ASK A SCIENTIST
Question: How does a curveball work?
When I was a young man growing up in Upstate New York, I was a devoted fan of the Cleveland Indians. At that time, the Indians had an awesome pitching staff and if they won games, they won them 1-0 or 2-1. Their staff included players named Sonny Siebert, Luis Tiant and my favorite, Sudden Sam McDowell. I was in awe of Sudden Sam who was reputed to have the fastest fastball and the most wicked curveball in the major leagues. Yet all three pitchers had great curveballs though they were all very different. Sudden Sam's seemed as if it 'fell' off a table when it neared the plate. Tiant's sidewinder-style pitch moved in a level arc, either moving in towards the batter or so far away it that the batter looked incredibly foolish swinging. Sonny Siebert's curveball was somewhere in the middle; it would both 'fall' a bit and curve towards or away. I remember wondering at the time, how is this all possible? Little did I know that the answer lie in a strange field known as aerodynamics! What is aerodynamics? The word comes from two Greek words: aerios, concerning the air, and dynamis, which means force. Aerodynamics is the study of forces and the resulting motion of objects through the air. Humans have been interested in aerodynamics and flying for thousands of years, although flying in a heavier-than-air machine has been possible only in the last hundred years. It is fun to look back at all the attempts we have made to fly over the course of history. Check out the following website produced by a professional engineering society dedicated to the study of flight. (http://www.aiaa.org/content.cfm?pageid=260) So how does the curve ball actually work? It's actually the same principle we find enabling an aircraft to fly. On a plane's wings, the air moves faster over the top of the wing as compared to the lower surface. Faster air means lower pressure on the top, slower moving air generates higher pressure on the bottom and the resulting pressure difference lifts the aircraft off the ground. This is known as the Bernoulli Principle. A wing generates this lift due to its curvature as it is relatively flat on the bottom but has a noticeable curve on top. OK, so you are probably thinking to yourself this is all well and good but a curveball is a ball, isn't it? It's a sphere so there is neither top nor bottom. This is where the rotation of the ball becomes important. As a German named Gustav Magnus figured out in 1852, the reason curveballs curve is because the air flows differently on the side of the ball spinning toward the plate than the side spinning away from the plate. Here's how it works. The speed of the air at the top of the ball is equal to the lateral speed of the ball (from the pitch being thrown) minus the speed caused by the ball spinning (since the ball is spinning opposite to the way the air is flowing). The speed of the air at the bottom of the ball is equal to the sum of the two speeds. Again the pressure at the top of the ball is therefore greater than the pressure at the bottom of the ball. So, the ball drops. This may explain why Sudden Sam's curveball 'fell' off a table but what about Sonny Siebert and Luis Tiant? That explanation lies in how each of them were holding the ball. Sudden Sam was holding the ball with his palm as if he were about to shake hands, that is, vertical while Luis had his palm outstretched, that is, horizontal, with Sonny somewhere in between. The ball curves in an arc at right angles to the plane of the pitcher's palm. Each pitcher rotated the ball in the very same way with the very same motion but it was the orientation of their palms, which determined the direction of the curve. I did not realize that I was learning about the magical, mysterious science we call aerodynamics when I was watching my favorite pitchers. I thought I was simply having fun. So maybe my teachers were right. Maybe the two aren't in conflict after all!
Ask a Scientist appears Thursdays. Questions are answered by faculty at Binghamton University. Teachers in the greater Binghamton area who wish to participate in the program are asked to write to Ask A Scientist, c/o Binghamton University, Office of Communications and Marketing, PO Box 6000, Binghamton, NY 13902-6000 or e-mail firstname.lastname@example.org. Check out the Ask a Scientist Web site at askascientist.binghamton.edu. To submit a question, download the submission form(.pdf, 460kb).