Putting a new spin on the football spiral – ScienceDaily

Only a handful of researchers have studied why an American football flies such a unique trajectory, hurtling through the air with remarkable precision, but also dodging, wobbling, and even tumbling as it dashes into the field. Now, for the first time, ballistics experts at Stevens Institute of Technology have applied their knowledge of artillery shells to explain this unique motion, creating the most accurate model yet of a spiraling soccer ball’s flight.

“When a quarterback makes a good spiral pass, the trajectory of the ball is remarkably similar to that of an artillery shell or a bullet, and the military has poured tremendous resources into studying the way these projectiles travel,” explained John Dzielski, a researcher by Stevens professor and mechanical engineer whose work is reported The Open Journal of Engineering of the American Society of Mechanical Engineers. “With well-understood ballistic equations, we were able to model a soccer ball’s flight more precisely than ever before.”

In fact, Dzielski said, the ballistic equations themselves aren’t particularly complex, but the motions they predict might be. The equations contain many terms that represent all of the ways air can affect the movement of a shell. The first challenge was to look at each variable in turn to determine which are important when used in a new or different context.

Dzielski and co-author Mark Blackburn, a senior research scientist at Stevens, first took an exhaustive approach — they modeled everything from a quarterback’s handedness to the effects of crosswinds to the effects of the Earth’s rotation — and then derived equations that removed factors that didn’t significantly affect the trajectory of a football. For example, during a 60-yard pass, the earth’s rotation changes the end point of the pass by only 4 inches. “Turns out the Earth’s rotation doesn’t have much of an impact on a football pass — but at least we know that for sure now,” Dzielski said.

Modeling the flight of a soccer ball sheds light on what separates good passes from bad ones. Dzielski and colleagues not only showed that a spiral pass can wobble slowly or quickly (or a combination of both), they were also the first to calculate what those frequencies are for a soccer ball. If the football starts to wobble, then it was thrown well. If it wobbles quickly, the quarterback twisted (like twisting a screwdriver) or sideways their wrist when the ball was released. The wrist may have twisted because the quarterback was hit.

“Quarterbacks and coaches already know this intuitively, but we were able to describe the physics at work,” Dzielski said.

Another, more surprising finding was that the Magnus effect, which causes a spinning baseball to skid or dodge due to changes in air pressure, has remarkably little effect on a spinning soccer ball. A soccer ball spins on the wrong axis to trigger the Magnus effect, so any deviations in trajectory must come from some other source, such as the lift created when a ball flies through the air, Dzielski explained. “A lot of people think that footballs swerve left or right because of the Magnus Effect, but that’s not the case at all. The effect of the Magnus force is about twice the effect of the Earth’s rotation,” he said.

Additionally, Dzielski and Blackburn first showed that this evasion is closely related to why the ball lands nose-down at the end of the pass when thrown nose-up.

Although Dzielski and Blackburn’s work represents the most accurate model of a soccer ball’s trajectory yet, Dzielski cautioned that more work was needed. Because a soccer ball rotates and falls as it moves, it is almost impossible to use wind tunnel studies to accurately record the aerodynamics of a moving soccer ball. “That means we don’t have good data to feed our model yet, so it’s impossible to create an accurate simulation,” he said.

In the coming months, Dzielski hopes to find funding for instruments that can collect aerodynamic data from a free-flying soccer ball in real-world environments, not just wind tunnels. “It’s the only way we can get the data we need,” he said. “Until then, a truly precise – and accurate – way to model a football’s trajectory will remain elusive.”

Leave a Comment