# Friction Properties

1. Jan 18, 2016

### rcummings89

My understanding through most of college has been that friction occurs because of the surface roughness of two materials rubbing together; the smoother you make the surfaces, the less friction you have.

Upon further research I can see that is not the entire case. Another problem for me is that I read many assumptions about how friction works with little to no explanation or rationale (effects of temperature, velocity, and surface area being prime examples). If you would bear with me I would like to present my understanding of friction properties and welcome corrections to any misconceptions I have:

1. The case I mention above, about friction being solely a function of surface roughness is actually most applicable to static friction. Only in cases of considerable roughness will it apply to dynamic friction as well (see Assumption No. 2)

2. Dynamic friction is primarily caused by adhesive forces between the two materials rubbing against each other. As you polish the two surfaces the static coefficient of friction tends toward the value of the dynamic coefficient because it isn't being compounded by surface roughness.

3. (Trying to infer from what I have read...) Friction is inversely proportional to temperature. As temperature increases the molecules are at a higher energy state and are more likely to "give up" their adhesive bonds to the neighboring molecule.

4. (Again, me trying to make sense of what I have read) Friction is independent of sliding velocity (for dry friction only, ignoring speeds at which drag is a factor) because based on Assumption No. 2 the bonds formed and destroyed as the object slides is happening nearly instantaneously compared to sliding velocity.

5. Friction is independent of surface area because surface irregularities only allow for a certain number of bonds to be formed. The exception being if an exceptionally high normal force is applied (forcing more material together) or if the surfaces are sufficiently polished to allow for better contact.

I know I am oversimplifying a complex process, but I appreciate any feedback you might have.

2. Jan 20, 2016

### jambaugh

I would add that it is not only surface to surface adhesion that is effected but also subsurface adhesion which holds each material together. The effect of roughness on moving friction would be that each surface acts like sandpaper to the other gouging it and abrading it and otherwise moving molecules around. This is why you typically see wear associated with friction (though not necessarily).

But as I understand it friction science, (which has a name... looking it up) that is to say Tribology is a very rich field of study with many as yet unanswered questions.