ENERGY AND ITS RELATION TO USE OF MACHINES 549 



locomotive were attached to a train requiring a force of more 

 than 60 tons to move it,*the locomotive would be unable to 

 start the train because its drive wheels would then slip on 

 the rails. A bicycle or an automobile is likewise propelled by 

 the friction between its wheels and the ground. Men work- 

 ing upon the ice wear ice creepers on their shoes to increase 

 friction between their shoes and the ice. After an ice or sleet 

 storm, horses are almost useless for hauling loads unless they 

 are sharp shod. Why? Give as many cases as you can where 

 friction is of service to us. How do the brakes of a railroad 

 train bring the train to rest? How is friction involved in 

 feeding the paper through a typewriter or printing press? 

 Explain how a leather belt is used to drive a sewing machine 

 or a threshing machine. 



649. Friction Produces Heat. All energy lost due to 

 friction reappears in the form of heat. It is the friction of the 

 head of the match upon a rough surface which produces 

 sufficient heat to ignite the match. What evidence is there 

 that a large amount of heat is produced by the brakes of a 

 railroad train when it is brought to a sudden stop? What 

 effect is produced upon one's hands when sliding down a rope 

 too rapidly? If the bearings of a machine are not frequently 

 oiled they are likely to become "heated" and possibly ruined. 

 Explain why this is so. 



V. ENERGY AND ITS RELATION TO THE USE OF MACHINES 



650. What is Meant by Energy? Anything which is capa- 

 ble of doing work possesses ENERGY. ENERGY is the capacity 

 for doing work (Art. 85). Man, by putting forth effort, does 

 work; therefore he possesses energy. A horse may do work; 

 therefore, he possesses energy. A steam engine, as long as it is 

 supplied with fuel and water and is properly controlled, can 

 do work; therefore it possesses energy. The machine by itself, 

 i.e., without fuel and water, can do no work. It is the fuel 

 which gives it the ability to do work. The energy comes from 



