# Friction - its root cause? **URL:** \ **Category:** Factual Questions\ **Created:** [August 31, 2007, 3:43am UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578 "2007-08-31T03:43:43Z")\ **Posts on this page:** 19\ **Page:** 1
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [August 31, 2007, 3:43am UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578/1 "2007-08-31T03:43:43Z")
At a fundamental level (i.e. at the atomic level), what is friction? Obviously it is not a fundamental force, so I assume it has its origins in electromagnetism. Further, it must be more than a manifestation of hydrogen bonding since friction affects all matter, yet not all matter contains hydrogen. Is it due to a van der Waals’ force? (I suppose that’s possible, but would such a force _always_ be attractive? i.e. friction _is_ always attractive, at least insofar as it prevents things from slipping past each other). OK, I give up - what _is_ going on at the atomic level when we have friction at the macro level? (And, while we’re at it, let me ask - is viscosity essentially the same as friction?, except that it refers to a fluid whereas the term ‘friction’ is used to denote the same phenomenon occurring with solids) Thanks! ---
**Author:** ![Keeve](https://avatars.discourse-cdn.com/v4/letter/k/f07891/32.png) [@Keeve](https://boards.straightdope.com/u/Keeve)\ **Post date:** [August 31, 2007, 3:54am UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578/2 "2007-08-31T03:54:12Z")
Great question. I’ve wondered this too. Here’s my guess: No surface is perfectly smooth. If two perfectly smooth surfaces _would_ rub against each other, they’d just glide off, frictionlessly. But in the real world, the surfaces are not perfectly smooth, they have little bumps on them. What happens when a bump from surface A hits a bump on surface B? The harder one knocks some atoms off of the softer one, and then it can keep going. And the energy lost in pushing those atoms off is friction. When the bumps are so big and hard that they can’t be smoothened out, the surfaces don’t have enough energy to overcome the friction, so they stop moving. Am I close? ---
**Author:** ![Askance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/askance/32/8281_2.png) [@Askance](https://boards.straightdope.com/u/Askance)\ **Post date:** [August 31, 2007, 4:21am UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578/3 "2007-08-31T04:21:56Z")
Sorry **Keeve** but Wikipedia has a different take: > [@](#): > > Friction is the force that opposes the relative motion or tendency toward such motion of two surfaces in contact. It is not a fundamental force, as it is made up of electromagnetic forces between atoms. When contacting surfaces move relative to each other, the friction between the two objects converts kinetic energy into thermal energy, or heat. Friction between solid objects and fluids (gases or liquids) is called drag. Friction in an electronic circuit is called resistance. **Contrary to popular belief, sliding friction is not caused by surface roughness,** but by chemical bonding between the surfaces - by the stickiness of the two surfaces. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [August 31, 2007, 4:30am UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578/4 "2007-08-31T04:30:50Z")
I agree with **Keeve**. I have always been taught tthat friction occurs at the macrscopic level. Surfaces are not smooth and surfaces consist of electrons so they try to repel each other. Each surface has little bumps that repel, and thus retard motion, when they encounter the bumps in the other surface. Friction is a companion field to [lubrication.](http://en.wikipedia.org/wiki/Lubrication) Lubricants work because they form a thin film that separates the surfaces by a greater distance than the roughness of the surfaces. This means that the only retarding force is the shear within the lubricating film. ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [August 31, 2007, 4:44am UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578/5 "2007-08-31T04:44:36Z")
**Keeve** ’s idea is very interesting (i.e. loss of energy due to “knocking off” of atoms). I had looked at the Wiki article, **Askance**. In the part that you quote, there is actually very little said about the (putative) mechanism of friction other than, “chemical bonding between the surfaces - by the stickiness of the two surfaces”. So friction is “stickiness”. Not too helpful! At best, that Wiki statement is pretty nebulous. In fact, it really just substitutes one mystery for another. In place of “friction” we have “chemical bonding” or “stickiness”. My question remains, what is the nature of friction (or its possible equivalent, “chemical bonding”). ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [August 31, 2007, 4:44am UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578/6 "2007-08-31T04:44:43Z")
[QUOTE=David Simmons] I have always been taught that friction occurs at the macroscopic level. [/QUOTE] But don’t forget the [van der Waals forces](http://www.chemguide.co.uk/atoms/bonding/vdw.html). Without the van der Waals forces, [geckos would slide down off walls.](http://www.eurekalert.org/pub_releases/2006-08/uoc--ecg082206.php) ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [August 31, 2007, 4:51am UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578/7 "2007-08-31T04:51:45Z")
After reflecting a bit more on **Keeve** ’s idea, it occurs to me that atoms wouldn’t need to be actually knocked off. Simply having one molecule bump into other would start both of them vibrating. Wouldn’t the vibrations carry away energy (i.e. some of the mechanical energy being put into the system is lost via vibration, hence more mechanical energy must be put in; if not, the movement induced by the mechanical energy must diminish). ---
**Author:** ![Askance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/askance/32/8281_2.png) [@Askance](https://boards.straightdope.com/u/Askance)\ **Post date:** [August 31, 2007, 4:54am UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578/8 "2007-08-31T04:54:27Z")
[QUOTE=KarlGauss] **Keeve** ’s idea is very interesting (i.e. loss of energy due to “knocking off” of atoms). I had looked at the Wiki article, **Askance**. In the part that you quote, there is actually very little said about the (putative) mechanism of friction other than, “chemical bonding between the surfaces - by the stickiness of the two surfaces”. So friction is “stickiness”. Not too helpful! At best, that Wiki statement is pretty nebulous. In fact, it really just substitutes one mystery for another. In place of “friction” we have “chemical bonding” or “stickiness”. My question remains, what is the nature of friction (or its possible equivalent, “chemical bonding”). [/QUOTE] Sorry, I didn’t think I had to explain [chemical bonds](http://en.wikipedia.org/wiki/Chemical_bonds): > [@](#): > > A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds. The explanation of the attractive forces is a complex area that is described by the laws of quantum electrodynamics. In practice, however, chemists usually rely on quantum theory or qualitative descriptions that are less rigorous but more easily explained to describe chemical bonding. In general, strong chemical bonding is associated with the sharing or transfer of electrons between the participating atoms. Molecules, crystals, and diatomic gases—indeed most of the physical environment around us—are held together by chemical bonds, which dictate the structure of matter. ---
**Author:** ![Sublight](https://avatars.discourse-cdn.com/v4/letter/s/439d5e/32.png) [@Sublight](https://boards.straightdope.com/u/Sublight)\ **Post date:** [August 31, 2007, 4:55am UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578/9 "2007-08-31T04:55:27Z")
[QUOTE=Keeve] Great question. I’ve wondered this too. Here’s my guess: No surface is perfectly smooth. If two perfectly smooth surfaces _would_ rub against each other, they’d just glide off, frictionlessly. But in the real world, the surfaces are not perfectly smooth, they have little bumps on them. What happens when a bump from surface A hits a bump on surface B? The harder one knocks some atoms off of the softer one, and then it can keep going. And the energy lost in pushing those atoms off is friction. When the bumps are so big and hard that they can’t be smoothened out, the surfaces don’t have enough energy to overcome the friction, so they stop moving. Am I close? [/QUOTE] Something I recall from my chemistry textbooks: With some metals (such as copper), if you polish two pieces of very high purity as smooth as you can make them, they will have an extremely high co-efficient of friction when brought together (to the point of actually bonding together), much higher than two rough or impure pieces. In this case, the friction comes much more from electromagnetic attraction than mechanical unevenness. ---
**Author:** ![Askance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/askance/32/8281_2.png) [@Askance](https://boards.straightdope.com/u/Askance)\ **Post date:** [August 31, 2007, 4:57am UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578/10 "2007-08-31T04:57:54Z")
[QUOTE=KarlGauss] After reflecting a bit more on **Keeve** ’s idea, it occurs to me that atoms wouldn’t need to be actually knocked off. Simply having one molecule bump into other would start both of them vibrating. Wouldn’t the vibrations carry away energy (i.e. some of the mechanical energy being put into the system is lost via vibration, hence more mechanical energy must be put in; if not, the movement induced by the mechanical energy must diminish). [/QUOTE] I’m afraid this whole idea of roughness at the atomic level is misconceived. The first thing you need to do when you’re down there is discard all sense of what you are used to. An atom is almost entirely empty space and, absent atomic forces, two could pass through each other virtually every time without even noticing each other. It is almost solely these atomic forces that generate friction and stickiness, as the gecko example shows. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [August 31, 2007, 5:00am UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578/11 "2007-08-31T05:00:49Z")
[QUOTE=Squink] But don’t forget the [van der Waals forces](http://www.chemguide.co.uk/atoms/bonding/vdw.html). Without the van der Waals forces, [geckos would slide down off walls.](http://www.eurekalert.org/pub_releases/2006-08/uoc--ecg082206.php) [/QUOTE] Right you are. And a followup question. Two gauge blocks can be wrung together and they will adhere. Is this van der Waals or air pressure, the air having been driven out from between the blocks? Never mind, Wiki says it’s both. Along with a light film of oil, wringing excludes all the air and brings the surfaces to close together that molecular interaction occurs. I assum the attractive part of the interaction is van der Waals force. ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [August 31, 2007, 5:05am UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578/12 "2007-08-31T05:05:56Z")
[QUOTE=Askance] Sorry, I didn’t think I had to explain [chemical bonds](http://en.wikipedia.org/wiki/Chemical_bonds): [/QUOTE] And I didn’t think I’d have to restate (see my OP) that, yes, it presumably _is_ due to some form of chemical bonding. My question is _what type_ of bond. As an aside, why be so snide in GQ? Do you think that I would ask a question, or request a clarification, just for fun? ---
**Author:** ![Randy\_Seltzer](https://avatars.discourse-cdn.com/v4/letter/r/ee7513/32.png) [@Randy\_Seltzer](https://boards.straightdope.com/u/Randy_Seltzer)\ **Post date:** [August 31, 2007, 5:27am UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578/13 "2007-08-31T05:27:00Z")
[QUOTE=KarlGauss] At a fundamental level (i.e. at the atomic level), what is friction? [/QUOTE] No answers here. Sorry. I have always understood this to be one of the Great Mysteries of physics. A problem that is mind-bogglingly intuitive and simple, yet the greatest minds in history have been unable to crack it. I’m betting that you’ll get nothing in this thread but guesses, until a real physicist shows up and explains that no one currently knows the definitive answer. (The wiki paragraph quoted a couple of times already comes from this (dubious?) [source](http://amasci.com/miscon/miscon4.html#fric), which also states…) > [@](#): > > The subject [friction] is still full of unknowns, and new discoveries await those who make surface science their profession ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [August 31, 2007, 5:59am UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578/14 "2007-08-31T05:59:41Z")
Then there’s [Casimir friction](http://cerncourier.com/cws/article/cern/29655), which likely has large effects on both protein dynamics and [nanomachine design](http://www.technologyreview.com/Nanotech/19240/). -[Casimir effect](http://en.wikipedia.org/wiki/Casimir_effect) ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [August 31, 2007, 5:59pm UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578/15 "2007-08-31T17:59:43Z")
I think the interactions atom-by-atom amount to high and low spots in a force field, and atoms going over the high and low spots vibrate and transmit their vibration back into the solids, where they amount to heat. The high and low spots in a force field are the stuff of the van der Waals interactions, and perhaps other interactions too, but primarily van der Waals. On a small enough scale, and with a physics-oriented mindset, I think the distinction between different kinds of chemical bonds, and indeed between chemical bonds and other interactions, breaks down. It’s all electrostatic attraction and repulsion. Note that extremely smooth surfaces like crystalline faces and polished silicon wafers and flame polished glass still exhibit plenty of friction. ---
**Author:** ![flex727](https://avatars.discourse-cdn.com/v4/letter/f/258eb7/32.png) [@flex727](https://boards.straightdope.com/u/flex727)\ **Post date:** [August 31, 2007, 6:36pm UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578/16 "2007-08-31T18:36:11Z")
[QUOTE=KarlGauss] …it presumably _is_ due to some form of chemical bonding. My question is _what type_ of bond. [/QUOTE] I would presume covalent bonding (which is the sharing of pairs of electrons between atoms, typical chemical bonding). Your OP mentions “hydrogen bonding” - what did you mean by that? ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [September 1, 2007, 11:26pm UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578/17 "2007-09-01T23:26:42Z")
The problem in offering a discrete answer to this question is that the concept of friction covers a wide variety of phenomena at various macroscopic and microscopic levels (down to the level of electrochemical bonds) that may or may not be interrelated and thus, no one explanation is going to suffice to describe all surface friction phenomena. To compound this, the term “friction” is also sometimes used to describe internal continuum mechanical losses (hysteresis) in solids and Newtonian fluids and lossy behavior in aeroelastic, turbulant conditions with non-Newtonian fluids. I consider this use of the term friction to be inappropriate, or at least highly confusing, but it is frequently use in technical jargon and therefore an accepted use of the term. If you want a simple answer for solid surface-to-surface static friction can be broken into two essential domains; that caused by surface roughness which results in normal forces between “interlocking” surface features (think of dragging two pieces of sandpaper against one another), and the tangential forces that result from having to break attractive electrical bonds between two surfaces, or between a surface, a adhesive medium, and another surface (like two plates of glass with a thin layer of water in between them). When you get to dynamic friction things get far more complex; surfaces that had low friction statically can suddenly have high friction dynamically (particularly in the solid-fluid-solid case where the internal viscosity of the fluid suddenly dominates the effect), and conversely, rough surfaces with high static friction may have much lower dynamic friction because of the intermittant contact permitted by surface roughness. In addition, the basic character of the interface may change; for instance, in a car engine, when it’s not running the shafts may sit directly on the bearings, but when it is spinning at high speed it is supported by a thin but pressurized layer of oil which prevents any surface-to-surface contact, minimizing wear. As a practical matter, at least on the high school physics or machine design level we represent solid surface-to-surface friction as a single coefficient for static friction and another one for dynamic friction, or perhaps a curve for dynamic friction. In reality, once you get into the nanomachine/cellular/molecular level, “friction” behavior dominates inertial mechanics and you have to cope with the actual individual mechanisms rather than just pretending that it’s all one big bag. The entire phenomena has to itself the field of study of tribology which is both absolutely fascinating and fantastically boring at the same time, depending on how deeply you delve into it and what your interest is in applied continuum mechanics. So…it’s complicated; too complicated to sum up in a couple of paragraphs. Anything important usually is. I wish I could offer up some general reference for the layman on the topic, but I don’t have anything in my library that breaks it out at that level. Stranger ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [September 2, 2007, 1:13am UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578/18 "2007-09-02T01:13:09Z")
If there’s a library handy get _The Feynman Lectures on Physics_ and look at volume I chapter 12 pages 3 to 5. Here’s a short excerpt: > [@](#): > > There is another kind of friction, called dry friction or sliding friction, which occurs when one solid body slides on another. In this case a force is needed to maintain motion. This is called a frictional force, and its origin, also, is a very complicated matter. Both surfaces of contact are irregular, on an atomic level. There are many points of contact where the atoms seem to cling together, and then, as the sliding body is pulled along, the atoms snap apart and vibration ensues; something like that has to happen. Formerly the mechanism of this friction was thought to be very simple, that the surfaces were merely full of irregularities and the friction originated in lifting the slider over the bumps; but this cannot be, for there is no loss of energy in that process, whereas power is in fact consumed. The mechanism of power loss is that as the slider snaps over the bumps, thee bumps deform and then generate waves and atomic motions and, after a while, heat, in the two bodies. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [September 2, 2007, 2:29am UTC](https://boards.straightdope.com/t/friction-its-root-cause/417578/19 "2007-09-02T02:29:49Z")
I forgot to mention that the interactions that cause the “atoms to seem to cling together” are the van der Waal / London dispersion forces. ETA As I believe someone has already mentioned.