text: Solid ftictim is the resistance to relative 
  motion of two surfaces in contact. When 
  relative motion exists between the surfaces, 
  is termed 
  the resistance to relative motion 
  “kinetic” 
  friction; when no 
  relative motion exists between the surfaces, 
  the resistance to the impending relative mo- 
  The minute 
  tion is termed “static” 
  discontinuities of the surfaces in contact are 
  able 
  relative 
  motion impends rather than exists, so static 
  friction will generally exceed kinetic friction. 
  The magnitude of the friction 
  force between 
  two surfaces will depend in great part on the 
  types of surfaces in contact and the magnitude 
  of force pressing the surfaces together. 
  A 
  friction 
  convenient method of relating 
  is a 
  charactersitics of surfaces in contact 
  proportion of the friction 
  force to the normal 
  (or perpendicular) force pressing the surfaces 
  together. This proportion defines the coeffi- 
  cient of friction, ~1. 
  
  or  “sliding” 
  
  The effect of slip velocity on the coefficient 
  of friction 
  is illustrated by the graph of figure 
  6.11. The conditions of zero slip corresponds 
  to the rolling wheel without brake application 
  while 
  the condition of full, 100 percent slip 
  corresponds to the locked wheel where the 
  relative velocity between the tire surface and 
  the runway equals the actual velocity. With 
  the application of brakes, the coefficient of 
  increases but incurs a small but meas- 
  friction 
  increase in 
  urable apparent slip. Continued 
  friction coefficient is obtained until some max- 
  imum is achieved then decreases as the slip 
  increases and approaching the 100 percent slip 
  the peak value of co- 
  condition. Actually, 
  efficient of friction occurs at an incipient skid 
  condition and the relative slip apparent at this 
  point consists primarily of elastic shearing 
  deflection of the tire structure. 