The physics of baseball

Aeronautics expert Peters doesn't watch a game the way a casual fan would

Peters is an expert in applied aerodynamics, but he also knows a thing or two about baseball. (Courtesy photo)

David Peters is an expert on aerodynamics and aeroelasticity. He researches the dynamic motions that profoundly influence vehicle dynamics of airplane propellers, helicopters and tilt rotors, but what he really wants to know is how the Cardinals are going to do this season.

Peters, the McDonnell Douglas Professor of Engineering in the McKelvey School of Engineering at Washington University in St. Louis, has a body of work in applied aerodynamics and a host of academic honors, but he’s also a baseball fan. That’s why in addition to his academic work, he’s known as the engineer who says it’s better to slide headfirst into a base instead of with your feet, or why it’s better to be a lefthander in the game than a righty.

And it’s all because of Game 7 of the 2006 National League Championship Series, the game the Cardinals won in the bottom of the ninth with the bases loaded on a wicked curveball thrown by Adam Wainwright to strike out slugger Carlos Beltran, “the pitch that froze New York.”

“I was at home watching like every other fan,” said Peters, who grew up in Belleville, Ill., listening to baseball on a transistor radio and the voice of Harry Caray broadcasting Cardinals games. Because he was chair of the mechanical engineering department, he got the nod when a local TV station called the university to ask why that curveball curved.

After that, Peters started getting more media calls, both local and national. He heard from high school teachers who needed help with lesson plans; from the St. Louis Science Center to do community programs for school kids. Then he got more calls, and queries pop up from time to time because the beauty of both baseball and physics is this: It happens every spring.

So why does a curveball curve? It all has to do with the Bernoulli Principle, he said. As the ball leaves a pitcher’s hand, a good curveball pitcher, through his grip on the seams and the amount of snap in his wrist, puts a spin on it. “Here’s what happens to the air that is flowing over the ball: the top of the ball is slowing down the air by its spin toward the plate while the bottom of the ball speeds the air,” he said. “The Bernoulli Principle says the faster air on the bottom of the ball makes for lower the pressure on the bottom, causing the ball to move down.”

And the properties of the air in which the ball is thrown make a big difference on how a curveball is going to curve. “Curveballs don’t act the same in Colorado as they do in the middle of the summer in Texas,” he said. (Denver’s Coors Field sits about 1 mile above sea level.)

Watching like a scientist

Peters is both scientist and fan, and when he’s at the ballpark, he’s looking for things a casual fan may not. Factors such as the temperature and humidity; wind velocity; the positioning of the players on the field in relation to who’s throwing on the mound; the exact time it takes from a pitcher’s release point to the time a ball hits a batter’s bat — or a catcher’s glove. And he is more than just fascinated with a ball thrown at 105 mph, as the Cardinals’ Jordan Hicks did last season. Peters knows the scientific reason why that ball is almost impossible to hit.

Peters (left) takes in a game at Busch Stadium with his brother-in-law Matt Domescik and sister Carol. (Courtesy photo)

“It’s 60 feet, 6 inches to home plate,” Peters said. “But by the time the pitcher lets go of a ball, it’s actually about 55 feet. Human reaction time for the best athletes is between 3/10ths and 5/10ths of a second.

“A ball thrown at 85 mph takes about 0.44 seconds to reach to the plate. If you throw a ball at 105 mph, it’s 0.36 seconds. With Hicks, the ball barely leaves his hand and the batter has to decide what he’s going to do.”

That release point, from a pitcher’s hand in relation to where a batter’s eye is seeing it, is key. A left-handed pitcher, for example, has an advantage over a right-handed batter due to something called parallax, which is why it’s so important to have a switch-hitter in your lineup.

“Small animals who were prey evolved with eyes on the side of their heads so they could watch for predators,” he said. “Human eyes are in the front of our head so we could judge distances (parallax).

“That distance between where a batter’s eye is (seeing the ball come off the release point) makes all the difference — even that tenth of a second,” he said.

And this scientist doesn’t dig the long ball — even knowing the laws of physics that elevate a baseball out of a ballpark. “A home run is the least exciting thing in baseball,” Peters said. “A properly executed hit-and-run is much more exciting to watch, and it’s a great thing to behold when plays are executed well in the field.

“When (Hall of Fame shortstop) Ozzie Smith played, every game was a physics lesson,” he said.