Jeff Francis and the physics of baseball

Jeff Francis pitching
Jeff Francis pitching for the Rockies
Photo credit: JamieSchwaberow/ Rich Clarkson and Associates, LLC.

The Magnus force has made Jeff Francis what he is today: a 25-year-old left-handed pitcher on the rise in his second full season with Major League Baseball's Colorado Rockies.

A former physics and astronomy major at the University of British Columbia, Francis could clarify what puts the curve in a curveball, the “hop” in a fastball, the slide in a slider, the sink in a sinker. But he won't be giving that talk any time soon.

“As much as it might seem contradictory,” Francis says, “physics knowledge does not help much on the field. So much of playing baseball is 'feel' that explaining to someone what makes a ball curve would be almost meaningless. I get asked that a lot, and sometimes I say: 'I never met him, but I bet Einstein couldn't throw a curveball.’ ”

On the other hand, Einstein did toss a memorable and gigantic curve at physicists’ concepts of matter, space, and time a hundred years ago. Francis is good, but he hasn't yet matched that impact.

But delivering a pitch is all about physics: the most efficient transfer of momentum from body to baseball; the maximum effectiveness of the arm as a lever; the rotational dynamics of the baseball leaving the fingertips. And within four-tenths of a second after Francis delivers a pitch, the batter faces his own challenge of physics and mechanics.

Yale professor emeritus Robert K. Adair says a batter must react in less than one-fourth of a second. In the one thousandth of a second of bat-ball contact, a superlative hitter such as Albert Pujols of St. Louis will deliver some 8000 pounds of force, compressing the ball to about half its original diameter–that is, if Pujols meets the ball precisely on the bat's “sweet spot,” or vibrational node (point of no vibration), after analyzing and reacting to the Magnus force effects on the pitch thrown by Francis.

The Magnus force was identified in 1852 when the German physicist Gustav Magnus demonstrated that a spinning object moving through a fluid experiences a sideways deflection in its path. A twist of the wrist, and a pitch with sideways spin will deflect in the direction of the baseball’s spin.

On a pitch with backspin–the conventional fastball–the Magnus force acts upward; not sufficient to lift the ball, but sufficient to deliver a perceived “hop” on its way to the plate.  The ball rising is an optical illusion. Normally, the ball drops a certain amount in the quarter-second or so that it's in the air on its regular flight towards home plate. If the right backspin is obtained, the air can hold it up just a bit longer on the way to home plate, dropping a certain fraction of the regular distance. Since the eye is so used to seeing it drop the regular amount, the ball gives the illusion of rising on the way towards the batter.

“I think all baseball players, whether they're superstars or not, are aware of certain physical aspects of the game by just being around baseball and observing,” Francis says. “For example, you'll always see hitters tapping their bat with their hand and then listening to it like a tuning fork, knowing that a higher sound means a higher frequency, which means harder wood, which, in turn, means the ball will jump off the bat more.”

Francis says teammates knowing about his physics background often tease him good-naturedly about being an intellectual, although “I can't get any more crossword clues than anyone else.”

Adapted from the June/July 2006 issue of Symmetry magazine ( with permission.

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