baseball

This is an application of Bernoulli's Principle. The stitches on a ball actually make it curve. The pitcher's fingers hold the ball along a seam, so when the ball is thrown with a snapping motion, it has topspin. Friction provided by the stitches causes a thin layer of air to move around the spinning ball in such a way that air pressure on top of the ball is greater than on the bottom, causing the ball to curve downward and the batter to miscalculate the position of the ball.

You may also refer to the detailed explanation given in the web-page below:

http://home.frognet.net/~ejcov/curve.html