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It’s amazing how professional baseball players can throw very fast curveballs, but do you know how do curveballs change direction in midair?

Hosted by: Stefan Chin
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Sources:
http://spiff.rit.edu/richmond/baseball/traj/traj.htm
https://www.eurekalert.org/pub_releases/2013-05/uoc--ha9050213.php
http://large.stanford.edu/courses/2007/ph210/pelc1/

Image Sources:
https://commons.wikimedia.org/wiki/File:12-6_Curveball.gif
It takes about a quarter of a second for the average person to react to something they see.

That’s how long it takes for a signal to travel from your eyes to your brain and then back down to your muscles. A quarter of a second might not seem very long, but that lag time actually makes some things, like hitting a baseball, really complicated.

At the professional level, it only takes half a second for the ball to get from the pitcher’s mound to home plate. So to hit the ball, you have to start swinging when it’s only halfway to the plate. Your brain can make up for this by predicting where the ball will go before it actually gets to you.

But that means a pitcher who can throw the ball so that it curves a few centimeters away from where the batter is expecting it to be has a huge advantage. That kind of throw is called a breaking pitch. And there are lots of different types of breaking pitches, but one of the most popular ones is the curveball, where the ball curves directly downward faster than normal.

Breaking pitches take advantage of what’s known as the Magnus effect, where the ball’s spin interferes with the air around it in a way that changes its direction. Here’s how it works: As an object moves through the air, the air’s molecules stick to it in a thin layer called the boundary layer, which is dragged along the surface for a bit, then separates. The point where that separation happens is an important part of how curveballs drop faster than you’d expect.

When you throw a curveball, you give it topspin, meaning that the ball is spinning so that the top of the ball moves away from you, in the direction you threw the ball, and the bottom of the ball moves toward you. Meanwhile, air flows over both the top and bottom of the ball. The bottom of the ball is spinning with the air flowing past it, so the boundary layer sticks longer and curves around toward the back of the ball.

The top of the ball, on the other hand, is spinning against the air flowing past it, so the boundary layer separates much earlier, before it can curve behind the ball. The overall effect is that the ball drags air around itself, deflecting the air behind it in the direction it’s spinning. Since a curveball has topspin, that means the air behind it gets deflected upwards.

And pushing the air upward forces the ball in the opposite direction, changing its flight path. That’s the Magnus effect. A talented pitcher can spin the ball in different directions to use the Magnus effect to their advantage.

Giving the ball topspin will cause it to fall dramatically downward into a curveball, but giving it backspin will cause it to resist gravity and fly in a straighter path. Now that’s what I’d call putting a positive spin on physics. Thanks for asking, and for more about the science of sports, check out our video about why golf balls have dimples.

Spoiler: you’d be surprised at how much thought people put into what they carve into golf balls.