EFFECT OF CENTRIFUGAL FORCE 35 



total sliding velocity of belt corresponding to a fixed per- 

 centage of the belt speed would also be incorrect. The rea- 

 son for this is that even a very low percentage would mean 

 a high sliding velocity with a high-speed belt, while a high 

 percentage would mean only a moderate sliding velocity in 

 the case of a low-speed belt. For example, a slip of i per 

 cent in the case of a belt velocity of 4000 ft. per minute would 

 be an actual slip of 40 ft. per minute, while a 5 per cent slip 

 at a belt velocity of 300 ft. per minute would be but 15 ft. 

 per minute of actual slip. 



The studies all pointed to the fact that the coefficient of 

 friction is a quantity that varies with the velocity, and its 

 value was finally fixed by Mr. Earth, after considerable study, 

 by the empirical formula 



, in which V is the velocity of the belt in feet per minute. 



Further studies of Mr. Lewis's work convinced Mr. 

 Barth that it was best, due to conflicting results, to !eave out 

 of consideration entirely the effect of initial tension on the 

 coefficient of friction, and to ignore the conclusion that it 

 depends in part on the intensity of the pressure. The values 

 of / according to the above formula for velocities ranging 

 from o to 6500 ft. per minute, are given in the table on 

 page 38. 



Effect of Centrifugal Force. A heavy particle moving 

 in a curved path of radius R exerts a force on the arm, cord 

 or other medium which constrains it to move in the curved 

 path. This force is termed centrifugal force and is repre- 

 sented by the formula 



, Wv* 



1 W - 



in which W is the weight of the body in pounds, V the linear 

 velocity in feet per second, R the radius, in feet, of the path 



