336 TABLE 338.— FRICTION 



The required force F necessary to just move an object along a horizontal plane = fN 

 where N is the normal pressure on the plane and /the "coefficient of friction." The angle 

 of repose # (tan # = F/N) is the angle at which the plane must be tilted before the object 

 will move from its own weight. The following table of coefficients was compiled by Rankine 

 from the results of General Morin and other authorities and is sufficient for ordinary 

 purposes. 



Material / 



Wood on wood, dry 25-.50 



soapy .20 



Metals on oak, dry 50-60 



" " " wet 24-.26 



" " soapy .20 



" elm, dry 20-.25 



Hemp on oak, dry .53 



" " " wet 33 



Leather on oak 27-38 



" metals, dry .56 



" " wet .36 



" " greasy .23 



" oily 15 



Metals on metals, dry 15-20 



" " " wet .3 



Smooth surfaces, occasionally greased 07-08 



" continually greased .05 



best results 03-.036 



Steel on agate, dry .20 



" " " oiled 107 



Iron on stone 30-70 



Wood on stone About .40 



Masonry on brick work, dry 60-.70 



" " " damp mortar .74 



" dry clay .51 



" moist clay .33 



Earth on earth 25-1.00 



" " " dry sand, clay, and mixed earth. . . .38-75 



" " " damp clay 1 .00 



" " " wet clay .31 



" " " shingle and gravel 81-1.11 



SMITHSONIAN PHYSICAL TABLES 



