PHYSICAL SCIENCE 33 



of length and time, we have the complete Newtonian 

 framework of mechanical science. From these 

 three units all other mechanical units may be 

 derived. 



For instance, force, as we have seen, is a direct 

 sense-perception, and therefore might be made a 

 fundamental physical concept; but it is treated in 

 practical science as a unit derived from the unit of 

 mass, which, though theoretically derived, is more 

 convenient to take as fundamental. The mass of 

 a body is defined by the force required to produce 

 a given acceleration, and so force is measured by 

 the product of the acceleration and the mass of the 

 body on which it acts, or F = Ma. 



Again, work or energy is defined as the product of 

 a force into the distance the force moves its point 

 of application, so that, in terms of fundamental units, 

 energy is FL = MaL, or since a is L/T 2 we get 



ML 2 

 Energy = -?p- 



Newton's formulation of dynamics has held undis- 

 puted sway till, in recent years, it has been shown 

 that the effective mass of an electrically charged body 

 in motion increases as the velocity of light is ap- 

 proached. But, whatever be the outcome of recent 

 discussion, at all velocities possible in mechanical 

 experiment, Newtonian results are in accordance 



with practical observation. 



o 



