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# Professor Dave Peters discusses the science of baseball on St. Louis Public Radio

4/13/2012

### For St. Louis Public Radio’s Véronique LaCapra, baseball season means it’s time to talk about the science behind America’s national pastime.

And Washington University in St. Louis aerospace engineer David Peters was happy to join in.

Here's a summary of their conversation, plus, some web extras for you.

PETERS: I'm a baseball fan from day one. I was at Stan Musial's last game here in St. Louis. Turns out I was in Cincinnati and happened to go to the game when Pete Rose broke Ty Cobb's record. I was at the game where Roger Maris' home run record was broken by Mark McGwire. I've just been a baseball fan, always have been.

LACAPRA: I can imagine that there’s a lot of physics involved in pitching. Can you talk about some of the main types of pitches — the curveball — the physics behind them?

PETERS: The basic principle's called the Bernoulli principle. And the Bernoulli principle says that when air speeds up, its pressure goes down, and when air slows down, its pressure goes up.

So the top of the ball is spinning towards the batter; the bottom of the ball is spinning away from the batter. So the top spinning towards the batter slows down the air on the top of the ball, and since the air slows down, the pressure goes up. The bottom of the ball is spinning away from the batter, actually speeds the air up.

So there's high pressure on the top, low pressure on the bottom, it pushes the ball down. Ball falls much faster than just gravity would make it. And they call it a 12-to-6 curve ball, 'cause it goes from 12 o'clock to 6 o'clock on the clock dial. You can't believe it. It's unmerciful.

Almost every pitch — fast ball, curve ball, knuckleball — they move because of air pressure. They move because of Bernoulli's principle and the different ways the pitcher makes them spin.