When an MLB pitcher takes the mound, he nods or shakes his head in response to the catcher’s signals to agree upon the type of pitch he’ll throw. The catcher needs to know what’s coming so he can be ready. The pitcher needs to decide on a pitch so he’ll know how to hold the ball when he makes the throw. Yes. The way a pitch moves largely comes down to the position of the stitches when the ball is released because the science behind the movement of a baseball in flight is the science of fluid dynamics – more specifically, aerodynamics.
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Aerodynamic Forces In Action
Back in 1799, George Cayley identified the four fundamental forces of flight for a powered fixed-wing plane. The aerodynamic forces of lift, thrust, drag, and weight are recognized to this day.
But, any object in flight without a power source has only three forces in play: lift, drag, and weight.
Whether powered or not, according to Newton’s first law of motion, an object (in a vacuum) will stay at rest or in motion in a straight line until it is acted upon by another force.
However, anyone who has seen a baseball game knows that’s not what happens.
Instead, a baseball speeds directly toward the batter and across the plate, or it heads directly toward the batter before curving away just before it crosses the plate, or it sinks at the last minute, or it does none of the above.
Why?
You could chalk it up to wind or a misshapen ball, but that wouldn’t explain it all. In fact, the behavior of the ball is the result of the forces of lift, drag, and weight at work on the ball in combination with the position of the raised stitches on the ball.
The raised stitches are at the heart of the matter. Every MLB baseball is stitched by hand and has 108 double stitches in a figure-eight pattern. Since the surface of the ball is slick leather, the stitches act as mini airfoils for the air around the ball, causing the air to go up and over the stitches as the ball moves. To achieve this up-and-over motion, the air moves faster as it goes up and over, resulting in lower pressure above the stitches. The result of this motion is the lift that keeps the ball airborne, and the with the pressure greater on one side of the ball than the other, the greater pressure moves the ball in the direction of lesser pressure.
Drag – the force of friction on the ball as it moves
Weight / Gravity – the pull of the Earth as the ball is airborne
Position of the Stitches
For a curveball, the pitcher positions his fingers so the middle finger is on the right horseshoe seam with the index finger between the seams. The wrist is angled so the thumb is on top to give the ball a downward spin – top to bottom – upon release. The ball will curve just as it reaches the batter. How much it curves depends upon the extent of the imbalance in forces and speed of the ball.
For a four-seam fastball, the ball rotates bottom to top, with the index and middle finger placed across one side of the horseshoe portion of the seam as it’s thrown at full velocity.
A slider calls for the index and middle finger to be positioned close together and off-center as the eye runs down the length of a horseshoe portion of the seam. With the tight spin, it looks like a fastball but breaks down and away when it reaches the plate. It’s the aerodynamics of the seams that brings the spin to this ball, with the ball breaking due to the off-center spin.
A knuckleball is a rarity because the ball is pushed straight out at release. The pitcher positions the tips of his index and middle fingers on one side of the narrow portion of the seam and releases it with as little spin as possible. Without the spin, the ball gets pushed around by the wind as it travels.
What Piqued My Interest
I’ve been curious about the way a pitch moves ever since a friend’s dad with box seats took us to my first MLB ballgame when I was nine. When I finally got serious about understanding how a baseball pitch moves, I began with a jewel of a book, The Physics of Baseball, by Robert K. Adair. Not only does it include a dedication to his son that breaks my heart every. single. time. but it also includes a chapter on air resistance, the Magnus Coefficient, the distance of flight of a batted ball, other factors affecting the path of the ball, and the Reynolds Number (Re). I happened to be writing a book about fluid dynamics and modeling of ships and spacecraft at the time, and I was thrilled to find the concepts had considerable overlap. So, up to Cooperstown I went – in the name of research but really, who wouldn’t want to go to Cooperstown? The rest, as they say, is history.
Check This Out!
It’s largely about the position of the stitches and the action of the wrist upon release.
And yes, every MLB baseball is rubbed with Lena Blackburne Baseball Rubbing Mud, harvested from a secret location in New Jersey, before it enters play. The mud doesn’t stink up or discolor the ball; what it does is make the slick leather on a new ball easier for the pitcher to handle. But that’s another story …
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