1 10 STATICS. [184. 



surfaces at this moment, and this resistance is called the limiting 

 static friction. 



Careful experiments have shown this force to be subject to 

 the following laws : 



(i) The magnitude of the limiting friction F bears a constant 

 ratio to the normal pressure N between the surfaces in contact ; 

 that is 





where //, is a constant depending on the condition and nature of 

 the surfaces in contact. This constant which must be deter- 

 mined experimentally for different substances and surface 

 conditions is called the coefficient of static friction. It is in 

 general a proper fraction ; for perfectly smooth surfaces /JL = O. 



(2) For a given normal pressure the limiting static friction, 

 and hence the coefficient of static friction, is independent of the 

 area of contact, 



184. The frictional resistance between two surfaces in rela- 

 tive motion is called kinetic friction. It is subject, in addition 

 to the two laws just mentioned, to the third law : 



(3) Kinetic friction is independent of the velocities of the bodies 

 in contact. 



The coefficient of static friction is generally slightly greater 

 than that of kinetic friction. 



It must not be forgotten that these so-called laws of friction 

 are experimental laws, and therefore true only approximately, 

 and within the limits of the experiments from which they were 

 deduced. When the relative velocity of the surfaces in contact 

 is very high, and when, as is usually the case in machinery, 

 lubricating material is introduced between the two surfaces, th 

 frictional resistance is found to depend on a number of other 

 circumstances, such as the temperature, the form of the sur^ 

 faces, the velocity, the nature of the lubricator, etc. 



185. Consider again a body resting on a horizontal plane 

 (Fig. 55), and acted upon by a horizontal force PJust large- 



i 



