The Science of the Hail Mary

Fall means many things to many people in Wisconsin. School is in full swing, the weather is cooling down, and the leaves are changing. But perhaps most importantly, it means football is back.

For many though, seeing the pigskin fly through the air over Lambeau Field is not just a sign of the changing season. Instead, it brings back memories of autumns past: tailgates, family and friends, learning to throw a spiral (or trying). And if you’re one who remembers the last of those, you probably remember how hard it was to do. If not, you may not realize it, but there’s a lot that goes into a good throw. So let’s take a break from environmental issues for a minute, and put the pigskin under the lens!

First, you may know that a football thrown halfway between straight out and straight up will fly the farthest. You may also know that a lot of spin is key to a good football throw, called a “spiral.” That’s because if it’s done right, as the ball spins around quickly while flying through the air with the nose pointed ahead, the laces trace a long and arcing spiral through the air. The spin is what keeps the ball stable against puffs of wind in flight, kind of like the spinning wheels of a bike help keep it upright, or the spin of a Frisbee keeps it level.

What you may not know is that the spin of the ball is what causes the nose to drop and follow the angle of flight. With the nose pointed slightly up, the spin means one side of ball is traveling more with the wind coming at it, and the other side is traveling with more against it. This makes a difference in relative wind speeds on each side, resulting in different amounts of wind drag. That difference in drag then causes a force called “torque” on the ball, which pulls the nose down until it’s straight into the path of travel. Without the spin, the ball would stay pointing at an upward angle even as it comes down, both slowing it down and making it harder to catch.

The spin also does something else: It makes a ball thrown hard enough (like by a professional quarterback) veer to one side. The fast-moving ball can lift like a wing, which interacts with the spin to again cause a torque. This time though, the torque turns the nose slightly outward, which then causes the same wing-like lift off to the side. For a right-handed quarterback this makes the ball veer right, versus left for a left-handed quarterback.

As it turns out though, a tiny bit of wobble can help by creating even another torque that helps keep the ball from veering, while too much wobble can cause instability in the flight. No wonder it’s so hard to get it just right!