## Archive for March, 2011

### Baseball bats

Friday, March 25th, 2011

It’s spring, when a young man’s fancy turns to baseball.

The NCAA (and high schools, I think) changed their standards for baseball bats this year, apparently in response to safety concerns about people (especially pitchers) being hit by fast-moving balls. The change took effect back in January and was decided on long before that, but I just heard about it.

Descriptions of the change are very confusing, at least to a naive physicist. An example:

So they adopted a standard called the Ball-Bat Coefficient of Restitution (BBCOR), which provides a more accurate measure of bats in lab tests than the old standard, the Ball Exit Speed Ratio (BESR). Rather than measure the ball’s speed off the bat, BBCOR testing measures the collision between the bat and the ball to see how lively the bat is.

That distinction is way too subtle for me! What does “how lively the bat is” mean, if it doesn’t mean how fast the ball leaves the bat?

To be more specific, the coefficient of restitution, by definition, is a measure of what fraction of the mechanical energy that was present before a collision remains after the collision. Having a standard that restricts theĀ  speed of the ball (following a collision under controlled circumstances) is precisely the same thing as having a standard that restricts the energy after the collision (i.e., the coefficient of restitution).

Where’s the Physicist to the National League when you need him?

Of course, even if the two standards are essentially equivalent, changing from one to the other might be a way to tighten up the standard, without making it explicitly obvious that that’s what you’re doing. Maybe that’s all that’s going on here.

You can actually read the old and new standards, equations and all, if you feel like it. It turns out, as far as I can tell, the headline change, from “exit speed” to “coefficient of restitution,” really is a bit of a red herring. The COR is a bit of a cleaner thing to measure, because the old standard had to be measured on a sliding scale for different bat sizes, and the new one doesn’t, but fundamentally they’re measuring essentially the same thing.

The more important point is that they’ve also added an accelerated break-in procedure to the protocol. Apparently composite bats get springier over use (I guess as the materials get compressed). The old procedure tested them new; the new procedure breaks them in first, so that you can’t buy a standards-compliant bat and later end up with one that’s too springy for the standard.

### Publications

Friday, March 25th, 2011

1. This paper has now been officially accepted by Physical Review D.

2. This one has been accepted to Astronomy & Astrophysics.

3. The New York Times published my letter to the editor.

Needless to say, the number of readers of #3 will be orders of magnitude greater than the combined number of readers of 1 and 2.