CHAPTER III 

 THE GIFTS OF THE SUN TO THE EAKTH 



22. Energy. The capacity for doing work, for over- 

 coming resistance, for causing change, is called energy. 

 If the position of a body or its composition is such that it 

 can exert force or overcome resistance, its energy is called 

 potential. If the body is actually moving, it is said to have 

 kinetic energy. 



When a pendulum bob is pulled aside and held higher 

 than the lowest point of its arc, it has potential energy. 

 If it is allowed to swing, it will change this 

 potential energy into kinetic. The potential 

 energy it had when at its highest point 

 is changed into the same amount of kinetic 

 energy when it reaches the lowest point 

 in its swing. 



When a gun is loaded the powder has 

 potential energy due to its composition, but when it 

 explodes this is changed into kinetic energy which is 

 imparted to the bullet. The smallest possible piece of 

 nitroglycerine has potential energy on account of the 

 arrangement of its molecules, and this is capable of being 

 readily changed into kinetic energy. 



The sun throughout its existence has been sending vast 

 quantities of energy to the earth. This energy has been 

 mostly in the form of heat and light. The ability of the 

 earth to support plant or animal life or to furnish man 

 the power necessary to carry on his industries is due to the' 



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