PHYSICS 59 



certainly has not always seemed so. The ancient Greeks did 

 not so regard it. To them circular motion was nobler than 

 linear motion and a body, if no force is acting on it, should 

 continue to move in a circle. This principle of Newton is 

 not self-evident and, since we cannot prove it, we assume its 

 truth. 



In this way, with the aid of the imagination, a few simple 

 and fundamental relations are set down. Thus is laid the 

 foundation of physics. 



So, strictly speaking, this foundation rests on pure as- 

 sumptions. These assumptions are, however, not arbitrary, 

 for they must in so far as we can test them agree with the 

 facts of nature. Experience and very searching criticism have 

 shown us that all deductions from these assumptions are, in 

 so far as we can test them, in agreement with observation. 



But I am not giving you the attitude of the founders of 

 this science. It certainly is not true that our early workers 

 in mathematics and physics started out to consciously build up 

 science from assumed relations. In nearly all cases they 

 thought they had found truths. Many times proofs of these 

 relations were offered and only in comparatively recent times 

 have we found fallacies in their arguments. Newton cer- 

 tainly believed that he had experimentally proven the law of 

 inertia. Many have thought that they had worked out cor- 

 rect and independent definitions of force and mass. One emi- 

 nent physicists, after hunting vainly for a sound definition of 

 mass, offers as a confession of failure the following: Masses 

 are coefficients which it is found are convenient to introduce 

 into calculations. 



From these concepts and relations are derived others, for 

 example, velocity, momentum, work, and energy. It is not 

 always easy to form clear concepts of what the physicist 

 means by these terms. For these terms are used in a techni- 

 cal sense. But on account of the many and important uses 

 of the terms work and energy, I shall point out some relations. 



