The physics of pitching a baseball
Recently, I stumbled on a story about New York Yankees pitcher Freddy García. In april of 2011, García was pitching against the Toronto Blue Jays. In the top of the 5th, Garcia threw this pitch to batter Juan Rivera. Now to the casual baseball observer, you might think this was just was an exception throw, which it was, but then move on to the rest of the game. However, to the folks who watch these events carefully, there was something unusual about this pitch. So unusual in fact, that there existed at the time no physical explanation for that observed movement of a baseball.
There are a number of forces acting on a pitch. The most basic ones we can initially think of are 1) the forward force given to the ball by the pitcher and 2) the force of gravity. The less force a pitches gives to the ball, the longer it will take the ball to arrive to the catcher and the more time gravity will have to accelerate the ball toward the ground.
Typically, to get movement on a baseball that deviates from this gravity dependant sinking, a pitcher will alter his grip to deliver the pitch with different kinds on spin it. This spin alters the way air flows around the ball as it heads toward the catcher. This 3rd force, caused by the the Magnus Effect, tells us that there will be a force on the ball perpendicular to the axis of rotation. If a pitcher throws a 4-seam fastball with enough backspin, the magnus force be greater then the force of gravity (at least over the 60 feet 6 inches between the pitchers mound and the catcher) and cause the ball to rises up on the batter. This is due to how air flows around a spinning ball. Much like how the wing of an airplane alters airflow, the spinning ball causes differences in air pressure around in, resulting in a new force component.
These three forces are all well and good for explaining most movement on a baseball, but they dont explain García's pitch. There is only a small amount of backspin on a relatively slowly thrown ball, which explains why gravity has time to pull it down. However, the ball cuts to the pitchers left, while the Magnus effect would predict rightward motion due to its spin. So what's going on??
Australian physicist (and apparent baseball and cricket enthusiast) Rod Cross discovered an explanation for this effect by testing polystyrene balls, which are lighter and show exaggerated movements. In his paper published in American Journal of Physics earlier this year, Rod demonstrated how the seam of a cricket ball, along with surface difference that accumulate during a game, affect the movement of the ball when bowled.
But a cricket ball has its seam straight down the middle, while a baseball has its seam in a figure eight pattern. So how are the two connected? If you watch the original video of García's pitch closely, you'll notice that the axis of rotation of the ball is such that the smooth face of the ball is always toward the axis of rotation (top left of ball). Most of the time this isn't the case, and the ball rotates in a way to average out its smoothed and seamed faces. Because of that, this effect is almost never observed.
Check out the man himself explaining it:
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