Friction. 539 



of friction between solids. Because of -the molecular 

 structure of bodies, no matter how smoothly the two fric- 

 tion surfaces are polished, there must still remain eleva- 

 tions and depressions which permit interlocking. There 

 is also a slight adhesion between the two surfaces which 

 adds a small amount to the friction ; from this it follows 

 that 110 two surfaces can slide over one another without 

 developing the resistance known as friction. 



677. Friction of Rest or Static Friction Between Solids. 



Whenever one body is brought to rest upon another over 

 which it is sliding, the jarring which takes place at the 

 time causes the upper body to fall from the higher inequal- 

 ities into the depressions of the lower one, thus developing 

 the maximum interlocking due to this cause. Then again, 

 after long standing, if there has been considerable pres- 

 sure, the plasticity of the substances will cause them to in- 

 terlock still more completely, and so it happens that the 

 amount of force required to overcome friction between 

 two solid surfaces at rest is always greater than the force 

 required to keep the body moving after the static friction 

 has been overcome, and Thurston states that it is commonly 

 40 per cent, greater. The case is not profoundly unlike 

 the difficulty in starting a loaded wagon after it has stood 

 over night and formed a depression under each wheel. 



678. Friction of Motion Between Solids. After one solid 

 surface is once started sliding over another time enough 

 does not intervene in passing from inequality to inequality 

 to permit maximum interlocking to take place, and the re- 

 sult is the smaller amount of friction of motion, compared 

 with that of rest, stated in (677). 



The amount of sliding friction can be simply illustrated 

 by using inclined planes with different kinds of surfaces, 

 by first placing the plane and then putting upon this the 

 object whose friction is to be determined. By gradually 

 elevating the plane and jarring it a little an inclination 

 will be reached at which the body will slide down the sur- 



