MECHANICAL PARADOXES. 



which the ball is thrown. At R the spin is 

 in the reverse direction. 



In the case of spinning like that at Q it 

 is clear that the front of the ball, which presses 

 hardest against the air, is moving downwards 

 and so throwing the air downwards. Conse- 

 quently the reaction tends to keep the ball up 

 that is, to prevent it from falling so fast as 

 it otherwise would. In other words, it has a 

 longer and more shooting flight than it would 

 otherwise have at the same speed. In R the 

 opposite is the case ; the reaction from the 

 front friction due to its spin helps the force 

 of gravity to pull it downwards, so that it 

 drops more rapidly than gravity alone would 

 make it drop. 



In various games with balls these spins 

 play an important part in deceiving the bats- 

 man as to the probable flight of a ball. The 

 effect is usually complicated by combining 

 two of the spins described above ; and in cricket 

 and tennis it is further complicated by the 

 influence of spin on direction at the moment 

 of bouncing. In cricket the break off the 

 ground is more important than the swerve in 

 the air, developing later and more suddenly, 

 so as to be more difficult to see, and producing 

 a greater change of direction in a given distance. 



This section, however, is concerned only 

 with the swerve produced by spin during the 

 flight through the air. It is this effect of 



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