i 7 6 GENERAL SCIENCE 



ing machine, street car, telephone, all came into use 

 during the past century. Without modern machinery 

 it would not be possible to take coal, petroleum, and gas 

 from the earth. To the scientists, the thinkers, whb 

 brought all these modern mechanical devices into exist- 

 ence, the world owes a debt which it never can pay. 



117. Definitions. In order to understand the princi- 

 ples of some of the simple machines, it is necessary to 

 get a definite idea of a few words which are used in 

 discussing them. 



(A) Energy. In the chapters on heat we often used 

 the word energy, and we have some idea of its meaning. 

 Coal and wood have energy which can be changed into 

 heat by oxidation. Hot iron has more energy than 

 cold iron, and this energy can be removed from the iron 

 by plunging it into cold water. The water then has the 

 energy which the iron had. These objects have energy 

 by virtue of their condition. 



We also have energy in our bodies and can use it at 

 will. A boy coasting down a hill on a bicycle has enough 

 energy to coast part way up another hill. A stone thrown 

 into the water makes the water splash and wave, because 

 of its energy. The energy of a falling hammer will drive 

 a stake into the ground. These objects have energy because 

 of their motion. 



A boy on a sled on the top of a hill in winter has enough 

 energy to take him to the bottom. A book held above 

 the desk has sufficient energy, if permitted to fall, to 

 shake the desk. Water in a city reservoir has enough 

 energy to cause it to flow through the pipes to the houses. 

 These things have energy because of their position. 



So objects may have energy by virtue of their condi- 

 tion, motion, or position. Energy which is due to the 



