CHAPTER V. 

 INERTIA. FORCE. GRAVITATION. 



11. THE NATURE OF FORCE. 



i. To show that a mass at rest tends to remain so. Tie a piece of cotton 

 or thread to the handle of a heavy door and then attempt to move the door 

 by pulling the thread sharply to one side. The thread will break and the 

 door will hardly be moved. The door can, however, easily be pulled to one 

 side if the pulling force is gradually applied. 



ii. A moving mass requires force to stop it. Swing the heavy door used 

 in the preceding exercise to and fro, keeping loose the thread attached 

 to it. While it is swinging away from your hand, stop your hand suddenly. 

 The thread will break and the door will proceed on its swing. 



Inertia. Common experience tells everyone that things do not 

 move of themselves. An object at rest remains at rest until it is forced 

 to move. Moreover, if it is moving it tends to go on moving in the 

 same direction and with the same velocity until made to change by 

 the application of force. In a word, dead matter is helpless and con- 

 servative. It does nothing by itself, and objects to alter its condition 

 whatever that condition may be. The inability shown by a material 

 body to change by itself its condition of rest or of uniform motion in a 

 straight line is called its inertia. It is exemplified when a cyclist is 

 suddenly stopped, for the tendency to continue moving is so great 

 that the cyclist, if he is travelling quickly, is shot over the handle-bar 

 of his machine. This law of inertia is often spoken of as Newton's 

 First Law of Motion. 



First law of motion. Every object remains at rest or moves with 

 uniform velocity in a straight line until compelled by force to act 

 otherwise. 



This law, which Newton first stated as being always obeyed by 

 bodies in nature, means, first, that if a body is at rest, it will remain 

 still until there is some reason for its moving until some outside 

 influence, which is called a force, acts upon it. In fact, the law really 



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