just couldn't stop thinking about.

• 1:06Speaker 03

  These tight spiral passes, and why balls sometimes in a punt? For example, why do they turn over sometimes and why do they not turn over?

• 1:16Regina Barber?

  You know the spiral pass? Those perfect throws where the football leaves a player's hand and tightly spins as it arcs through the air? So as an adult, like any physicist would, Tim looked to science for the answers, but quickly realized that studying a seemingly simple part of the game, like the flight of a ball through the air, raised loads of other questions.

• 1:38Speaker 03

  On occasion, for example, in a kickoff, the ball will actually rise. If you were kicking a football in vacuum, it would simply be a parabolic arc. But with air, again, you get interesting effects like lift. The ball can actually curve up instead of curving down for a brief moment as the aerodynamic forces push it up.

• 2:06Regina Barber?

  And Tim, asking and answering so many questions about physics and football, caught the attention of Nobel laureate Bill Phillips. And Bill was so intrigued by Tim's work that he invited Tim to give a Christmas lecture on physics and football at the National Institute of Standards and Technology. Everything seemed to be going well, up until the end of the talk during Q and A.

• 2:29Speaker 03

  Bill Phillips had been sitting in the front row, stood up and said, well, I've got a question. And I had 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. So I was a little petrified. He said, I really don't understand why, when a quarterback throws a tight spiral pass, it turns over.

• 2:53Regina Barber?

  In other words, Bill wanted to know why, in these tight spiral passes, the front nose of the football points up when it leaves the quarterback's hand, and then tilts down when it lands in the hands of the receiver. Fundamental ideas in physics tell us that the ball should either rotate in the air or just stay mostly upright. But it doesn't. And so, when Bill asked this question, Tim, I assume, had heart palpitations and sweaty palms, as he racked his brain for why the ball tilted down, before he finally looked at Bill and said.

• 3:24Speaker 03

  I don't know. I have no idea.

• 3:27Regina Barber?

  So of course, Tim starts searching. But he keeps running into one very big roadblock.

• 3:33Speaker 03

  There were a fair number of papers in the literature about this phenomenon. And it turned out, they weren't correct.

• 3:45Regina Barber?

  So today on the show, we're celebrating the return of Monday Night Football the best way we know how. We kick off with a short physics lesson before unraveling this football mystery that has plagued him for years. I'm Regina Barber, and you're listening to Short Wave, the science podcast from NPR.

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• 5:39Regina Barber?

  OK, before we get back to the mystery, I'm not going to pass up an opportunity to share some fundamental physics with you. Because regardless of whether it's the Chiefs playing or the Bills, every team that plays obeys Newton's laws of motion.

• 5:54Speaker 03

  When you're watching a game of football, basically, everything you see is rooted in classical physics, and more specifically, Newton's three laws of motion.

• 6:06Regina Barber?

  Newton's first law of motion says an object in constant motion will remain in motion. A force like you physically pushing is needed to get something to move. Another force like friction is needed to stop it from moving. And Tim says that this law-

• 6:20Speaker 03

  It illustrates Newton's genius. Because in Newton's time, and indeed in our time, when you, you know, experience real life, things naturally slow down. If you're playing billiards, you hit the cue ball and it rattles around for a while, but it slows and stops. Or if you're driving a car and you take your foot off the gas, it will slowly come to a stop or run into a tree and stop, but it'll stop.

• 6:49Regina Barber?

  And in football, you can see this clearly when two players on opposite teams run into each other.

• 6:55Speaker 03

  A good example would be when a fast defensive end wipes out a quarterback who's been standing in the pocket trying to pass.

• 7:05Regina Barber?

  This example also nicely illustrates Newton's third law, which says that each force has an equal and opposite force. That's because, kind of counterintuitively, our running defensive lineman and our quarterback-

• 7:18Speaker 03

  The forces that they exert on each other are equal. It's surprising, but even the smaller quarterback exerts the same force on the big, fast lineman as the lineman exerts on him.

• 7:30Regina Barber?

  But in all likelihood, the quarterback is going to go flying through the air once the two make contact. The reason is because of Newton's second law. Newton's second law is summed up by an equation. Force equals mass times acceleration. In other words, force is related to both the mass of an object and how fast the object is speeding up or slowing down, meaning its acceleration. And our quarterback, well, he's generally smaller than our defensive lineman.

• 7:57Speaker 03

  And he's more easily accelerated. So he's the guy that goes flying through the air after the hit.

• 8:03Regina Barber?

  But what about the football itself? Why does the ball travel through the air in certain ways? These questions brought Tim to the concept of air resistance or drag, which affects everything, even how far players can throw or kick the ball at different altitudes.

• 8:20Speaker 03

  So whenever you have a body moving through space-

• 8:26Speaker 03

  If there's air involved, in other words, if you're not doing this in a vacuum, the air will resist the motion of that body and ultimately will slow it down.

• 8:37Regina Barber?

  The thing that causes the ball to fall back to earth is gravity, a downward force, but air drag can be just as big of a force. So that brings us back to our mystery. Why does the nose of the football change direction as it flies through the air? Why does it go from pointing up when thrown and down when caught? It turns out, this was a much harder problem to solve than Tim and other physicists had expected.

• 9:02Regina Barber?

  Some physicists had tried to figure it out, and the theories that did exist weren't fully accurate. Like, some researchers thought that the football was like a perfectly upright spinning top. The axis is vertical, and even if you tap the top, so the axis moves, friction will restore the vertical axis.

• 9:19Speaker 03

  And that sounds good, and you say, well, that's what's going on with the football. The football is spinning, and so it's always going to try to maintain its axis along the direction that it's moving.

• 9:32Regina Barber?

  The problem is, that's not really what's happening with the football.

• 9:36Speaker 03

  When you throw a football, it starts out vertical, and 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. Typically, the front of the ball would be pushed up due to the air drag.

• 9:53Regina Barber?

  And so, if the front of the ball should be pushed up by drag, how is it ending up turned down, pointing at the receiver? Tim still knew air drag was important. He just had to figure out how. Other theories did mention air resistance and said maybe the football was like a weathervein, lining up with the direction of the wind.

• 10:13Speaker 03

  That has two problems with it. One is, a football is not a weathervein. A weathervein is asymmetric.

• 10:23Speaker 03

  And the wind pushes more effectively on the back than the front, and so that lines the thing up. But that's not true of a football. A football is front-back symmetric. And it also turns out that if you do an experiment in a wind tunnel.

• 10:37Regina Barber?

  Which you were doing.

• 10:38Speaker 03

  Which I did. It turns out that the darn ball lines up perpendicular to the wind. So the axis of the ball is perpendicular to the direction of the wind.

• 10:48Regina Barber?

  A football in a wind tunnel doesn't align with the wind like a weather vane would, which means there had to be something else making it turn. So Tim knew that the rotation of the ball and air drag were both important, but they didn't completely answer his question. He started to wonder about another important concept: torque, which is how much a force makes an object rotate. Like, if you throw a pencil across the room, the torque from you throwing it causes it to flip over itself in the air as it flies. So in theory, the torque from a throw and maybe even the air itself should cause the football to tumble over itself. And that makes sense. If the ball isn't spinning, but in a spiraling forward pass.

• 11:29Speaker 03

  Instead, it seems to be causing the ball to tilt down.

• 11:32Regina Barber?

  That perfect spiral travels in a beautiful projectile motion. So again, there was only a partial explanation. Tim decided it was time to call in for backup. So he brought in two theoretical physicists, Richard Price from MIT and William Moss from Lawrence Livermore National Laboratory.

• 11:51Speaker 03

  We spent the next three years yelling at each other over Zoom about the problem.

• 11:57Regina Barber?

  Together, they thought of another potential solution. What if, in addition to this tight spiral motion of a thrown football, there might be another kind of spinning? Gyroscopic precession.

• 12:12Regina Barber?

  It's basically a second rotation. Think back to that spinning top from earlier and imagine it's no longer perfectly vertical, but slightly tilted. This top is now also tracing out a circle. Gyroscopic precession describes the way the axis of a spinning top, or a football, makes a cone shape as the ball spirals really quickly. For example, are spinning top circles around an invisible vertical line running through its point of support due to gravity? But for a football flying through the air, gravity isn't a support point. It's the air flowing around the ball as it travels.

• 12:47Speaker 03

  For the ball in flight, the thing that defines vertical, or the relevant line about which to process, is not gravity, but the onrushing air.

• 12:59Regina Barber?

  With gyroscopic precession, Tim thought he was on to something. So the three physicists came back together and...

• 13:06Speaker 03

  Richard had done a theoretical calculation and Willie did a computer simulation, and they matched perfectly. And I brought in this idea of the gyroscopic precession, and it all clicked. And we said, yeah, this is, we've got it. We've nailed it.

• 13:22Regina Barber?

  And with that, after 20 years of working late nights on and off around his real job, Tim could finally put the mystery to bed. This episode was produced by Rachel Carlson, edited by our showrunner and team coach, Rebecca Ramirez, and fact-checked by Britt Hanson. Gilli Moon was the audio engineer. I'm Regina Barber. Thank you for listening to Short Wave from NPR.