252 



Our Surroundings 



cover the roughnesses and prevent the metals from actually touch- 

 ing. Because rolling contact causes less friction than sliding con- 

 tact, tiny metal balls or rollers, called roller bearings, are often 

 used between moving metal surfaces of machines. 



Under certain circumstances, friction is helpful, even neces- 

 sary. As the locomotive engineer sands a slippery track, so we 

 sprinkle ashes on an icy sidewalk, to make the friction greater. 

 The friction between a pencil and our fingers enables us to hold it. 



Brown Brothers. 



A GIANT CRANE 



A complex machine made up of many simple machines. 



We would be unable to start or stop an automobile without fric- 

 tion between the tires and the road. Without friction we would 

 be helpless. 



Energy and Machines. In all machines, the principle of 

 the conservation of energy is illustrated. W T hen energy is used 

 in the work of a machine none of it is lost. Some of it may 

 disappear in friction, which changes it to heat, but much of it 

 always appears as work accomplished. Remember that energy 



