CHAPTER XV 

 WORK AND ENERGY: GRAVITATION 



Energy Defined. Behind work lies energy. Work is 

 what is done. Energy is what does it. Energy is that by 

 means of which anything moves or changes other things. 



The word force has a meaning similar to that of energy, 

 but there is a difference you should note. Energy is a 

 more general word than force. By force we usually mean 

 a specific kind of energy. Thus we speak of the force of 

 gravity rather than of the energy of gravity. Energy is 

 a quality which all forces possess. Forces differ in their 

 nature. Energy is the quality which is common to them 

 all. 



Forms of Energy. Doubtless you have heard the ex- 

 pression "forms of energy." Heat, light, electricity, and 

 the fall of bodies are manifestations of different forms of 

 energy. We call them forms of energy, and we find that 

 they can be transformed into each other. This you have 

 learned, as in the case of transforming water-power into 

 electricity (see page 70). You have also learned of the 

 law of conservation of energy (see page 71). This law 

 may also be stated as follows: no form of energy can be 

 increased or diminished except when some other form of 

 energy is diminished or increased. The same quantity of 

 energy is present in the world to-day as existed ages 

 ago. You cannot increase this total amount of energy and 

 you cannot take from it. Whenever energy is expended 

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