LAWS OF BONE ARCHITECTURE 193 



is said te produce a moment about that point, equal to the mag- 

 nitude of the force (in pounds) multiplied by the perpendicular 

 distawe (in inches) from the line of action of the force to the 

 point considered. The point about which the force tends to 

 cauai rotation is called the center of moments. 



A general application of moments is shown in figure 4. The 

 fo*3e F acts at a distance m, from the center of moments, A. 

 The distance m, is called the lever arm. The moment of the 

 force F about the point A is equal to F times m, the result being 

 expressed in inch-pounds according to the units of force and 

 length herein used. 



19. There may be any number of forces tending to rotate a 

 body about a given point, either in the same or in the opposite 

 direction. If the body is in equilibrium the total moment in a 

 clock-wise (-f) direction must be equal to the total anti-clock- 

 wise ( — ) moment, otherwise rotation would occur. 



20. A lever may have any shape. Regardless of the actual 

 length of the lever, the true lever arm is the perpendicular dis- 

 tance from the center of moments to the line of action of the 

 force. Figures 5 a and h illustrate this principle. 



21. A couple, in mechanics, is a system of two parallel and 

 equal forces acting in opposite directions. If each of these 

 forces is represented by F, and the perpendicular distance be- 

 tween them is m, the moment of the couple is F'm. A couple 

 tends to revolve the body upon which it acts and equilibrium 

 can be established only by a couple producing the same moment 

 in the opposite direction. Figure 5 c illustrates the action of a 

 couple. 



Theory of beams 



22. Reactions. Bending stress occurs in a bar resting in a 

 horizontal position upon one or more supports. The loads on 

 the bar and its own weight cause it to bend and produce in it 

 complex stresses and elastic deformations which may be re- 

 solved into stresses of tension, compression and shear. 



Throughout this discussion of beams, paragraphs 22 to 4^ 

 inclusive, it is assumed that the cross section of the beam is uni- 

 form throughout its length. 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 21, NO. 2 



