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Thursday, May 31, 2007

The Physics of Drag

My column today on Baseball Prospectus delves once again into the PITCHf/x data tracked by the new Gameday application. This time I take a look at the drag on a pitched ball and square the data with the description of the model discussed by Robert Adair in The Physics of Baseball.

To answer the most frequently asked question thus far - no, I haven't looked at Tim Wakefield in any depth. I did see, however, that his average pitch (and I have 346 to look at) lost exactly 10% of its velocity. Overall though that percentage decrease is in line with the following chart (a version of this chart is also in the original article) since his average pFX (which is a measure of the break of the pitch) was 8.6 and his average start speed was 68.5 miles per hour as shown in the chart below.

However, that percentage does not seem to differ by the break length (a different measure of break introduced this year) nor the pFX value. It's also interesting to note that all but one pitch came out of his hand at less than 79 miles per hour. I think it's likely that the Magnus force placed on a knuckler as it moves in various directions tends to slow it down more than one would other think based on the slow speed and lack of spin.


John said...


More great work. I still think the data vary sytematically by park. Why do pitches in SDN slow down so much more than other parks.

For me the atmospherics market doesn't hold water. On that basis I'd expect Seattle to be closer to SDN as it is colder. I would have thought you'd have seen some difference but nowhere near what you find.

This strikes me as a callibration issue.


Dan Agonistes said...

Thanks. My first thought as well was that the differences are too large and that seems to be backed up by a few other sources on air resistance I've read. Now if SDN was at the bottom and Texas at the top then we wouldn't be doubting it. I'm giving the benefit of the doubt at this point. It'll be interesting when Coors goes online later this year but the preliminary data shows it as lower than Texas.

Anyway, I'm trying to think of some other way to test for the calibration issue. Any ideas would be appreciated.

John said...


I did some work on callibration on THT, which you have probably seen. This is 100% conclusive but is suggestive of small calibration issues (although that is permissible given the system is new)

However, this wouldn't be able to account for the SDN, TEX difference in your chart. I think the only way to investigate that is to try to isolate the factor eg, air pressure/ density/ temp and look at similar parks.

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