OK TASSIVZ STRENGTH AND FRICTION. 15* 



With a few exceptions, the friction of all solid bodies is, either perfectly, or 

 Very nearly, a uniformly retarding force, neither increasing nor diminishing 

 when the relative velocity of the bodies concerned is changed. The friction 

 of some rough substances is a little increased with the velocity, but, as they 

 become more polished, this variation disappears. When, however, the mo- 

 tion is wholly extinct, and the bodies remain in contact with each other, 

 their adhesion is usually greater than the friction, and by a continuation of 

 the contact, it may become twice or even thrice as great, especiidly where the 

 surfaces are large, and the substances but moderately hard. 



The truth of tlie assertion, that friction is a uniformly retarding force, may 

 be shown very conveniently by means of Atwood's machine for experiments 

 on accelerated motion. By suffering the axis of the pulley to rest on the 

 surface of any fixed substance, we may subject it to a friction of which the 

 magnitude may be varied by diflerent methods; and we shall find that the 

 motions of the boxes still indicate the action of a uniformlv acceleratinir 

 force,' the spaces described being always proportional to the squares of the 

 times of descent; it follows, therefore, that since the operation of gravity is 

 uniform, th^t of friction which is deducted from it at each instant, must 

 also be uniform, in order that the remaining acceleration may follow the 

 same law. 



The uniformity of the force of friction may also be shown by the descent 

 of a flat substance on an inclined plane: if the body be caused to begin its 

 descent with a certain velocity, it will be retarded, when the resistance is- 

 greater than the relative force of gravity : in this case the retardation will 

 continue until it is wholly stopped, the resistance not diminishing with the 

 velocity. If, on the contrary, the relative weight overpowers the resistance 

 at first, the motion will be continually accelerated, the resistance not being 

 increased by the increase of the velocity. But since every experiment of 

 this kind must be performed in the presence of the air, the resistance of this 

 fluid, which follows another law, will in the end prevent the acceleration. 



It may in general be asserted, with some exceptions, that the force of 

 friction is simply proportional to the weight or pressure that brings the sub- 

 stances concerned into contact, independently of the magnitude of their sur- 



VOL. I. X 



