200 SCIENCE AND METHOD. 



A. The motion of a material point, isolated and un- 

 affected by any exterior force, is rectilineal and 

 uniform. This is the principle of inertia; no accelera- 

 tion without force. 



B. The acceleration of a moving point has the same 

 direction as the resultant of all the forces to which the 

 point is subjected ; it is equal to the quotient of this 

 resultant by a coefficient called the mass of the moving 

 point. 



The mass of a moving point, thus defined, is con- 

 stant; it does not depend upon the velocity acquired by 

 the point, it is the same whether the force is parallel 

 to this velocity and only tends to accelerate or retard 

 the motion of the point, or whether it is, on the con- 

 trary, perpendicular to that velocity and tends to 

 cause the motion to deviate to right or left, that is to 

 say to curve the trajectory. 



C. All the forces to which a material point is sub- 

 jected arise from the action of other material points ; 

 they depend only upon the relative positions and 

 velocities of these different material points. 



By combining the two principles B and C we 

 arrive at the principle of relative motion, by virtue of 

 which the laws of motion of a system are the same 

 whether we refer the system to fixed axes, or whether 

 we refer it to moving axes animated with a rectilineal 

 and uniform forward motion, so that it is impossible 

 to distinguish absolute motion from a relative motion 

 referred to such moving axes. 



D. If a material point A acts upon another material 

 point B, the body B reacts upon A, and these two 

 actions are two forces that are equal and directly 

 opposite to one another. This is the principle of the 



