# What is the cause of friction exactly?

1. Jul 31, 2014

### quantumcat

I've been thinking about friction(qualitatively) for some time now. I've not got a satisfactory explanation about the nature of the frictional force yet. My physics professor says its due to crests and troughs on the interacting surfaces. Resnick and Halliday(Chapter 6 Page 118 - Principles of Physics- Extended Ninth Edition) doesn't mention peaks and troughs at all. According to them, cold welding is responsible for friction. If the surfaces are very smooth and free of impurities, the contacting surfaces will weld.
"If the applied force is great enough to pull one surface across the other, there is first a tearing of welds at breakaway and then a continuous re forming and tearing of welds as movement occurs and chance contacts are made. The kinetic frictional force that oppose the motion is the vector sum of the forces at those many chance contacts.
Intuitively, this doesn't seem right. There is considerable friction if you try to drag a tire across a road without rolling it. If this is due to adhesion primarily, wouldn't it be difficult to lift the tire as well? Also, since the contact patch has adhered to the road surface rolling should be much more difficult too.
I'm starting to think its a mixture of both. If the surfaces are very smooth or very rough, there is a lot of friction, but if its somewhere in the middle, friction is less. Also, is cold welding only possible between similar kinds of atoms?

2. Jul 31, 2014

### Wes Tausend

...

My personal opinion is that close contact of surfaces slightly disrupts the outer electronic orbit configuration of the atoms of adjacent surfaces. Merely placing two surfaces close together causes a minor amount of disruption of atomic deformation from any tiny measure of "placement impact" and therefore a small amount of orbit restoration when the surfaces are again separated. Upon any impact, however soft, heat is generated. Upon "lifting" apart, very small amounts of heat are again generated by what amounts to departure from initial contact. I believe both rolling resistance, and vibrating contact demonstrate this slight surfaces heating.

Sliding the surfaces demands some additional extra energy to be expended (frictional resistance) to both deform many new, and restore previous, electron orbits. This is re-emitted as heat radiation in turn, delivering much greater heat by the slide-disturbed electrons over that of mere contact. Thus emerges what I imagine as the heat of frictions at an atomic level. But this is only a reasoned personal opinion, so beware adopting it over some more conventional explanation. I would like to hear the more conventional explanations myself.

Here are two wikipedia explanations: Cold Welding & Friction

I have a more conventional general explanation for tires. Normally the coefficient of friction of materials is quite linear. As an example, increasing the surface area of a small object on a larger surface produces the same total amount of friction as long as the compression, or weight, is the same. In other words two identical weighted objects of identical matter have the same friction, even if one has a greater surface area. And in addition, doubling the contact compression, or weight, increases the friction linearly by two.

For a practical engineering lesson, this is not quite true for rubber tires. Friction increases non-linearly (less than 1:1) with additional weight. This means that a wider tire (increased contact patch) delivers greater friction for equal weight than a small tire, which is why racecars use wider tires. It also means that when cornering, some traction is lost as an inside tire becomes lighter during centrifugal weight transfer, but the outside tire (which receives this lost weight) does not gain all the lost traction back. This is because the rubber/asphalt traction friction does not increase linearly. For this reason, the preference is to design automotive chassis so that the least amount of weight transfer takes place in order to maintain the maximum amount of cornering (and stopping) traction. A low center of gravity is paramount but there are other dynamic factors as well. Besides power, racing is very much a friction science.

Wes
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3. Aug 11, 2016

i dont think that these two are useful

4. Aug 11, 2016

### phinds

Those two WHAT are not useful? Please be more clear in your posts.

5. Aug 11, 2016

### 256bits

It is all about the chemical bonding between atoms of the two surfaces, and there is no hard and fast rule to explain all about friction.
For chemical bonding between the surfaces:
Some of the bonds can become very strong, as atoms from one material can be thought of as becoming part of the "lattice" network of the other.
Cold welding would be of that type.

rolling friction
Rather than having to displace the whole contact patch, only part of it between the 2 surfaces is being separated.

rough surfaces
If one material surface can deform slightly conform to the highs and lows of the other surface, then to displace the two surfaces, there is an additional vertical movement out of the lows and over the highs.

A material with a really high friction factor