The Physics of Broken Bats

Unless you live too far from town to learn baseball, which I do, you know that the San Francisco Giants beat the St. Louis Cardinals to make it into the World Series last night. And you probably have also heard that there was some bat shattering hitting last night as well. Check out this super slow motion video of Hunter Pence hitting an inside pitch past the short stop and breaking his bat in the process.

hunter pence broken bat slow motion

hunter pence broken bat slow motion

Now, there’s a lot of superstition in baseball, and the sports writers at ESPN laid it on thick:

No one will ever be able to fully explain it. Not Pence, the man who hit it. Not Joe Kelly, the man who threw the pitch. Not Pete Kozma, the shortstop who couldn’t catch it. Not even Sir Isaac Newton, who probably thought the law of gravity would some day apply to baseball. [ESPN]

But even in foggy San Francisco, the physics of baseball is the same as the physics of anything else. And the broken bat here is pretty simple. Pretend that instead of swinging the bat at a tiny moving baseball, Hunter Pence was swinging the bat at a steel pole cemented into the ground. Like the kind they put next to gas pumps to keep careless drivers from running the pump over.

If you take a swing at that one of two things is going to happen. First, if you hit the fat part of the bats head, so that most of the bat’s mass is between you and the post, you’re going to get some intense vibrations in the handle of the bat as the bat and your hands screech to a stop.

When you hit a baseball or softball, the bat imparts kinetic energy onto the ball, causing it to launch onto the field. However, the ball also imparts energy onto the bat! Some of this energy goes into rotating the bat about a pivot point. Imagine a stick floating in a tub of water. If you push down on the stick near one of its ends, it will rotate. The same thing happens to a bat when it comes into contact with a ball. It tries to rotate and in the process, it pushes against your hands! [Cornell Center for Materials Research]

But if you hit the post so that most of the bat’s mass is on the other side of the post, chances are that the bat will bend as momentum drives the bat’s mass bast the post and your hands drive the handle past the other side of the post. Assuming the post doesn’t move, and you are strong enough, you will be able to break the bat. What happened to Hunter Pence last night was that the ball hit the bat too close to the handle causing the head of the bat to keep rotating right away from the handle of the bat. The excerpt from Robert K. Adair’s book The Physics of Baseball explains what is happening when a bat breaks in this way.

The Physics of Baseball Book

The Physics of Baseball Book

So that’s how a hand crafted spindle of super hard ash wood can be split in two by a tiny little baseball.