THE PRINCIPLES OF DYNAMICS. 

 CHAPTER V. 



MASS AND FORCE. 



67. We have seen in the last Chapter how a description of 

 the motion of a point can be furnished by a statement of its 

 acceleration, we have now to see how this knowledge can be made 

 available for a complete description of the motions of bodies. To 

 this end it is necessary to form a conception of bodies and of the 

 nature of their mutual actions. The elements that enter into 

 this conception have all been suggested by experience, but the 

 conception is much more precise in detail than the observations 

 that suggested it. It will therefore be best in the first place to 

 regard the conception as purely ideal, and afterwards to explain 

 how it is applied to obtain a description of the motions of natural 

 bodies. 



The first point to which attention must be directed is the 

 distinction between homogeneity and heterogeneity. If we divide 

 a body into a number of parts of exactly equal size and of like shape, 

 then it may happen that we are unable to distinguish the parts one 

 from another by any difference of quality. The parts may be all of 

 the same weight, as tested by a balance, they may all be equally 

 hard, and so on. When this is the case the body is said to be homo- 

 geneous, otherwise it is said to be heterogeneous. Now experience 

 shows that a heterogeneous body can be divided into parts each 

 more nearly homogeneous than the body, and this suggests that 

 a heterogeneous body may be regarded as made up of homogeneous 

 parts, each part being itself a body. 



The first step in forming our ideal conception is to acquire the 

 notion of a material figure, suggested as a representation of a 

 homogeneous body or of a homogeneous part of a body. 



