FRICTION. 



113 



motion after it is started. The weights measure the friction. Place 

 a second similar brick upon the first ; the moving force must be 

 doubled. Place another similar brick upon the other two ; the 

 original moving force must be tripled. 



(5.) Friction is not affected by extent of surface 

 except within extreme limits. In the case of 

 the brick above mentioned, the moving force will 

 be the same whether the brick lie on its broad face 

 or on its side. 



(6.) Friction is greater between surfaces of the 

 same material than between those of differ- 

 ent kinds. 



(a.) Bodies of the same material have the same molecular struc- 

 ture ( 10, a). Hence their little projections and cavities mutually 

 fit each other as would the teeth of similar saws. A very little re- 

 flection will show that the element of similarity in molecular struc- 

 ture (just as with the saws) is very important in determining the 

 amount of friction. For this reason, the axles of railway cars being 

 made of steel, the " boxes " in which they revolve are made of brass 

 or other different metal. Hence the advantages of a watch "full- 

 jewelled," and hence the swiftness of the skillful skater. 



(7.) Rolling friction is less than sliding friction. 



(8.) Friction is diminished by polishing or lubri- 

 cating the surfaces. An unequalled example of 

 friction reduced to its minimum is in the case of 

 the joints of animals. 



EXERCISES. The Screw. 



