50 



APPLIED SCIENCE 



slide them. This is done first by polishing the surfaces 

 until they are as smooth as possible and then by inserting 

 some lubricant, such as oil, grease, or black lead which fills 

 up the little holes and thus reduces friction. Friction is 

 also reduced by having two different substances or metals 

 in contact, as, for example, the brass or sometimes jeweled 

 boxes in which the steel axles of wheels in clocks and watches 

 revolve. The greatest amount of friction arises just before 

 motion takes place, because the inequalities of the upper 

 surface sink into those in the lower more completely at rest 

 than in motion. 



In going down a hill, drivers of heavy vehicles pass a 

 chain through a spoke of the wheel to increase the friction, 

 and thus prevent the wheel from turning. Friction between 

 the ground and the shoe enables us to walk. Shoes with 

 hob nails are dangerous on a smooth 

 iron plate because the two iron surfaces 

 give little friction. 



51. Use of Ball Bearings. Rolling 

 friction is friction due to a solid rolling 

 over a smooth surface, as in the case 

 of a car wheel moving over a rail, while 

 a sliding friction is due to the sliding of 

 the same particles of a wheel over a 

 rail. Sliding friction is greater than 

 rolling friction, and in the case of iron 

 it is 100 times greater. Hence the use 

 of ball bearings (Fig. 35). 



FIG. 35. Ball Bear- 

 ings. 



52. Measurement of Friction. In all machines there is 

 more or less friction. The work done by the acting force 



