In simplistic (K-8) physics classes, it seems to be generally instructed that the friction between two moving surfaces is due to the unevenness of each surface and the microscopic roughness. However, I have recently read that this is in fact the cause of what is called traction, but that kinetic friction is possible even with completely smooth materials.

Given two atomically smooth, nonpolar surfaces in a vacuum sliding past each other with a non-zero normal force $F_{N}$, how can I determine, based on the material, what the coefficient of friction $\mu$ will be? Given the propensity of metals to cold weld, it might be easier to only consider nonmetals.

As far as I can tell from various sources, London dispersion forces/ VDW forces might have something to do with it, but I have no good sources. What about chemical covalent bonding, ionic bonding etc? Are there often full and separate ions formed (what defines a true ionic bond anyway?), or perhaps just a smaller electrostatic attraction? Is there any complete set of equations or items to consider to obtain a reasonable answer without resorting to experimentation?

Edit: For example, Wikipedia contradicts itself (I think) on the Friction page, first stating that

"While it is often stated that the COF is a "material property," it is better categorized as a "system property." Unlike true material properties (such as conductivity, dielectric constant, yield strength), the COF for any two materials depends on system variables like temperature, velocity, atmosphere and also what are now popularly described as aging and deaging times; as well as on geometric properties of the interface between the materials."

and then

"New models are beginning to show how kinetic friction can be greater than static friction.[26] Kinetic friction is now understood, in many cases, to be primarily caused by chemical bonding between the surfaces, rather than interlocking asperities"


2 Answers 2


Friction is due to electrostatic repulsions that take place at a very small number of contact points between two rough surfaces. Microscopically, kinetic friction converts the driving energy into heat. There are many atomic models developed by studying trends to theoretically predict the coefficient of friction such as:

Theoretical Calculations of Coefficient of Friction

Coming to the second question, Wikipedia isn't contradicting itself. It is just reinforcing the fact it stated earlier. Coefficient of friction does not depend on material properties. It depends on system properties. That is why, coefficient of kinetic friction is less than the coefficient of static friction.


Ringlein and Robbins, American Journal of Physics


To determine the coefficient of friction, (in an ideal situation, of course) push a known mass of the object with a given force F. Also the frictional force has a numerical value of (approximately) coeff. times normal force. So you can calculate the coeff. this way.

There are momentary 'bonds' formed between the surfaces. These bonds arise due to Van der Waals forces and other molecular effects. Since the 2 surfaces will not have the SAME number of electrons, there arises electrostatic forces. So some molecules get a little displaced (they still are attached to the object/surface they belong to). Because of this, some nearby molecules also get an even smaller distortion (dipole) or may even get a bit displaced. Molecules of the other surface/body will experience forces due to all these effects, and collectively constitute 'Friction' macroscopically. I didn't get how Wikipedia contradicts itself, so no comment there.

Hope I helped :)


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