Curveballs: illusions and reality

A few years ago, I got to teach a fun freshman seminar course that I called The Science of Baseball.  We covered baseball from a number of scientific approaches: physics, biology (PED's!), psychology, economics, etc.  It was a blast...someday I need to do that again.  I still have the reading list, though it's a bit outdated.

In any case, Michael Maffie had a nice piece today at Redleg Nation highlighting something that is right up that course's alley.  He highlights a PLOS One study that noted an optical illusion that could affect batters' ability to track curveballs.  In the comments, there was also some discussion about why breaking pitches curve.

I wrote this in response, built largely on what I gleaned from Robert Adair's The Physics of Baseball.

Cool stuff. I remember seeing that PLOS One study when it was published, but I’d completely forgotten about it. :)

A few thoughts:
* I think sliders are very interesting in light of this. Unlike the fastball and curve, which will flash as the ball tumbles through space, slider spins are such that the stitches do not “tumble.” Rather, batters see a white “dot” created by the spin as the slider spins on its axis. Therefore, maybe this is yet another difference between what a batter perceives when a pitcher throws a slider vs. other pitches? 

* I have a very basic understanding of curveball (and fastball) physics. But this is the basic breakdown, I think, based on what I read in Adair’s Physics of Baseball. Hopefully it’s correct. 

- The stitches cause the surface of the ball to be “rough.” Rough surfaces create the opportunity for the ball to collect a layer of air around it as it travels at high velocities. This protective layer of air helps to allow for much lower drag than a smooth ball would experience. that effect bottoms out at ~80 mph and then increases again as you increase in velocity. At velocities relevant to baseball, therefore, higher velocity = higher drag = more force applied to baseball. 

- The fact that a curve ball is spinning results in different effective velocities on the top and bottom of the ball. The top part of the ball in a curveball is spinning “into” the wind, and thus has higher velocity, and thus more drag. The bottom part of the baseball, spinning “away” from the wind, experiences less drag. This difference in drag results in more force (called the magnus force) on the top compared to the bottom of the baseball. Therefore, curveballs drop at a (slightly) faster rate than expected by gravity. 

- Fastballs work the opposite way. A four-seam fastball rotates such that the the seams on the bottom of the baseball are rotating into the “wind,” such that the bottom of the baseball experiences greater magnus force than the top of the baseball. As a result, four-seam fastballs (at least) fall less than would be expected by gravity. They still definitely fall (there is no true “rising” fastball), but since we “expect” a normal ballistic path (more or less), they look like they’re defying gravity. 

You can see this deflection on pitchf/x graphs. Brooks Baseball isn’t working for me right now(?!), but if you go here....
Look at the Vertical vs. Horizontal movement graph. Fastballs show up as having “positive” vertical movement, whereas curveballs have “negative” vertical movement. Zero, in that case, would be no deflection from what you’d expect by gravity alone.
Fun stuff!!
-j