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Separating the useful spin from the rest.

Alan M. Nathan
University of Illinois

Ever since the early days of PITCHf/x, we have had unprecedented information about the movement of pitches. We now have a precise quantitative measure of how much and in what direction a pitch moves—i.e., deviates from a straight-line path. The movement is the result of the combined forces of gravity pulling the ball downward and the so-called Magnus force on a spinning baseball. It has become conventional to remove the effect of gravity, which is easily calculable, so that the resulting movement—pfx_x and pfx_z in PITCHf/x lingo—is due only to the Magnus force. I will utilize that convention in this article. It seems sort of reasonable that there ought to be some simple relationship between the movement to the spin rate. For example, if a pitch is spinning at a higher rate, the expectation is that there will be more movement. But is that expectation correct? In fact, it is not correct because, as the title of this article suggests, all spin is not alike. And that is the issue I want to discuss here.

So why is it that all spin is not alike? The reason has to do with the vector nature of the spin: It has a magnitude and a direction. The magnitude is pretty simple, since it is just the number of revolutions per minute, or rpm. Let’s talk about the direction. The easiest way to determine the direction of the spin is to use a right-hand rule: Wrap the fingers of your right hand around the ball so that they point in the direction that the ball is turning. Your thumb will then point in the direction of the spin axis.

Here are some examples. A straight overhand fastball has pure backspin and the spin axis points to the pitcher’s right. An overhand “12-6” curveball has pure topspin and the spin axis points to the pitcher’s left. A ball thrown with pure sidespin has its spin axis pointing up or down. In all these examples, the spin axis is perpendicular to the direction of motion. On the other hand, a gyroball is a pitch thrown with the spin axis perfectly aligned along the direction of motion, much like a spiral pass in football. Indeed, it is often called “bullet spin”, since that is how a bullet will spin when shot from a rifle. All of these pitches are special cases, since in general the spin axis could be pointing in any direction whatsoever.


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The distance a fly ball travels depends to a large degree on which lot of baseballs it came from.

How Far Did That Fly Ball Travel (Redux)?

Alan Nathan#, Jeff Kensrud*, Lloyd Smith*, Eric Lang#

#Department of Physics, University of Illinois

baseball.physics.illinois.edu

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Quantifying the degree of difficulty of Yoenis Cespedes' incredible outfield assist.

Alan Nathan is Professor Emeritus of Physics at the University of Illinois at Urbana-Champaign. After a long career doing things like measuring the electric and magnetic polarizabilities of the proton and studying the quark structure of nucleons, he now devotes his time and effort to the physics of baseball. He maintains an oft-visited website devoted to that subject: go.illinois.edu/physicsofbaseball.

When I woke up on Wednesday morning and checked my overnight Twitter timeline, I found considerable buzz about an incredible throw made by Oakland A’s left fielder Yoenis Cespedes on Tuesday night.


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Ben discusses the origins and implications of baseball's skyrocketing strikeout rate with a panel of experts including Harry Pavlidis, Rob Neyer, Brian Bannister, and Alan Nathan.

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An expert on the physics of baseball tackles a common question.

Alan Nathan is Professor Emeritus of Physics at the University of Illinois at Urbana-Champaign. After a long career doing things like measuring the electric and magnetic polarizabilities of the proton and studying the quark structure of nucleons, he now devotes his time and effort to the physics of baseball. He maintains an oft-visited website devoted to that subject: go.illinois.edu/physicsofbaseball.
 


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Does speed off the bat determine how far a fly ball goes?

Most of our writers didn't enter the world sporting an @baseballprospectus.com address; with a few exceptions, they started out somewhere else. In an effort to up your reading pleasure while tipping our caps to some of the most illuminating work being done elsewhere on the internet, we'll be yielding the stage once a week to the best and brightest baseball writers, researchers and thinkers from outside of the BP umbrella. If you'd like to nominate a guest contributor (including yourself), please drop us a line.

Alan Nathan is Professor Emeritus of Physics at the University of Illinois at Urbana-Champaign. After a long career doing things like measuring the electric and magnetic polarizabilities of the proton and studying the quark structure of nucleons, he now devotes his time and effort to the physics of baseball. He maintains an oft-visited website devoted to that subject: go.illinois.edu/physicsofbaseball.
 


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To what extent can changes in scoring be traced to temperature?

Believe it or not, most of our writers didn't enter the world sporting an @baseballprospectus.com address; with a few exceptions, they started out somewhere else. In an effort to up your reading pleasure while tipping our caps to some of the most illuminating work being done elsewhere on the internet, we'll be yielding the stage once a week to the best and brightest baseball writers, researchers and thinkers from outside of the BP umbrella. If you'd like to nominate a guest contributor (including yourself), please drop us a line.

Alan Nathan is Professor Emeritus of Physics at the University of Illinois at Urbana-Champaign. After a long career doing collisions of subatomic particles, he now spends his time studying the collision of ash with cowhide. He maintains an oft-visited website devoted to all things related to the physics of baseball: go.illinois.edu/physicsofbaseball.
 


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Our latest guest contributor returns from the lab with exciting findings about home runs.

Believe it or not, most of our writers didn't enter the world sporting an @baseballprospectus.com address; with a few exceptions, they started out somewhere else. In an effort to up your reading pleasure while tipping our caps to some of the most illuminating work being done elsewhere on the internet, we'll be yielding the stage once a week to the best and brightest baseball writers, researchers and thinkers from outside of the BP umbrella. If you'd like to nominate a guest contributor (including yourself), please drop us a line.

Alan Nathan is Professor Emeritus of Physics at the University of Illinois at Urbana-Champaign. His principal area of research is the physics of baseball. He maintains a web site devoted to this topic at go.illinois.edu/physicsofbaseball. His younger colleagues at Complete Game Consulting have bestowed upon him the exalted title of Chief Scientist.

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August 2, 2007 12:00 am

Schrodinger's Bat: Interview With a Physicist

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Dan Fox

Extending his own interest in the physics of baseball, Dan sits down with a lauded physicist and baseball fan to discuss the topic.

"I think physicists are the Peter Pans of the human race. They never grow up and they keep their curiosity." - Nobel Prize winner Isidor Isaac Rabi

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