A baseball moving through the air is the same as air moving past the baseball as far as the forces which the air exert on the ball are concerned. A ball thrown straight (without rotating) through the air can
be pictured as air moving past the ball with the same velocity on all sides of the ball which is shown by the equal density of stream lines above and below the ball in Fig. 5. According to Bernoulli's principle,
there are equal pressures (equal velocities) of the air on all sides of the ball and it does not curve.
If the ball is rotating as it moves through the air, its spin will increase the velocity of the air past the ball on one side and retard the velocity of the air past the ball on the opposite side as is indicated in Fig. 6 by many stream lines on one side and few on the other. The higher pressure (low velocity) on the one side pushes the ball toward the low pressure (high velocity) region and it curves as shown by the heavy arrow in Fig. 6. If the ball had been rotating in an opposite
