CHAPTER III 

 ENERGY AND FORCE 



ENERGY appears in so many different forms and its 

 changes from one form to another are made so rapidly, 

 that it is sometimes hard for us to believe that we are 

 dealing with varied forms of the same thing. 



The usual definition of energy is capacity for doing 

 work, and we say that work is done when force acts 

 through space. The words energy and force are often 

 misused for each other in ordinary speech, but in 

 science these two words have distinctive meanings. 

 Force is any cause which alters a body's state of rest 

 or of uniform motion in a straight line. It requires force 

 to stop a moving body just as it requires force to set it 

 in motion when it is in a state of rest. Considerable 

 force must be exerted by a team of horses to start in motion 

 a loaded wagon. After it is started, however, just as 

 much force must be exerted to stop it, but the force nec- 

 essary to stop it will not all need to come from the horses. 

 There is another force at work which causes the wagon 

 to rub very hard on the surface over which it passes. 

 This force is called gravity or the force of gravitation. 



Gravity. Sir Isaac Newton (1642-1727) first an- 

 nounced the law which we now call the law of gravitation, 

 although it is now quite certain that the much abused 

 Galileo had a very definite knowledge concerning this 

 force. The law states that every body in the universe 

 attracts every other body with a force which varies 



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