I’m lousy at baseball. Fly balls fly right over me, line drives make me duck, and I can’t run the bases worth a darn–but that’s all right, since I seldom hit the ball.

So to write this column about the science of pitching, I turned to an expert: Robert K. Adair, Sterling Professor of Physics at Yale University, whose 1990 book The Physics of Baseball provided all the information I needed. If you’re a serious baseball fan (or a serious physics fan), I highly recommend it.

As every batter knows, the baseball does not follow a perfectly straight line between the pitcher and the plate (more’s the pity). In fact, the pitcher has a number of weapons in his arsenal, all designed to confuse the batter, beginning with the curve ball.

Curve balls are thrown with a spin, which means one side of the ball spins against the flow of air, and one side with it. Air resistance is higher on the side spinning into the wind, and that pushes the ball in the other direction. This is called the Magnus Effect.

A “typical” wide-breaking curve ball might be thrown at 70 miles per hour, spinning at 1600 rpm, and would cross the plate about six-tenths of a second later at about 61 mph. Although the path of the ball is actually a smooth curve, like a small piece of a much larger circle, to the batter it appears to “break” because halfway to the plate it’s only 3.4 inches away from the initial, straight path, but by the time it gets to the plate it’s curved 14.4 inches–the distance from the inside corner to the outside corner.

Most curve balls wouldn’t actually be thrown like that, because major league pitchers are more interested in making the ball curve downward than to the side. There is, however, a kind of fast curve ball with its direction of spin parallel to the ground. Because it’s fast, it doesn’t curve as much (because there’s not as much time for the Magnus Effect to act on it), and all of that curve is to the side. It’s known as the slider.

The screwball is a reverse curve thrown by a right-handed pitcher to break away from a left-handed batter. (Or, presumably, vice versa.)

An even more confusing pitch, for everyone concerned, is the knuckle ball, which, though it curves, is quite different from the curve ball. Whereas a curve ball’s curve is generated by its spin, the knuckle ball (usually thrown off the fingertips, not the knuckles) is thrown with almost no spin: maybe just a half-turn between pitcher and batter.

Its trajectory is therefore influenced most by the difference in air resistance between the stitched and smooth parts of the ball. The slow rotation of the ball moves that area of drag around, so the ball can be pushed in more than one direction. One simulated knuckle ball in a wind-tunnel had moved a full 11 inches off-centre when it was only 20 feet from the plate–then ducked back in to pass right through the strike zone. A batter would relax long before that ball turned into a strike, and a catcher would gather himself to chase a wild pitch. The trouble is, tiny changes in the initial ball orientation give wildly different results at the plate, which makes the knuckle ball very hard to control.

Of course, the key baseball pitch is the fastball, which sizzles across the plate at between 90 and 100 mph. Since the ball loses about one mile per hour every seven feet due to air resistance, it’s actually leaving the pitcher’s hand at well over 100 mph.

Gravity insists that a fastball drop about three feet in the 56 feet it travels from pitcher to batter, which means the pitcher actually has to throw the ball at an upward angle. To the batter, however, a perfectly thrown fastball will appear to travel in a straight line, because the upward angle the pitcher applies and the gravitational effect cancel each other out. The pitcher can also put a “hop” on the ball by throwing it with a backspin, which increases air resistance on the bottom of the ball and forces it up four or five inches.

The “split-finger” fastball, which is legal, has a lot in common with the spitball, which isn’t. Both are delivered with very little spin (the spit, or other lubricant, allows the ball to slip smoothly out of the pitcher’s hand), which means they travel slower and arrive lower than a regular fastball–but the pitcher throws them just as hard, which means the batter is expecting a regular fastball, and is likely to swing too soon and too high.

Scuffing the ball (which is also illegal) can increase drag on one side and cause the ball to move in that direction. It takes a skilled pitcher to make effective use of scuffing, since the effect is small, but even a small effect can be the difference between a home run and a pop fly. That’s because baseball truly is a game of inches–and that, by the way, is why I didn’t use metric measurements in this column.

Whoever heard baseball called a game of centimetres?

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