550 MACHINES, WORK, AND ENERGY 



the fuel, not from the mechanism of the engine. We have 

 seen in Chap. VI that from which nian or the horse gets his 

 supply of energy. What is it? 



651. Motors. Any machine or animal used to transform 

 energy into work is called a MOTOR. A steam engine trans- 

 forms the energy in coal into work; it is a COAL-STEAM MOTOR. 

 A gasoline engine transforms the energy in gasoline into work; 

 it is a GASOLINE MOTOR. A machine which transforms the 

 energy in an electric current into work is an ELECTRIC MOTOR. 

 To the extent that any man or animal simply transforms the 

 energy in the food eaten into mechanical work, he is a motor. 

 Agriculturists speak of the work horse as an ANIMAL MOTOR. 



A steam engine or gasoline engine is capable of doing work 

 only as long as it is consuming fuel; as soon as it ceases to 

 consume fuel it ceases to be able to do work. It is equally 

 true that a man or a horse soon ceases to be able to do work 

 unless supplied with food. It is the energy in the food 

 eaten which enables man or animal to do work. Thus we see 

 that man may differ little from a machine if he consumes food 

 merely for the purpose of transforming the energy in the food 

 into mechanical work. 



652. One Difference between an Animal Motor and a 

 Mechanical Motor. While a mechanical motor, such as a 

 steam engine or a gasoline engine, ceases to be able to do work 

 almost at the instant that the fuel supply is exhausted, an 

 animal motor can continue to do work for some time after 

 its food supply is exhausted. This is because the fat and 

 other tissue of the body can be converted into energy when 

 necessary. Any animal, however, which is obliged to do 

 hard labor without a sufficient supply of food will lose weight 

 rapidly and will soon die. 



653. Efficiency of Various Motors. Many experiments 

 performed show that the average efficiency of man as a motor 

 is about 20 per cent. That is, it has been found that a man 

 is able to convert about 20 per cent, of the energy in the food 

 he eats into mechanical work. Similar experiments show 



