Unraveling the physics of one of America’s most popular sports

Crisp morning air, the start of the school year, and the return of football — no matter what your favorite part of the season is, fall is finally here.

Apart from being one of America’s favorite autumnal pastimes, football has some serious physics behind it — something Timothy Gay knows well as both a former college football player and now the Willa Cather Professor of Physics at the University of Nebraska-Lincoln.

In 1999, Gay launched his career as an expert in the physics of football, starring in video segments aired during Cornhusker home games where he explained physics concepts and how they manifest themselves on the field. Since then, he’s served as NFL Films’ “Football Physicist”; collaborated on an Emmy-winning documentary on the 1972 “Immaculate Reception,” one of the game’s most famous plays; written a book on the physics of football; and collaborated with theorists to explain “the paradox of the tight spiral pass.”

APS News, posing as a football coach seeking to enhance their team’s season with the power of physics, spoke with Gay about the game’s uniquely shaped ball, the advantages of different turf types, and more. This interview has been edited for length and clarity.

Tell me about the shape of the ball.

The first football games were played with essentially a rugby ball. That ball morphed slowly into the shape that we're familiar with today, for the express reason that it was easier to pass — coaches got the idea that passing might be a good strategic plan of attack. Up until that point, the game had been entirely running-based.

Now, you have a ball that’s easier to pass, and pass long. Quarterbacks like to throw a tight spiral pass, because it reduces drag on the ball, which means it can go farther, and it tends to stabilize the ball in flight, which means it’s less susceptible to random air drafts.

What makes American football so interesting is that the ball has two axes of symmetry with two different lengths. Its prolate shape makes it impossible to predict how it’s going to bounce, because you have the spin as well as the shape introducing uncertainty as to which path it will take.

My team can train in either Denver, Colorado or Miami, Florida. Does it matter?

The advantage of training in Denver is that you get used to not having enough air, so it helps develop stamina. A disadvantage is that it would make your kickers and passers think they can kick or throw farther than they could if they were going to play, for example, in the Super Bowl coming up in Santa Clara, California, which is basically at sea level. Depending on the conditions, a football will go about five yards further in Denver than it will in Santa Clara simply because of the atmospheric pressure.

Humidity does not have a big effect. You always hear baseball commentators saying, “It's really muggy tonight, so nobody can hit a home run,” but that turns out to be wrong; water molecules are lighter than nitrogen or oxygen molecules. Extreme cold makes the air denser, so the ball wouldn't go as far, but that turns out to be a small effect, too.

We can practice on a field with either synthetic turf or real grass. Which one should we pick?

The gold standard is natural grass which is well-maintained. Many NFL stadiums have two natural grass fields so players can practice on one while they get rid of all the divots on the other.

AstroTurf was a once popular artificial turf that's now obsolete; it was very bad in terms of injuries. What replaced AstroTurf is FieldTurf, made of tiny granules of plastic that act as a dirt stabilizer and little slivers of green-tinted vinyl that look like grass.

There is some evidence that natural grass is better than FieldTurf — non-contact foot and ankle injuries are slightly more prevalent with FieldTurf. Keep in mind that a running back who wants to change his direction by 90 degrees can do that remarkably quickly, but the force going through their ankle is about 800 pounds. This is why ankles get blown out, and that problem appears to be exacerbated with FieldTurf, which is a bit more grabby.

What does physics say about how to keep my players both safe and agile?

In the beginning, there were no helmets, and the death rate was so bad. Then they gave them leather skull caps, which were still bad. In 1912, 22 football players died of skull fractures; this is what caused the formation of the NCAA, because Theodore Roosevelt said we can't have 22 deaths a year due to this game.

The polycarbonate helmet came out in about 1935, and that essentially eliminated skull fractures. A stiff outer shell distributes the force of a hit over the surface of the head, so it's not focused at one point.

You look at the helmets today and they're beautiful marvels of engineering — except that they don't eliminate concussions. In football, nobody's going to eliminate concussions simply due to the laws of physics: conservation of momentum and energy means you've got to have some recoil of the head when it's hit with another head.

Now, if you look at videos of high school and college practices, they're wearing these big foam head pieces on top of their helmets — which shocks me, because back in the early nineties, similar foam rubber caps were being used. While more foam on your head reduces the concussion problem, it increases neck injuries.

There's a reason they polish helmets: If you have a blow to the head, you want it to glance right off, and a smooth helmet will help with that. But with foam rubber helmet coverings, a guy’s neck can get pulled around as they’re getting hit. And it turns out that a rotational torque to the head is more dangerous in terms of producing a concussion than a direct translational blow to the head.

When I was playing ball, those of us in the “pit” wore “horse collars,” foam tubes that go around your neck. They have versions of this today, but players don't like horse collars because they limit your ability to rotate your neck and see what the guy next to you is doing. But they also minimize the rotation of your head, and that reduces concussions.

My kicker is convinced that he can only kick when the ball’s laces face away from him. Is he right?

It is true that it's better to kick with the laces away. The lacing represents an asymmetry, so if your foot is not perfectly placed, you're going to give the ball transverse momentum, and it's not going to go where you want it to.

In the old days, they used to kick footballs with the tip of their toe. Now they do soccer kicks, so you're kicking it with the side of your foot. You get much better aiming capability.

How precise does my quarterback need to be to complete a long-distance pass?

For an effective pass, you have to throw the ball at the right speed, at the right time, and in the right direction. A receiver basically has a sphere in space with perhaps a diameter of five feet where the ball has to end up, and the receiver is moving through that space pretty quickly.

The timing of the throw has to be within a couple tenths of a second, and if you’re throwing the ball nominally at 50 miles per hour, you’ve got to have a half mile per hour level of control on how fast it gets there.

The precision of that is remarkable. You get that [precision] not because you have an array of stopwatches and meter sticks with you, but because you've practiced the heck out of it.