Things giving rise to static and dynamic coefficients of friction?

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Frabjous said:
No, relative velocity between the surfaces. There are plenty of measurements of different friction coefficients at different velocities for a variety of systems.
Don't those depend on the masses of the surfaces though, and therefore momentum? Or is it simply velocities and topography?
 
I am not a friction expert. Most general theories take adhesion and plastic deformation into account. (To account for different materials having different properties, one could look at suface/interfacial energies.) So anything that effects these things matters.

Look at a frictional system of two different materials. I can choose either system to be at rest. How does one define “momentum” in this case?
 
Frabjous said:
I am not a friction expert. Most general theories take adhesion and plastic deformation into account. (To account for different materials having different properties, one could look at suface/interfacial energies.) So anything that effects these things matters.

Look at a frictional system of two different materials. I can choose either system to be at rest. How does one define “momentum” in this case?
In the case of dynamic friction, you'd look for which one is doing the disturbing of the topography of the other one. That one would have more momentum, or non-zero momentum.
 
jaketodd said:
In the case of dynamic friction, you'd look for which one is doing the disturbing of the topography of the other one. That one would have more momentum, or non-zero momentum.
I can measure the force of friction in either frame and it is identical. Saying that there is a preferential frame that depends on microstructural behavior is extremely cumbersome. What if I heated the ”hard” plate so that it became the ”soft” plate?

We general(edit: +ly) like physics to be frame independent.
 
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Frabjous said:
We general like physics to be frame independent.
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Frabjous said:
What if I heated the ”hard” plate so that it became the ”soft” plate?
Then they would both be frictionless surfaces to one another and there would be no coefficients of friction between them.
 
jaketodd said:
Then they would both be frictionless surfaces to one another and there would be no coefficients of friction between them.
No, there would still be friction between them.
 
Frabjous said:
No, there would still be friction between them.
I'm sorry; I thought you meant that the plates would be separate.

But in discrete space and discrete bodies, there will always be friction, because there is a smallest unit, always a "roughness." But if possible, continuous for both, I suppose total frictionless is possible, at least in theory.
 
jaketodd said:
I'm sorry; I thought you meant that the plates would be separate.

But in discrete space and discrete bodies, there will always be friction, because there is a smallest unit, always a "roughness." But if possible, continuous for both, I suppose total frictionless is possible, at least in theory.
There are also adhesive forces so probably not.
 
Frabjous said:
There are also adhesive forces so probably not.
I suppose in discrete space, smallest units, it's possible for there to be perfect smoothness, but not if near a gravitating body, because then all the quanta would not match up perfectly. Warped discrete spacetime.

Adhesive forces, good point, like gravity of the two surfaces, however weak. Unless there's a smallest unit of gravity, and the two surfaces are not at the threshold to have any gravity. But then there's all the other factors mentioned in this thread that can increase the friction coefficients.
 
jaketodd said:
Adhesive forces, good point, like gravity of the two surfaces, however weak
Or chemical bonding.