THEORETICAL MECHANICS. 



CHAPTER III. 



INTRODUCTION TO DYNAMICS. 



I. Mass ; Moments of Mass ; Centroids. 



I. MASS J DENSITY. 



1. In the first part of this work only the geometrical and 

 Idnematical properties of motion have been considered, the 

 moving object being regarded as a mere point or as ^a geo- 

 metrical configuration. It is, however, known, from observation 

 and experiment, that the motions of actual physical bodies are 

 not fully described and determined by those properties alone. 



Physical bodies are distinguished from geometrical configura- 

 tions by being possessed of mass ; and this property as affecting 

 their motion must be taken into account in dynamics. 



2. In physics the mass of a body is usually defined as the 

 .quantity of matter contained in the body. Postponing for the 

 present the full discussion of the idea of mass in its relation to 

 .acceleration and force, and of the methods for comparing and 

 measuring masses, it will suffice for our present purpose to 

 think of the mass of a body as a certain constant quantity, inde- 

 pendent of the body's position or motion with respect to the 

 earth or other bodies, as an indestructible something underlying 

 every physical body. 



The student must be warned not to confound mass with 

 weight. The weight of a body, as we shall see later, is the 

 force with which the body is attracted by the earth ; it varies, 



PART II I I 



