54 



THE HUMAN MOTOR 



into frictional engagement with the pulley so that the engine 

 assumes a steady speed equal to that which it would have if it 

 did a definite work, against the resistances which oppose it. 

 Therefore the work done by an engine in overcoming the friction 

 equals the ordinary capacity of the engine for doing work. In 

 order that the lever L shall not be dragged round by the rotation 

 of the brake-band, weights Fj are placed in the scale G, which 

 hold it horizontal, but allow slight oscillations. The counter 

 weight D allows the lever and the scale pan to be in equilibrium 

 before each experiment, the movements being limited by the 

 stops T and T'. Thus, in equilibrium, the force of friction which 

 acts at the extremity of the radius r of the pulley, will have the 

 same moment as the force F, acting at the extremity of the lever 

 L, from which : 



* X r = F! X L. 



The work done by friction 2 K r x $ per revolution will be the 

 expression 2 n FjL. The work done 2 n F X L x n, per n revolu- 

 tions, can be estimated from the values of F found by experiment. 

 It is well to remember, on the one hand, that a Prony brake 

 gives the work done by an engine making n revolutions in a known 

 time ; whilst, on the other hand, that the friction has allowed the 

 production of a static effort (raising of a loaded scale) at the ex- 

 pense of mechanical energy. 



Sliding friction has many practical applications, whether it be 



to maintain a ladder in position when 

 leaning against a wall with its base 

 on the ground, to retain the work in 

 a joiner's vice, or to rotate pulleys 

 by means of belts. It can be recti- 

 lineal in the case of sledges, con- 

 centric to an axis as in a pivot, or 

 circular as in a spindle. 



When a point M slides with friction 

 on a surface the force F, which pulls 

 it along, has a component N which 



