180 



RESISTANCE OF MATERIALS 



of P' and E 4 will give the line of action, as well as the magnitude, of 

 the resultant of the original system P^ P 2 , P^ P 4 . The closed figure 

 A'B'C'D'E'f* obtained in this way is called an equilibrium polygon. 



E 



FIG. 118 



For a system of parallel forces the equilibrium polygon is con- 

 structed in the same manner as above, the only difference being 

 that in this case the force polygon becomes a straight line (Fig. 119). 



FIG. 119 



Since P' and P" are entirely arbitrary in both magnitude and 

 direction, the point 0, called the pole, may be chosen anywhere 

 in the plane. Therefore, in constructing an equilibrium polygon 



