1910] 



Oil the Dynamics of a Golf Ball. 



805 



Fig. 16 ; 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 



Fig. 16. 



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

 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 bait 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 o-olf 

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

 and the magnitude of tlie force proportional to the velocity of spin. 



Fig. 18. 



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

 magnetic force we shall get an indication of the behaviour of the 

 spinning golf ball. Let us first consider the effect of under-spin on 

 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 



