EMILY KWONG, HOST:
It's Super Bowl Sunday, and whether you are a Patriots or a Seahawks fan, the rules of physics equally apply. And one of the most beautiful things to watch in the sport is the spiral pass, where the football leaves a player's hand, and it tightly spins as it arcs through the air. Patriots quarterback Drake Maye recently threw a bunch in a blizzard. Here's one on CBS Sports.
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UNIDENTIFIED ANNOUNCER: Back to Maye, who throws it down the field. It's caught.
KWONG: And this pass - the spiral pass - has actually baffled physicists for decades, including Regina Barber, my friend and cohost on Short Wave, NPR's science podcast. Hey, Gina (ph).
REGINA BARBER, BYLINE: Hey, Em (ph).
KWONG: Why has the spiral pass confused physicists?
BARBER: Yeah. It's because of how the ball changes direction in the air. So in a spiral pass, the front nose of the football is pointed up when it leaves the quarterback's hand, and then it tilts down when it lands in the hands of the receiver.
KWONG: Oh, yeah.
BARBER: But that change in direction clashes with fundamental ideas in physics, which say that the ball should either, like, rotate in the air, like, tumble over itself, or the nose should stay straight up, like, as...
KWONG: Oh.
BARBER: ...It's going down. But it doesn't.
KWONG: That sounds like a serious mystery. Who has tried to figure it out?
BARBER: Yeah. So a couple years ago, I met this physicist who spent 20 years trying to figure out why - serious hobby research. His name was Tim Gay, and he's actually an experimental atomic physicist, but he does love football.
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.
BARBER: And so he set up a bunch of experiments in a wind tunnel to get to the bottom of this. And at first, he considered torque.
KWONG: Remind me what torque is.
BARBER: Yeah. So torque is related to a force making something rotate. So like, think of a door. The handle is pretty far away from the hinge or that axis of rotation.
KWONG: Yeah.
BARBER: But if that handle was in the middle, like a hobbit door, that door...
KWONG: (Laughter).
BARBER: ...Would be way harder to open. So these two situations experience different torque from you opening the door.
KWONG: Right. OK.
BARBER: So now let's think about something moving in the air.
KWONG: OK.
BARBER: So torque from the force of the air can cause the football to tumble over itself. You can throw it in a way that it won't tumble. But if you get the football to spin, that really helps a lot, like a gyroscope.
KWONG: Right. The football is also spinning as it's arcing. That's what makes it the spiral pass.
BARBER: Yeah. And for Tim, he realized that torque didn't fully explain why the football tips down in the path. So he had to call in backup. These two theoretical physicists, Richard Price from MIT and William Moss from Lawrence Livermore National Laboratory.
GAY: We spent the next three years yelling at each other over Zoom about the problem.
BARBER: And together, they came up with this idea, Em. OK, so what if, in addition to this tight spiral motion as you throw the football, there was this second kind of spinning - gyroscopic procession?
KWONG: Gyroscopic procession.
BARBER: Yeah.
KWONG: What's that?
BARBER: OK. So it's the second rotation that's happening. This actually happens to Earth. Earth spins about an axis, you know, a line through its center. But that axis actually circles around. Like, think of a spinning top, that toy, like, slowing down, wobbling. So for the football spinning in the air, the ball doesn't have really a table to support it, like a top.
KWONG: Right. Right.
BARBER: But that air is, like, making - is kind of that support.
GAY: For the ball in flight, the thing that defines vertical or the relevant line about which to process, is not gravity, but the on-rushing air.
KWONG: Wow. So the air is providing the point of support for the gyroscopic procession.
BARBER: Yeah. Yeah.
KWONG: OK.
BARBER: So he called up his buddies to test this idea.
GAY: 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. This is - we've got it. We've nailed it.
KWONG: (Laughter).
BARBER: And that's how, after over 20 years, the mystery of the spiral pass was solved. And this is a pass we will definitely see on TV tonight.
KWONG: Boom. Well, I will be watching.
BARBER: Yeah. Go Seahawks, my second favorite team after the Bills.
KWONG: Go physics.
BARBER: Yes. Go Physics.
KWONG: That is Regina Barber. We host Short Wave, NPR's science podcast. Check it out.
BARBER: Yeah.
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