PART I. 



THEORY OF NEWTONIAN FORCES. 

 CHAPTER I. 



PRINCIPLES OF MECHANICS. UNITS AND DIMENSIONS. 



46. Matter and Energy. Dynamics. Physics is the 

 science of Matter and of Energy. Its laws are found to be 

 invariable and capable of exact statement, that is of presentation 

 in the language of Mathematics. The application of mathematical 

 analysis to the treatment of physical phenomena, enabling us to 

 deduce general laws from the results of experiment, and to infer 

 the consequences of general laws, forms the subject of Mathe- 

 matical or Theoretical Physics. 



Matter has the essential property of occupying space. It has 

 in addition universally only the property of Inertia, to be defined 

 below. In order to define Energy, we must consider the motion of 

 matter in space. That portion of mathematical physics which 

 treats of the motion of matter is called Mechanics, or Dynamics. 

 It is the object of physicists to reduce the explanation of all 

 physical phenomena to descriptions of motion of matter, and 

 accordingly the study of the principles of Dynamics is indis- 

 pensable to the study of any branch of theoretical physics. Before 

 considering the nature of electrical and magnetic phenomena we 

 shall therefore devote a few chapters to Dynamics. 



47. Scalar and Vector Quantities. Physical quantities 

 are of two kinds. Quantities whose complete specification involves 



