FORCES AND MOTIONS IN DAILY LIFE 



149 



faces are put together, the tiny irregularities seem to 

 fit together. They hook on to one another and thus 

 resist any tendency for one surface to slide over the 

 other one. This is illustrated by Figure 233. 



FIC.. 233. EVEN SMOOTH SURFACES ARE ROUGH 



There are two kinds of friction. Everyone knows 

 that it is much easier to draw a cart than to draw a 

 sled over a pavement. The reason for this is that with 

 the sled we are dealing with sliding friction and in 

 the wagon with rolling friction. In the first case the 

 surface irregularities slide over each other and show 



FIG. 234. ROLLING FRICTION IS LESS THAN SLIDING FRICTION 







a greater tendency to hook and resist. In the case 

 of rolling friction in the cart there is less surface 

 in contact with the pavement to begin with, and then 

 the surfaces tend to roll or slip by each other with 

 less chance for the little projections of one surface 

 to hook to those of the other. 



Friction helps us in many ways. If our shoes did 

 not have friction with surfaces we should be unable 

 to walk or run. In playing athletic games in the gym- 

 nasium we use rubber-soled shoes because they have 

 greater friction with the floor. Football and baseball 

 players wear shoes with projections on the bottom, 

 which tend to increase the friction with the earth and 

 prevent the athlete from slipping. Wherever power is 

 transmitted from one pulley wheel to another by 

 means of a belt it is made possible only by the friction 

 of the belt with the pulley. In the automobile, besides 

 the use of friction in the tire treads and chains, it is 

 used in the clutch and in the brakes. Friction is used 

 so much in the brakes to stop the automobile that we 

 must frequently have new asbestos linings put in 

 them to increase the friction and make them safer. 



Man has learned how to reduce friction. Wherever 

 moving parts of machines which are doing useful work 

 rub together, friction is a hindrance. Great industries 

 have been built up in an endeavor to reduce the hin- 

 drance of friction to the lowest possible point. One of 



the best ways to reduce friction is by lubrication with 

 some substance such as oil. This tends to fill the little 

 irregularities in the surface and keep them from hook- 

 ing on to each other. 

 If you have ever tried 

 to push a lawn mower 

 before and after oiling, 

 you know how much 

 harmful friction can 

 be reduced. The oil in- 

 dustry is maintained 

 partly by man's en- 

 deavor to reduce fric- 

 tion. 



The fact that roll- 

 ing friction is less 

 than sliding friction 

 has led to the develop- 

 ment of another great 

 industry. This is the 

 roller and ball bearing 

 industry. The use of 

 little rollers and balls, 



made of hard steel, between surfaces which are in 

 contact, greatly reduces the amount of friction. In 

 Figure 235, which shows roller bearings, you will 

 notice that the rollers are held in frames. 



Friction is usually associated in our thinking with 

 hard surfaces, but in recent years air friction has 

 come to play an important part in transportation. Au- 

 tomobiles, buses, and trains have adopted stream- 

 lined designs which tend to reduce air friction and 

 thus make possible greater speeds. Airplanes also 



FIG. 235. ROLLER BEARINGS 



Courtesy Chrysler Corporation 



FIG. 236. STREAMLINED AUTOMOBILE 



have been streamlined to reduce air friction, and 

 planes are now flown at greater altitudes where the 

 air is less dense and therefore produces less frictional 

 resistance to retard the planes. Figure 236 is a pic- 

 ture of a modern streamlined automobile. 



Why do substances have weight? Weighing is one 

 of the most important activities in which we engage 



