270 MECHANICS 



SOLUTION. Since the neutral axis passes through the center 

 of the section, the distance c is in this case equal to one-half the 

 depth; that is, = 10. The section modulus is therefore 



c 10 



EXAMPLE 2. When subjected to loads perpendicular to the 

 cover-plates the outermost fibers of the section shown in Fig. 2, 

 are stressed to 16,000 Ib. per sq. in., What is the resisting 

 moment of the section? 



SOLUTION. The moment of inertia of the section has been 

 found to be 1,403.22 and the outermost fibers are 9.63 in. from 



1,403.22 

 the neutral axis; hence, the section modulus is equal to 



9.63 



= 145.7; this multiplied by 16,000 gives 2,331,200 in.-lb. 



Formulas for obtaining directly the section moduli of 

 sections frequently used are given in the table of Moments of 

 Inertia, etc. For rolled-steel sections, they are given in manu- 

 facturers' handbooks. _ 



FRICTION 



Friction is the resistance that a body meets from the surface 

 on which it moves. It depends on the degree of roughness of 

 the surfaces in contact, and is directly proportional to the per- 

 pendicular pressure between the surfaces. It is independent 

 of the extent of the surfaces in contact, so long as the normal 

 pressure remains the same. It is generally greater between 

 surfaces of the same material than between those of different 

 materials, and greater between soft bodies than hard ones. 



Coefficient of Friction. The ratio between the resistance 

 to the motion of a body due to friction and the perpendicular 

 pressure between the surfaces is called the coefficient of friction. 

 When the coefficient of friction between two surfaces is known, 

 the frictional resistance is obtained by multiplying the normal 

 pressure by the coefficient. 



EXAMPLE. What is the resistance per linear foot of a retain- 

 ing wall against sliding when the normal pressure on the foun- 

 dation is 10,000 Ib. per lin. ft. of wall and the coefficient of 

 friction of the masonry on the foundation is .65? 



