CHAPTER XII 

 FORCE, A SPACE RATE OF ENERGY 



IN our earlier discussion we have used the word 

 ''force" without definition, since we all have a con- 

 cept acquired by our own muscular experiences. The 

 physicist, however, has given to this term a technical 

 meaning. Thus as one of the definitions in the Century 

 Dictionary there is "the immediate cause of a change 

 in the velocity or direction of motion of a body." 

 This concept originated in 1647 with Newton, who 

 stated three fundamental laws as to the motion of 

 bodies, which he had deduced from the facts of astron- 

 omy and from various experiments. His first law says 

 that "every body continues in a state of rest or of 

 uniform motion in a straight line except in so far 

 as compelled by force to change that state." This 

 statement defines force, and implies a characteristic 

 of all bodies, inertia. Bodies are inert. 



That this statement of Newton is really a law we 

 all believe although we cannot prove it. We know 

 that bodies at rest remain at rest unless they are 

 forcibly moved and that it requires force to change 

 the direction of motion of a body. As to the idea 

 that a body would continue in uniform motion in a 

 straight line if no force were exerted upon it we have 

 no proof. If we start a body it comes to rest, sooner 



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