THEORETICAL MECHANICS 



CHAPTEE I 

 REST AND MOTION 



INTRODUCTION 



1. Uniformity of nature. If we place a stone in water, it will 

 sink to the bottom ; if we place a cork in water, it will rise to 

 the top. These two statements will be admitted to be true not 

 only of stones and corks which have been seen to sink or rise in 

 water but of all stones and corks. Given a piece of stone which 

 has never been placed in water, we feel confident that if we place 

 it in water it will sink. What justification have we for supposing 

 that this new and untried piece of stone will sink in water ? We 

 know that millions of pieces of stone have at different times been 

 placed in water ; we know that not a single one of these has ever 

 been known to do anything but sink. From this we infer that 

 nature treats all pieces of stone alike when they are placed in 

 water, and so feel confident that a new and untried piece of stone 

 will be treated by the forces of nature in the same way as the 

 innumerable pieces of stone of which the behavior has been 

 tested, and hence that it will sink in water. This principle is 

 known as that of the uniformity of nature ; what the forces of 

 nature have been found to do once, they will, under similar condi- 

 tions, do again. 



2. Laws of nature. The principle just stated amounts to say- 

 ing that the action of the forces of nature is governed by certain 

 laws ; these we speak of as laws of nature. For instance, if it 

 has been found that every stone which has ever been placed in 



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