on Static Friction. 37 



the curved is as it Were peeled off the flat surface, so that 

 the force required for an infinitely small displacement is 

 infinitely small. 



This answer is more general than would appear at first 

 sight. The range of the force of cohesion is so small that 

 all surfaces, no matter how carefully trued, may he con- 

 sidered to touch only at the summits of elevation. The play 

 •of Newtonian colours between glass plates affords pretty 

 ■testimony to this. 



The second answer is, that at the surface of any solid or 

 liquid the molecules are orientated by the normal component 

 of the forces of attraction, and that this contributes specially 

 to resistance to slip. 



It is not easy to see why cohesion should resist slip since 

 the potential energy of the forces of attraction will be the 

 same wherever contact is made. 



If both faces were formed of continuous solids there would 

 'be no initial force to resist slipping, though there would be 

 dissipation of energy in compression v\ aves during movement 

 if the bodies were elastic. 



Resistance to slip must be due to the discontinuity of 

 -matter. When the applied faces are at rest the molecules on 

 •either side of the interface take up the position in which 

 the potential of the attractive forces is minimal. Consider a 

 single molecule: it will " seize'"' in a position of least potential 

 and a tangential force applied to it produces a displacement 

 until the internal and external forces on it balance one 

 another. If the molecules of a solid were able to change 

 partners freely, as they can do in a fluid, slipping would occur, 

 as it of course does occur in a fluid. The fact that solid 

 faces will not slip past one another when external force is 

 applied, means that the uncompensated force on the molecules 

 at the surface increases rapidly for displacements from the 

 equilibrium position which are small in comparison with the 

 distances between their centres. Why, then, is there not 

 resistance to slip, that is to say, initial resistance as distinct 

 from the dissipation of kinetic energy during relative motion, 

 in a fluid ? The answer is to be found in the fact, which 

 van der Waals emphasized, that the problem is dynamical 

 and not statical. Increase in the heat energy decreases the 

 damping of the heat vibrations of a solid until a point is 

 rreaohed at which the molecules are able to change partners 

 freely. 



Resistance to slip is, however, not due merely to the short 

 range of the forces acting on the molecules, but to their 

 orientation. In 1913 one of us showed that the work done 



