MECHANICS 



95 



Resultant Moments. In Fig. 2 is shown a lever com- 

 posed of two arms at right angles to each other, and free to 

 turn about the center C. A force A acts on the horizontal 

 arm in such a manner that it tends to produce left-hand 

 rotation, its moment being 10X5 = 50 ft.-lb., which, since 

 it tends to produce left-hand rotation, will be called negative. 

 Another force B, whose moment with respect to the center C 

 is 12X3=36 ft.-lb., tends to produce right-hand rotation. 



FIG. 2 



Therefore, -50 + 36 =-14 ft.-lb., which is the resultant 

 moment, and has the same turning effect as the two moments 

 already given. 



If, instead of the two forces just considered, there is a body 

 that is acted on by any number of forces whose moments 

 about a given center are known, the resultant moment of 

 these forces will be the algebraic sum of the moments of the 

 given forces. 



CENTER OF GRAVITY 



The center of gravity of a body, or of a system of bodies, 

 or forces, is that point at which the body or system may 

 be balanced, or it is the point at which the whole weight 



