THE CURVING OF A BASEBALL 



201 



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 con- 

 cerned. A ball thrown straiglit (without rotating) through the air can 



H 



Mercury 



Fig. 4. 



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, 



Fig. 5. 



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 



Fig. 6. 



A'OL. LXXXIII. — 14. 



