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what does Feynman lecture 12 friction “outgas the materials in a vacuum” mean? Does it mean that the experiment is carried out in vacuum? What is the purpose of doing that? “It was pointed out above that attempts to measure μ by sliding pure substances such as copper on copper will lead to spurious results, because the surfaces in contact are not pure copper, but are mixtures of oxides and other impurities. If we try to get absolutely pure copper, if we clean and polish the surfaces, outgas the materials in a vacuum, and take every conceivable precaution, we still do not get μ.”

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Just as a film of oil changes friction, so does a layer of adsorbed gas from the atmosphere. No surface is really clean, until such material is driven off of the surface, usually by a vacuum bake cycle. At least, afterward the surface is cleaner, if not absolutely clean.

If surface composition is important, one can operate a lathe in vacuum, and observe the freshly exposed surface left behind the cutter. It's not clear how you could combine that with a friction measurement, though.

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  • $\begingroup$ Thanks so much! After the surface is cleaned by a vacuum bake cycle, the experiment of measuring μ should be carried out in vacuum? or it is OK to do the experiment in atmosphere? $\endgroup$ – Vivien dong Jun 25 at 0:31
  • $\begingroup$ @Viviendong It's always OK to do an experiment! If there's a good reason to want the surface to be clean, atmospheres without polar molecules (Argon, for instance) are less of a problem than oxygen. That's the principle behind MIG arc welding; the 'IG' stands for inert gas. $\endgroup$ – Whit3rd Jun 26 at 0:24
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With all respect to Feynman, I think he has got confused here. He seems to be assuming that there ought to be an "exact" law of friction like $F = \mu N$ and then complaining that you can't do an experiment that actually produces that result, however hard you try to eliminate "errors".

The Coulomb friction equation (which shouldn't be called a "law" at all IMO) is just an approximation that works fairly well for small contact forces, low relative velocities, large contact areas, and some pairs of materials in contact. Its main advantage is that it is easy to use in hand calculations compared with more sophisticated models of friction, and it is easy to do experiments in situations where it is approximately true (e.g. simple bench-top physics lab experiments). Unfortunately it is often taught in such a way that students think it is a "law" similar to $F = ma$ or the ideal gas laws. It isn't!

Engineers tend to take a more pragmatic approach, and say "sure, $F=\mu N$, where $\mu$ is a variable which depends on all sorts of parameters (including $N$, temperature, relative slipping velocity, etc, etc), and if you want an accurate value in any particular situation you forget about theory and just measure it."

There are more complex models of friction which reproduce real-world behaviour more accuarately, in situations that Feynman wasn't even considering. For example if there are oscillating forces producing very small amplitude slipping (of the order of $10^{-3}$mm) between two surfaces in contact, you can't classify the friction into two different types called "static" or "dynamic" at all. There is no force amplitude below which the friction becomes "static" and there is no relative motion at all. Situations like that important in the design of supposedly "rigid" joints between components subjected to alternating loads, for example.

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