204 LAWS 01 ION. 



Hows from these law /' be tin- normal 



n two surfaces, then tin- l'ricli"n is /*/', win 

 const ity for the same materials and in ealled ti. 



efficient of frit 



The following results, selected frm a table pven l>y i 

 fessor liankinc, will all'-rd an idea of tin- amount >f frieti 

 determin' 1 by ex: : these results apply t. ti 



of motion. 



1 : iron on stone n varies between " UM 



timber on timber .... ........ "J and *5. 



For timber on metals ..................... -' and -6. 



For metals on metals ..................... !"> and %&, 



For full particulars on this subject we refer the. reader to 

 Coulomb's papers, and to the Memoirs published in the Me- 

 moires de I'lnstitut, by M. M>riii ; see also Rankine 

 of Applied Mechanics, and Moseley's Mechanical J '/*'/ ij, I, * 

 of En<j (intring and Architecture. 



173. Angle of Friction. Suppose a body acted on only by 

 its weight to be placed on a horizontal plane and the plane, 

 to be turn- --I r^iind a horizontal line until the body 

 slide. Let IF be the weight of the body and a the angle tin- 

 plane makes with the horizon. The pressure of the body n 

 the plane will be equal to the resolved part of its -vv 

 perpendicular to the plane, that is to JFcos oc. The fri 

 is equal to the resolved part of the weight parallel to the 

 plane, that is to IF sin a. If ft be the coefficient of friction, 

 we 1 



TFsin a = yxWcosa; 



fore tan a = p. 



experiment will enable us to determine the value of the 

 coefficient of friction for dill- n nt substances. The inclination 

 of the plane when the body is just about to slide is < 

 the angle of friction. 



171. In Art. 32 we have found the condition of equilibrium 

 of a particle constrained to rest on a smooth curve ; we proceed 



