Molecular Attraction 53 



because of the lack of friction. If you scratch .the 

 soles, they are rougher and you no longer slip. If you 

 try to pull a stake out of the ground, you have to squeeze 

 it harder than the ground does or it will slip out of your 

 hands instead of slipping out of the ground. When 

 you apply a brake to an automobile, the brake must 

 press tightly against the axle or wheel to cause enough 

 friction to stop the automobile. 



There are always two results of friction: heat and 

 wear. Sometimes these effects of friction are helpful 

 to us, and sometimes they are quite the opposite. The 

 heat from friction is helpful when it makes it possible 

 for us to light a fire, but it is far from helpful when it 

 causes a hot box because of an ungreased wheel on a train 

 or wagon, or burns your hands when you slide down a 

 rope. The wear from friction is helpful when it makes 

 it possible to sandpaper a table, scour a pan, scrub a 

 floor, or erase a pencil mark ; but we don't like it when 

 it wears out automobile tires, all the parts of machinery, 

 and our clothes. 



Experiment 17. Hold a nail against a grindstone while 

 you turn the stone. Notice both the wear and heat. Let 

 the nail rest lightly on the stone part of the time and press 

 hard part of the time. Which way does the nail get hotter? 

 Which way does it wear off more quickly? Run it over a 

 pane of glass and see if it gets as hot as it does on the grind- 

 stone ; if it wears down as quickly. 



Why we oil machinery. We can decrease friction 

 by keeping objects from pressing tightly against each 

 other, and by making their surfaces smooth. The 

 most common way of making surfaces smooth is by oiling 



