PART II. 



STRESSES. 



CHAPTER V. 

 GRAPHIC STATICS. 



Equilibrium. Statics considers forces as at rest and therefore in 

 equilibrium. To have static equilibrium in any system of forces there 

 must be neither translation nor rotation and the following conditions 

 must be fulfilled for coplanar forces (forces in one plane). 



2 horizontal components of forces = o (a) 



3 vertical components of forces =: o (b) 

 2 moments of forces about any point = o (c) 



Representation of Forces. A force is determined when its 

 magnitude, line of action, and direction are known, and it may be rep- 

 resented graphically in magnitude by the length of a line, in line of 

 action by the position of the line, and in direction by an arrow placed 

 on the line, pointing in the direction in which the force acts. A force 

 may be considered as applied at any point in its line of action. 



Force Triangle. The resultant, R, of the two forces P and P 2 

 meeting at the point a in Fig. 8 is represented in magnitude and direc- 

 tion by the diagonal, R t of the parallelogram abed. The combining 

 of the two forces P 1 and P 2 into the force R is termed composition of 

 forces. The reverse process is called resolution of forces. 



