58 UNIVERSITY OF MISSOURI BULLETIN 



muscular force ; the word mass is used as meaning much the 

 same as the ordinary word matter. 



Starting with these concepts the physicist tries to find the 

 simplest relationships existing among them. But no direct 

 and simple relation can be found. All experiments bring in 

 disturbing factors. For example, the actual law of a falling 

 body is not simple. The facts are complicated by the friction 

 of the atmosphere. A sheet of paper flutters slowly to the 

 floor, but crushed into a compact mass it falls with a greater 

 speed. For falling bodies we must then have a very complex 

 law, one which would take into account the shape and density 

 of the falling body. But is this the kind of a law we want? 

 No, we wish to start with the simplest law, one where all the 

 disturbing and complicating factors are removed. We know 

 that the disturbing cause in this case is the air, so we either 

 remove the air or choose bodies of such shape and weight 

 that the air will have only a negligible effect. Then and then 

 only do we get a simple law. And so it is on every hand. 

 But partly by experiment and partly by hypotheses, certain 

 relations between our fundamental concepts are foimd. One 

 of these relations, sometimes called Newton's first law of mo- 

 tion, will be explained merely as an example of this process. 

 This law may be stated by saying that all matter has inertia. 

 A further statement, v/hen we define what we mean by inertia, 

 would include this — that a body would move on forever in a 

 straight line unless acted upon by some force. No one can 

 actually prove that by any experiment. We can never produce 

 a motion where all forces are absent. We always find some 

 forces present, for example friction, and observe that tlie 

 motion soon ceases. So in this case Newton imagined a 

 simple condition, one impossible to attain — that of a moving 

 body with no external forces acting on it. In this condition. 

 he said that the body would move forever in a straight line. 

 This seems simple enough. It may to us who have been 

 brought up on such principles appear self-evident. But it 



