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Common Science 



FIG. 25. The friction of the stone heats the nail and wears it away. 



or greasing them. A film of oil or grease makes things 

 so smooth and slippery that there is very little friction. 

 That is why all kinds of machinery will run so smoothly if 

 they are kept oiled. And since the oil decreases friction, 

 it decreases the wear caused by friction. So well-oiled 

 machines last much longer than machines that are not 

 sufficiently oiled. 



Why ball bearings are used. There is much less 

 friction when a round object rolls over a surface than 

 when two surfaces slide over one another, unless the 

 sliding surfaces are very smooth ; think how much easier 



