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3.27.2004

MIT grad trying out for San Diego Padres 

Jason Szuminski, MIT '00, is currently going through spring training with the Padres and has a nontrivial chance to make the roster.

I do wish that writers would give a complete explanation for why baseballs curve and airplanes fly, though:



Frankly, I hope Jason Szuminski cracks the Padres pitching staff to become the first M.I.T. graduate to play in the major leagues, if only for the following possible clubhouse conversation.

Szuminski: You know Boomer, the reason a curveball curves is because its spin creates a difference in airflow around the ball. It's the same principle as with an airplane or a Frisbee. Or it's like when you're rowing a canoe and you make the canoe turn by dragging one oar in the water.

David Wells: Hey, Professor, try this experiment. Pull my finger.



The principle that is usually quoted is Bernoulli's principle, which claims that a differential in air speed around the sides of a airplane wing or spinning baseball or other object causes a differential in air pressure that causes the desired deflection (upward for an airplane, sideways for a curveball, and so forth).

As any MIT grad knows, this Bernoulli effect is negligible compared to the other aerodynamic phenomena that are really at work. For example, take a piece of paper, curve it upwards and blow across the top of it. Bernoulli's principle claims that the faster airflow across the top of the paper will decrease the air pressure across the top of the paper and pull it up, but your piece of paper will actually be deflected downwards.

The correct explanation is given in terms of the Coanda effect; a good reference is this website. Suffice it to say that the incompressibility of air forces the airflow around an object to follow the motion of that object; the Coanda effect is then essentially a corollary of Newton's third law.

If you're flying a plane with wings curved downward, the airflow will be forced downward by the shape of the wings. If the plane is pulling the air down, the air must therefore pull the plane up. The reverse holds if the wings are curved upward.

The spinning motion of a ball adds a new wrinkle to the phenomenon, but it does seem to be understood. If you're a righty throwing a curveball, your middle finger snaps down the left (first base) side of the ball, imparting a slight spin to it. The left side of the ball will be rotating towards you, and the right (third base) side will be rotating away from you. Since the left side of the ball is spinning with the incoming airflow, the air on that side is more easily entrained to follow the curvature of the ball's surface. Since the ball is convex, the air gets pulled towards the ball, and the ball deflects to the left.

If one appealed to the Bernoulli principle alone, one would conclude that the airflow over the right side of the ball would be faster, since the right side of the ball is spinning into the incoming air. One would then conclude that the curveball would break to the right.

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