MECHANICS 



261 



on the line of action ot the force is called the lever arm, or simply 

 the arm, of the force with respect to the origin. 



A moment is expressed in foot-pounds, inch-tons, etc., 

 according to the units to which the force and its arm are 

 referred. 



The moment is either positive or negative, depending on the 

 direction in which the force tends to cause rotation. It is 

 positive for clockwise motion, and negative for counter-clock- 

 wise motion. Thus, the moment of F about C is positive and 

 the moment about C\ is negative, because, if the arms p and p\ 

 were bars, the force would tend to rotate p in a clockwise direc- 

 tion, and Pi in a counter-clockwise direction. 



CENTER OF GRAVITY 



The center of gravity of a figure or a body is that point upon 

 which the figure or the body will balance no matter in what 

 position it may be placed, provided it is acted upon by no other 

 force than gravity. 



If a plane figure is alike, or symmetrical, 

 on both sides of a center line, the latter 

 line is termed an axis of symmetry, and 

 the center of gravity lies in this line. If 

 the figure is symmetrical about any other 



FIG. 3 



FIG. 4 



axis, the intersection of the two axes will be the center of gravity 

 of the section; thus, the center of gravity of a parallelogram 

 is at the intersection of the diagonals and that of a circle 

 or an ellipse is at the geometrical center of the figure. The 



