CHAPTEE IV 



STATICS OF SYSTEMS OF PARTICLES 



44. So far we have been considering the action of forces on a 

 single particle. A different class of problems arises in considering 

 the action of forces on a body composed of a great number of par- 

 ticles, to which forces are applied in such a way as to act on the 

 different particles of the body. 



Consider what happens when a force F is applied to one parti- 

 cle A of a body which is composed of a great number of particles 

 A, B, C, D, -. If the particle A 

 were in no way influenced by the 

 other particles B, C, D, the 

 particle A would start into motion 

 under the action of the applied 

 force, and would soon become sepa- 

 rated from the other particles B, 

 C, D,--. If, however, the particles 



A, B y (7, D, - constitute a single 

 continuous body, this does not 

 happen. What happens is that as 



soon as the particle A begins to move relatively to the other parti- 

 cles, systems of actions and reactions come into play between the 

 particle A and the adjacent particles B, (7, X>, . Speaking loosely, 

 we may say that the forces acting on A tend to check the motion 

 of A, while the corresponding reactions tend to impart motion to 



B, C,D,--. When B, C,D,-- start into motion, further systems of 

 forces begin to operate on the particles next beyond B, C, D, -, 

 and so on. Thus all the particles are set into motion, and instead 

 of the particle A moving singly the complete body moves as a whole. 

 We have now to discuss whether such a body, or system of bodies, 



59 



FIG. 31 



