PULLEYS, INCLINED PLANKS, AND WEDGES 49 



49. The Cost of Mechanical Advantage. It has been 

 shown that by the use of tools and machines which are all 

 based on one of the six principles just described, it is possible 

 to apply a small force to overcome a large resistance. This 

 advantage is obtained by sacrificing either speed to gain 

 force or force to gain speed. The ratio of the resisting force 

 to the applied force is called the mechanical advantage of the 

 tool or machine. The advantage gained in all the simple 

 machines is lost in time. No machine will enable a given 

 amount of force to raise 2 Ibs. with the same velocity as 

 it can raise 1 lb. As a matter of fact, power is wasted by the 

 use of machinery because the in- 

 crease of friction adds to the amount 

 of force which has to be used. 



50. The Effect of Friction. Thus 

 far we have considered the relations 

 of speed, force, and resistance from a 

 somewhat theoretical standpoint; in 

 actual practice a deduction has to be 

 made from the advantage apparently 

 gained because of the resistance of 

 the machine to free motion. This 

 resistance is due to the rough surfaces 

 of the bearings of the machine, al- 

 though to the naked eye these bear- 

 ings may appear perfectly smooth. 

 When polished surfaces are inspected 

 or examined under the microscope 

 seen to have many inequalities and 

 rough. These inequalities fit into the 

 site surface, out of which it requires 



FIG. 34. Metal Under a 

 Magnifying Glass. Im- 

 aginative view of a 

 shaft showing micro- 

 scopic roughness that 

 causes friction. 



(Fig. 34) they are 

 to be comparatively 

 hollows of the oppo- 

 some force to lift or 



