462 TRANSFERRING AND TRANSFORMING ENERGY 



work. If we put rollers under the box there is less resist- 

 ance, but some resistance always develops when two sur- 

 faces are moved over each other. This resistance is 

 called friction. The rougher the two surfaces, the more 



the friction ; and the 

 smoother they are, 

 the less the friction. 



To lessen friction 

 we make surfaces 

 that slide over each 

 other very smoothly 

 and oil them. Roll- 

 ing surfaces are found 

 to have less friction 

 than flat surfaces, 

 and so we use ball 

 or cylinder bearings 

 BBI in bicycles, automo- 

 biles, and many other 

 machines. But no 

 matter what we do, 

 some of the work 

 exerted on a machine 

 is always used up in 

 overcoming friction. 

 In an efficient machine the friction is reduced in every 

 possible way in order to avoid as far as possible " loss of 

 energy." In some of the simple machines, especially the 

 wedge and the screw, friction is always so great that the 

 machines are not very efficient. 



The Lever. Experiment 145. (a) Bore a small hole through 

 a meter-stick at each of the decimeter divisions. Place on the table 



INDIAN WEAVING 



A form of skilled manual labor which modern 

 machinery has almost done away with. 



