THE DYNAMICS OF A GOLF BALL 187 



is acted on by a force in the direction in which the nose of the body 

 is moving relatively to its center; the direction of this force is thus 

 at right angles, both to the direction in which the center of the body 

 is moving, and also to the axis about which the body is spinning. 



Pig. 7. 



For this purpose a cylinder A (Fig. 7) is mounted on bearings so that 

 it can be spun rapidly about a vertical axis; the cylinder is attached to 

 one end of the beam B, which is weighted at the other end, so that 

 when the beam is suspended by a wire it takes up a horizontal posi- 

 tion. The beam yields readily to any horizontal force, so that if the 

 cylinder is acted on by such a force, this will be indicated by the 

 motion of the beam. In front of the cylinder there is a pipe D, 

 through which a rotating fan driven by an electric motor sends a blast 

 of air which can be directed against the cylinder. I adjust the beam 

 and the beam carrying the cylinder, so that the blast of air strikes the 

 cylinder symmetrically; in this case, when the cylinder is not rotating 

 the impact against it of the stream of air does not give rise to any 

 motion of the beam. I now spin the cylinder, and you see that when 



BUst 



Fig. 8. 



the blast strikes against it the beam moves off sideways. It goes off 

 one way when the spin is in one direction, and in the opposite way 

 when the direction of spin is reversed. The beam, as you will see, 

 rotates in the same direction as the cylinder, which an inspection of 

 Fig. 8 will show you is just what it would do if the cylinder were acted 

 upon by a force in the direction in which its nose (which, in this case, 

 is the point on the cylinder first struck by the blast) is moving. If I 

 stop the blast, the beam does not move even though I spin the cylinder, 

 nor does it move when the blast is in action if the rotation of the 

 cylinder is stopped; thus both spin of the cylinder and movement of 

 it through the air are required to develop the force on the cylinder. 



