G. F. Becker — Impact Friction and faulting. 195 



interlock and any force applied would deform the two plates 

 as a single elastic mass. It appears indeed not beyond the 

 bounds of possibility, that the inequalities of a surface might 

 be perfectly elastic but of such a form as to yield passage to a 

 body passing over them by bending and compression. Such a 

 case would correspond to the impact of perfectly elastic bodies 

 in which a portion of the energy is converted into vibrations ; 

 in other words to a case in which, in spite of perfect elasticity, 

 the coefficient of restitution is not unity. Such cases, if they 

 exist at all, must be very rare ; for it is a matter of daily ex- 

 perience that highly tempered steel-cutting instruments are 

 sharpened upon, and are therefore abraded by, friction upon 

 soft and greasy leather. If on the other hand the material 

 were inelastic, or partially elastic, work would be done on the 

 contact and relative motion would take place. This relative 

 movement would be greatest for total inelasticity. Partially 

 elastic sheets would not separate like partially elastic balls, 

 but would move relatively by an amount corresponding to the 

 work done in members of a system of partially elastic balls. 

 The material remaining the same and the coefficient of restitu- 

 tion constant, the number of minute projections on any pair of 

 contact surfaces would make no difference in the work done at 

 that contact though it would determine the ultimate relative 

 positions of the centers of inertia of two sheets. The tiny 

 projections of material surfaces are certainly not prisms with 

 vertical sides, but irregular in shape and larger at their bases 

 than at their summits. The closer surfaces are brought to- 

 gether therefore, the more such projections will interlock and 

 the greater will be the expenditure of energy requisite to grind 

 them off. 



Corresponding interpretation of Morin's laws. — The results ob- 

 tained by Morin from his own and Coulomb's experiments on 

 friction are expressed in rules known to be approximately and 

 only approximately true. They are that friction is propor- 

 tional to the area of the rubbing surfaces, proportional to the 

 pressure, and independent of the velocity. The first relation 

 is evident, the second states that the interlocking of projections 

 increases with the pressure, as it certainly must, and further- 

 more that this interlocking is such that the amount of work to 

 be done increases in simple, direct ratio to the pressure ; this is 

 only approximately true. The third relation, that the amount 

 of work done is independent of the velocity, manifestly im- 

 plies the assertion that the coefficient of restitution is indepen- 

 dent of the velocity, or that it is constant for partially elastic 

 bodies and beyond the limit of elasticity as well as under con- 

 ditions where no sensible deformation occurs. It was of course 

 never claimed that this rule was exact or that it was applicable 



