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Name: Ethan
Status: student
Grade: 9-12
Location: AL
Country: USA
Date: N/A


Question:
Why couldn't there be a frictionless surface?


Replies:
The first thing to keep in mind when thinking about friction is that it does not just depend upon the surface. Very often it is a combination of both the surface and the object that is sliding over it. We are capable of making surface and object combinations which have relatively low friction in some circumstances. In very special cases we can make the friction extremely low. However, it is always present to some extent.

The basic problem is that the object and the surface are going to interact with each other. That interaction will lead to some energy exchange. If you are thinking of an object sliding across a surface, then it amounts to an energy loss. The first thing you need to check is "roughness" of the surface and object. If they are "bumpy," you can have significant friction. Secondly there may be chemical bonding between an object and surface which can cause friction (or indeed just to stick them together). There will also be energy lost as the surface atoms rearrange, deform, and move in response to the collisions as the surface and object move.

What about if you made the object and the surface completely smooth? And chose the materials such that there would be no appreciable bonding? And completely rigid?

We can in some cases make a surface and object atomically flat. However, even in this case, on the small scale the objects will not appear "flat" to each other. Because both things are made of atoms, the variation in the distribution of electrons will lead to "bumpiness" in the form of places where the atoms will prefer to meet. So essentially there is nothing truly flat-smooth. There is also no way to completely eliminate atomic recoil from collision. And even without strong bonding, the atoms of the surface and object can interact, exchanging electrons, responding to local variations in electric (and magnetic) configurations. So nothing is truly flat and things tend to interact resulting in energy loss.

Perhaps the best way to keep there from being friction between an object and a surface is to never let them actually get close. Think of the magnetic-levitation trains operational in Asia. Here alternating, opposing magnetic fields keep the object, the train, from actually touching the surface, the tracks (in fact, that is also how the train can be propelled too). Here there is no contact friction. However, there is still a "frictional" electrodynamic drag that will sap energy(and of course the biggie is air resistance).

Michael S. Pierce
Materials Science Division
Argonne National Laboratory


It is a matter of entropy and conservation of energy.

Friction occurs because the molecules of two surfaces in sliding contact become displaced and, more importantly, set in motion. The energy for this motion, of course, comes from the kinetic energy of the two objects in contact. So, when the molecules in the two objects move faster, the objects slow down.

The energy transfer always happens in that direction (from the kinetic energy of the large objects to kinetic energy of many molecules) because the motion of the molecules quickly becomes randomized. The molecules will not spontaneously move in concert to push their surfaces along faster, any more than the constantly-moving molecules of air in a room will spontaneously move together and gather in the center of the room.

Richard Barrans
Department of Physics and Astronomy
University of Wyoming


Under "normal" conditions, the electrons in materials in close contact are moving around (even if they are bound to an atom or molecule). This induces a temporary distortion of the electrical charge on the surface of the material. The electrons in a material "close by" senses this electrical distortion and that in turn induces a temporary distortion of the electrical charge in the second material, but with opposite + / - direction. This interaction is ALWAYS attractive, so even if the two surfaces are completely smooth, i.e. with no dislocations, or surface unevenness etc., the two surfaces "attract" one another and that results in friction.

Having said all that some materials at low temperatures (about 2.4 kelvins for helium) become super fluid, that is there is zero viscosity -- viscosity is very closely related to friction. I would think (although I have not researched the question) that two surfaces separated by superfluid helium would be essentially friction-free. Of course you might have to do this in zero gravity which also ALWAYS attracts two bodies.

Another configuration would be to levitate the two surfaces with appropriate electric and/or magnetic fields in a vacuum, far enough apart that attractive forces are essentially zero, but this too is a complicated arrangement.

Finally, there is a semantic issue "friction free". The friction under some arrangement can be made very small, but it may be the case that in practice that there is always some "micro-friction". It is always tricky to say that some physical quantity is absolutely zero, because some more refined experiment may show that the quantity is absolutely zero. An example, two atoms attract one another by Newton's gravitational law, so that force is not absolutely zero. However, for most practical purposes it is sufficiently small that to a very good approximation it is zero.

Vince Calder

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