Why does a curveball curve?
The principle factor affecting a moving (and spinning) baseball is air drag. Imagine a standard fastball thrown with a straight overhead motion. The ball will be spinning naturally on a horizontal axis (with the top of the ball rotating back toward the pitcher). This spin causes the magnitude of the drag vectors to be different near the top half of the ball than they are on the bottom half. Without the spin, the drag force would be equal on both hemispheres of the ball…
With a standard, straight fastball the air resistance is greater on the side with the higher relative motion of the ball against the air. This increased (and decreased) drag will affect the motion on the vertical axis and cause the ball to either drop faster or slower than it would due to normal gravity. As an aside, there is no such thing as a “rising fastball.” What batters call a rising fastball is simply one that doesn’t drop as much as is expected.
Now on to the curve… most curve balls are thrown with more of a sidearm motion to give the ball a different, more lateral spin. Some pitchers have what’s called an “overhand curve” but it’s basically just an overhand motion in which they break their wrist at the last moment to give the ball the lateral spin.
This lateral spin changes the differential in the drag vectors from acting on the upper half and bottom half (as with the fastball) to the left half and right half of the ball. This causes the lateral movement of the curveball.
Another note - a rough ball will travel faster (and further) than a completely smooth one. The roughness of a baseball either from the seams or scuffed leather will create a small layer of turbulent air around the ball which reduces the air drag. This is the same principle that causes a dimpled golf ball to travel three times further than a smooth one would. I guess this is one of the reasons that pitchers like scuffed balls and umpires don’t…
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