64 



GENERAL SCIENCE 



If three fourths of the force applied to a machine is 

 available for useful work, while one fourth is used in 

 overcoming friction in the machine and moving its parts, 

 we say the efficiency of the machine is 75 per cent. 



Many ways have been devised for reducing friction 

 and thus increasing the efficiency of machines. Roller 



FIG. 61. A, Common Bearing; B, Ball Bearing; C, Roller Bearing. 



and ball bearings greatly reduce friction by substituting 

 rolling friction for sliding friction (Figure 61). 



Experiment 13. Determine the force necessary to slide a mass 

 of 500 grams over a level table top. Now determine the force 

 necessary to move a similar mass on wheels. Which is greater, 

 the sliding friction or the rolling friction? What effect has lubri- 

 cating oil on sliding friction ? 



EXERCISES 



1. What power must be exerted to roll a barrel weighing 300 

 pounds up a plank 10 feet long into a wagon 3 feet high? 



2. Mention three uses of the inclined plane. 



3. The diameter of the wheel on a letter press is 16 inches. 

 The pitch of the screw is one half inch. What, pressure will be 

 produced by a force of 100 pounds applied to the wheel? 



4. How may the pitch of a screw be determined? 



5. Why is it difficult to walk on highly polished floors? 



6. Has friction any value? 



7. Mention four uses of rollers to reduce friction. 



8. What causes bearings to become hot? 



9. How long must an inclined plane be so that a force of 100 

 pounds will roll a barrel weighing 400 pounds into a wagon 3 feet 

 high? 



