Pedro Martinez, Boston Red Sox

Image Source A Baseball with 216 raised red stitches hits the air and curves right under the batter, and the batter swings. Why did the ball drop? Why did the batter swing? What exactly happens to the ball as it is thrown? What happens to the ball depends on what spin was put on it. What causes the ball to curve, slide or stay in a strait pattern? This all has to do with the fact that there is drag force, or air resistance. A curve ball is created when a ball is spinning. The faster flowing air under the ball creates less pressure, which forces the ball to dive or break. Baseball would be a dull game without drag force because there would be no curves, sliders, or knuckle balls.

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Twisiting Motion of the wrist to cause a curve ball

Image Source So how exactly do pitchers throw the curve balls? They grasp the ball with the middle and index fingers on or near the stitching, with their thumb underneath. As they throw the ball, they snap their wrist in a turning motion, like turning a door knob, to make the ball spin in the direction of the throw. The stitching on the ball gathers up air as the ball rotates, creating higher air pressure on one side of the ball. The higher velocity difference puts more stress on the air flowing around the bottom of the ball. That stress makes air flowing around the ball "break away" from the ball's surface sooner. Conversely, the air at the top of the spinning ball, subject to less stress due to the lower velocity difference and can "hang onto" the ball's surface longer before breaking away. Therefore curveballs do most of their curving in the last quarter of their trip. Perception plays a big role in the curve ball: The typical curveball goes through only 3.4 inches of deviation from a straight line drawn between the pitcher's hand and the catcher's glove. However, from the perspective of the pitcher and batter, the ball moves 14.4 inches. This proves that a curve ball really curves.

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