Gravity dictates that a thrown baseball curves downward on its path through the air. Yet pitches are often described as "dropping" or "rising" as they reach home plate. As recently as Karnavas, Bahill and Regan in 1990, and Bahill and Karnavas in 1993, scientists said a rising fastball was impossible unless delivered from a low side-arm or underhand position. They contended the "curve" of a ball was an optical illusion and simply a batter overestimating the speed of a pitch and swinging at a higher spot than the ball's natural trajectory took it. However, a 19th century experiment shows a ball can deviate from its expected path.
Magnus Effect
Physicist Sir Isaac Newton wrote a paper in 1671 discussing the curved path of a spinning sphere, but it wasn't until 1852 that Gustav Magnus conducted an experiment demonstrating the principle behind modern baseball's breaking pitches. A thin boundary layer of air intermingles with the surface of an object flying through the atmosphere. Round objects are not very aerodynamic and cause the boundary layer to pull away from them, resulting in a low-pressure area -- or wake -- behind the sphere. This is known as the Magnus Effect.
Magnus Force
Drag is the effect of unequal pressure on the front and back of an object. It is the reason that all free-flying objects experience a decrease in forward motion. When a ball is spinning, as breaking pitches do, the boundary layer reacts to the surface spinning toward it as well as the surface spinning away from it. This produces a slight sideways deflection of the ball, creating an uneven wake. The pressure differences on the ball cause additional pressure at a right angle to the forward motion. This is the Magnus force and it changes the path of the ball.
Physical Influences
Pitch speed and the rate of spin on the ball have proportional effects on the Magnus force. The faster the forward speed of the ball, the greater the lateral force. If a pitcher doubles the rate at which the ball spins, the Magnus force is also doubled, resulting in twice the amount of curve. The force is also proportional to the thickness of the air through which it travels. Thinner air produces less break in the pitch. The Magnus force works equally from the release of the ball to the time it reaches the batter. However, the rate of curvature increases the farther the ball travels. Thus, breaking pitches curve more in the final approach to the plate.
Axis of Rotation
In addition to controlling the velocity and spin rate of the ball, the pitcher also affects the amount of break by how he orients the ball's axis of rotation. If the ball rotates on the axis parallel to the flight path, Magnus force has almost no effect. A rotation perpendicular to the flight path is better for ball movement.
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