1910] 



on the Dijnamics of a Golf Ball. 



801 



can lie raised or lowered so as to bring eitiier the smooth or the 

 braml^le-marked ball opposite to the blast. The pressure is measured 

 in the following way : L M are two tubes connected with the 

 pressure-gauge P Q ; L and M are placed so that the golf balls can 

 just fit in between them ; if the pressure of the air on the side M 

 of the balls is greater than that of the side L the liquid on the 

 right-hand side Q of the pressure-gauge will be depressed ; if, on the 

 other hand, the pressure at L is greater than that at M the left-hand 

 side P of the gauge will be depressed. 



I first show thtlt when the golf balls are not rotating there is no 

 difference in the pressure on the two sides when the blast is directed 

 against the balls ; you see there is no motion of the liquid in the 

 gauge. Next 1 stop the blast and make the golf balls rotate ; again 

 there is no motion in the gauge. Now when the golf balls are 

 spinning in the direction indicated in Fig. 11, I turn on the blast, 

 the liquid falls on the side Q of the gauge, rises on the other side. 

 Now I reverse the direction of rotation of the balls, and you see the 

 motion of the liquid in the gauge is reversed, indicating that the 

 high pressure has goue from one side to the other. You see that 

 the pressure is higher on the side M where the spin carries this side of 

 the ball into the blast, tlian on L where the spin tends to carry the ball 

 away from the blast. If we could imagine ourselves on the golf ball,, 

 the wind would be stronger on the side M than on L, and it is on the 

 side of the strong wind that the pressure is greatest. The case when 

 the ball is still and the air moving from right to left is the same from 

 the dynamical point of view as when the air is still and the ball 

 moves from left to right ; hence we see that the pressure is greatest 

 on the side where the spin makes the velocity through the air greater 

 than it would be without spin. 



Thus, if the golf ball is moving as in Fig. 12, 

 the spin increases the pressure on the right of the 

 ball, and diminishes the pressure on the left. 



To show the difference between the smooth ball 

 and the rough one, I bring the smooth ball opposite 

 the blast ; you observe the difference between the 

 levels of the liquid in the two arms of the gauge. 

 I now move the rough ball into the place previously 

 occupied by the smooth one, and you see that the 

 difference of the levels is more than doubled, showing 

 that with the same spin and speed of air blast the 

 difference of pressure for the rough ball is more than 

 twice that for the smooth. 



AYe must now go on to consider why the pressure of the air on 

 the two sides of the rotating ball should be different. The gist 

 of the explanation was given by Newton nearly 250 years ago. Writing 

 to Oldenburg in 1671 about the dispersion of light, he says, in the 

 course of his letter, " I remembered that I had often seen a tennis- 



3 CI 2 



O' 



Fig. 12. 



