INTRODUCTION 



Augustus Young, for many years professor of astronomy 

 at Princeton University. He says (General Astronomy, 

 Art. 400) : 



It has been customary with some writers to speak of a body 

 thus moving "uniformly in a straight line" as actuated by a 

 "projectile force", a very unfortunate expression, which is a 

 survival of the Aristotelian idea that rest is more "natural" to 

 matter than motion, and that when a body moves, some force 

 must operate to keep it moving. The mere uniform rectilinear 

 motion of a material mass in empty space implies no action of a 

 physical cause, and demands explanation only as mere existence 

 does. Change of motion, either in speed or direction this alone 

 implies force in operation. 



The second is from the pen of that noted mathe- 

 matician, Professor A. N. Whitehead (Introduction to 

 Mathematics, p. 43) : 



Galileo's successful experiment was not the result of a mere 

 lucky guess. It arose from his correct ideas in connection with 

 inertia and mass. The first law of motion, as following Newton 

 we now enunciate it, is Every body continues in its state of 

 rest or of uniform motion in a straight line, except so far as it 

 is compelled by impressed force to change that state. This law 

 is more than a dry formula ; it is also a paean of triumph over de- 

 feated heretics. The point at issue can be understood by delet- 

 ing from the law the phrase "or of uniform motion in a straight 

 line". We there obtain what might be taken as the Aristotelian 

 opposition formula : "Every body continues in its state of rest ex- 

 cept so far as it is compelled by impressed force to change that 

 state. 



In this last -formula it is asserted that, apart from force, a 

 body continues in a state of rest ; and accordingly that, if a f>ody 

 is moving, a force is required to sustain the motion ; so that when 

 the force ceases, the motion ceases. The true Newtonian law 

 takes diametrically the opposite point of view. The state of a 

 body unacted on by force is that of uniform motion in a straight 

 line, and no external force or influence is to be looked for as the 

 cause, or if you like to put it so, as the invariable accompaniment 

 of this uniform rectilinear motion. Rest is merely a particular 

 case of such motion, merely when the velocity is and remains 

 zero. Thus, when a body is moving, we do not seek for any ex- 

 ternal influence except to explain changes in the rate of the ve- 

 locity or changes in its direction. So long as the body is mov- 

 ing at the same rate and in the same direction there is no need 

 to invoke the aid of any forces. 



