What happens when the coefficient of friction is 0?

[GiriBang]

My physics teacher said something shocking a few months ago.
During a lecture, he asked what would happen if mu of a solid is zero?
We answered that the solid will slide very smoothly but he said the Electrostatic attraction would make it hard to slide as the distance between the two surfaces has decreased due to absence of irregularities.
This is what he drew.

When I tried to Google this all I get is info about super fluidity.
So is this possible?

 

[DaveC426913]

I'm not very knowledgeable about this but my first thought was:
how is mu reduced to zero in his scenario?

If it is zero as a consequence of being atomically smooth, then I can see his point,
But is he simply saying it becomes zero by fiat? i.e. by magic? If so, then I think friction would decrease.

 

[GiriBang]

I'm not very knowledgeable about this but my first thought was:
how is mu reduced to zero in his scenario?

If it is zero as a consequence of being atomically smooth, then I can see his point,
But is he simply saying it becomes zero by fiat? i.e. by magic? If so, then I think friction would decrease.

He was considering a hypothetical situation.

 

[kuruman]

I think you answered your teacher correctly. When μ = 0, the surface will no longer be able to exert a force parallel to it and in the opposite direction to the velocity of the object sliding on it. That's what would happen within the physical model in which μ scales the force that impedes the horizontal velocity of a sliding object.

Your teacher's response was related not to what would happen, i.e. the horizontal motion is unimpeded, but what physical model could conceivably be the case when the horizontal motion of the sliding object is unimpeded. It is true that if you have two perfectly smooth surfaces of the same material, they will bind and it will be hard to slide one mass on the other. However smoothness is not the only way to reduce μ; lubricants do an excellent job to that effect. Also, a solid does not have a μ. It takes two solids to specify the μ between them.

 

[GiriBang]

smooth surfaces of the same material, they will bind and it will be hard to slide one mass on the other.

Sorry I forgot to mention the two surfaces or of same material before. Also, is the reason of binding Electrostatic attraction?

 

[A.T.]

...he asked what would happen if mu of a solid is zero?...he said the Electrostatic attraction would make it hard to slide...

That makes no sense, since the coefficient of friction tells you how difficult it is to slide, per definition.

If he asked what would happen if the surfaces were perfectly smooth, that's a different story. In vacuum they might even weld together:
https://en.wikipedia.org/wiki/Cold_welding

 

[GiriBang]

https://en.wikipedia.org/wiki/Cold_welding

This was what I was looking for. Thank you.

 

[gmax137]

also search for "jo blocks" or "gauge blocks" -- machinists use them in combination by "wringing" them together.

 

[DaveC426913]

He was considering a hypothetical situation.

Nor am I suggesting otherwise.

A.T.'s post #6 expresses what I was getting at more eloquently.

 

[Lnewqban]

...
During a lecture, he asked what would happen if mu of a solid is zero?
... but he said the Electrostatic attraction would make it hard to slide...

The coefficient of friction is just a measured rate that quantifies proportion of resistive to normal forces between two surfaces.
Rather than tending to zero, the value of the coefficient that would make the perfectly smooth surfaces hard to slide respect to each other would be high, not only due to electromagnetic effect but due to atmospheric pressure.

Lubrication and magnetism are two tools to reduce the coefficients of friction in bearings and other mechanisms as much as possible.
Note that for practical cold welding, rather than perfectly smooth and flat surfaces, huge amounts of pressing forces are used (hydraulic presses and explosives).

 

[GiriBang]

The coefficient of friction is just a measured rate that quantifies proportion of resistive to normal forces between two surfaces.

So it's wrong to say "coefficient of friction of that surface is x" and right to say "coefficient of friction between those two surface is x" ?

 

[Lnewqban]

So it's wrong to say "coefficient of friction of that surface is x" and right to say "coefficient of friction between those two surface is x" ?

Exactly!
For same smoothness and flatness, surfaces of same material tend to have higher coefficient of friction than different materials.
I believe that fact is related to same molecular alignment of the material.