Short Wave - A (Monday Night) Football Mystery

Episode Date: September 1, 2025

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 [e-mail] 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.See 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

You're listening to Shortwave from NPR. 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. As a physicist, I tend to look at everything I observe through a physics lens, and that held true for football as well.

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, Why do they make the helmet that way? Why is the ball shaped that way? But more than helmets or footballs, there was one elegant move to the game that he just couldn't stop thinking about.

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? 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. 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.

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&A. 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.

in why when a quarterback throws a tight spiral pass, it turns over. 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 palpitation, and sweaty palms, as he racked his brain for why the ball tilted down.

Before he finally looked at Bill and said, I don't know. I have no idea. So, of course, Tim starts searching, but he keeps running into one very big roadblock. There were a fair number of papers in the literature about this phenomenon, and it turned out they weren't correct. 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 Shortwave, the science podcast from NPR. Okay, 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 in the Chief's plane or the bills, every team that plays obeys Newton's laws of motion. 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. 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... 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. And in football, you can see this clearly when two players on opposite teams run into each other. A good example would be when a fast defensive end wipes out a quarterback who's been standing in the pocket trying to pass. 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, are running defensive linemen and are quarterback?

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

Well, he's generally smaller than our defensive linemen. And he's more easily accelerated. So he's the guy that goes flying through the air after the hit. 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. So whenever you have a body moving through space,

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. 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.

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. And that sounds good, and you say, well, that's what's going on with the football. The football is spinning. it's always going to try to maintain its axis along the direction that it's moving. The problem is that's not really what's happening with the football. 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. And so, if the front of the ball should be pushed up, due to the air drag. 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 weather vein, lining up with the direction of the wind.

That has two problems with it. One is a football is not a weather vein. A weather vein is asymmetric. 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, which you were doing. 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. A football in a wind tunnel doesn't align with the wind like a weather vein would, which means there had to be something else making a 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... Instead, it seems to be causing the ball to tilt down.

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. We spent the next three years yelling at each other over Zoom about the problem. Together, they thought of another potential solution. What if, in addition to this tight spiral motion of a throne football,

there might be another kind of spinning, gyroscopic procession. 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 procession 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.

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. With gyroscopic procession, Tim thought he was onto something. So the three physicists came back together and... 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 procession, and it all clicked. And we said, yeah, we've got it. We've nailed it. 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 Hansen. Gilly Moon was the audio engineer. I'm Regina Barber. Thank you for listening to Shortwave from NPR.