THE DYNAMICS OF A GOLF BALL 



193 



this is the path of a ball without spin. I can imitate the effect 

 of spin by exposing the particles while they are moving to mag- 

 netic force, for the theory of these particles shows that when a mag- 

 netic force acts upon them, it produces a mechanical force which is at 

 right angles to the direction of motion of the particles, at right angles 

 also to the magnetic force and proportional to the product of the 



Pig. 16. 



velocity of the particles, the magnetic force and the sine of the angle 

 between them. We have seen that the force acting on the golf ball is 

 at right angles to the direction in which it is moving at right angles 

 to the axis of spin, and proportional to the product of the velocity of 

 the ball, the velocity of spin and the sine of the angle between the 

 velocity and the axis of spin. Comparing these statements you will see 

 that the force on the particle is of the same type as that on the golf 



Fig. 17. 



ball if the direction of the magnetic force is along the axis of spin 

 and the magnitude of the force proportional to the velocity of spin, 

 and thus if we watch the behavior of these particles when under the 

 masmetic force we shall get an indication of the behavior of the 

 sninning golf ball. Let us first consider the effect of underspin on 



Pig. 18. 



the flight of the ball: in this case the ball is spinning, as in Fig. 3, 

 about a horizontal axis at right angles to the direction of flight. To 

 imitate this spin I must apply a horizontal magnetic force at right 

 angles to the direction of flight of the particles. I can do this by 

 means of the electromagnet. I will begin with a weak magnetic 

 force, representing a small spin. You see how the path differs from 



