Things giving rise to static and dynamic coefficients of friction?

[jaketodd]

Topography of both the object and the surface.

Mass/inertia.

Moisture, but that can probably fall under topography.

I suppose atmospheric pressure, maybe. Or wind.

Magnetism.

Any others?

 

[Baluncore]

Dust.
Asperities.
https://en.wikipedia.org/wiki/Asperity_(materials_science)

 

[jaketodd]

Makes me think, is there anything with a perfectly smooth surface, other than in mathematics. Maybe spacetime, if spacetime is indeed continuous.

 

[hutchphd]

A more interesting question is "where does the energy go ?"

 

[jaketodd]

A more interesting question is "where does the energy go ?"

Not sure what you mean. If you're talking about between the object and the surface, it's probably mostly released as heat. Think of rubbing your hands together to get them warm.

If the object doesn't move, and there's some energy given to it, but not enough to make it move, then that would probably be heat as well, distributed throughout the object and the surface.

If you're talking about a frictionless interaction, then the energy would be heat but also motion, kinetic energy.

 

[Baluncore]

If the object doesn't move, and there's some energy given to it, but not enough to make it move, then that would probably be heat as well, distributed throughout the object and the surface.

I think that is static friction, where there is no relative motion. Energy would be invested in elastic deformation of the material near the contact, but insufficient energy to make it move. The heat would only appear if it became sliding friction, with a relative surface movement.

Sliding friction will cause local deformation and bond breaking, which will release IR heat energy, and radiate noise as phonons from the contact zone.

 

[jaketodd]

I think that is static friction, where there is no relative motion. Energy would be invested in elastic deformation of the material near the contact, but insufficient energy to make it move. The heat would only appear if it became sliding friction, with a relative surface movement.

Sliding friction will cause local deformation and bond breaking, which will release IR heat energy, and radiate noise as phonons from the contact zone.

Deformation is a form of heat. Kinetic energy is required to deform. That causes the deformation, and distributes throughout the object and surface, as well. Correct me if I am wrong.

 

[Baluncore]

A force that causes deformation, results in a flow of energy.

Elastic deformation stores some potential energy, and may return it later. That is the case with static friction.

Plastic deformation occurs when bonds are broken and energy is applied. Some of the energy used to break the bonds is released as heat. That is the case with sliding friction.

Deformation has units of ratio=length/length. Energy and heat have units of joules. The concepts cannot be the same.

 

[jbriggs444]

Kinetic energy is required to deform.

To be picky, energy is required to deform. But it need not be kinetic energy.

Yes, if you push on something with static friction to make it deform, the body that is doing the pushing must be in motion. But how much motion? If you perform the deformation arbitrarily slowly, you need arbitrarily little kinetic energy. So kinetic energy cannot be the important factor.

 

[Lnewqban]

Any others?

Lubrication.
Heat.
Pressure.

 

[Baluncore]

Any others?

Vibration.

 

[Lnewqban]

Hysteresis.

 

[jaketodd]

Hysteresis.

I love your quote:

"Mankind is composed of two sorts of men — those who love and create, and those who hate and destroy. Love is the bond between men, the way to teach and the center of the world." - José Martí"

I'm having trouble understanding Hysteresis. Help?

 

[jaketodd]

To be picky, energy is required to deform. But it need not be kinetic energy.

Yes, if you push on something with static friction to make it deform, the body that is doing the pushing must be in motion. But how much motion? If you perform the deformation arbitrarily slowly, you need arbitrarily little kinetic energy. So kinetic energy cannot be the important factor.

In order for something to move, even just a little bit, requires kinetic energy.

I think you're thinking of calculus. The limit of kinetic energy needed to create change, as the change approaches zero, equals zero. But that is a misconception because if there is no motion/change, then there's no energy, kinetic, required - it would not have an influence.

 

[Lnewqban]

I love your quote:
....
I'm having trouble understanding Hysteresis. Help?

Thank you.

This article explains that friction between tires and pavement is affected by the capability of the rubber to return to a previous relaxed state.

https://en.wikipedia.org/wiki/Hysteresis#In_mechanics

Motorcyclists are well aware of the fact that sudden and abrupt changes in steering and acceleration drastically reduce the "gripping" of the tires, especially on wet pavement: after those changes, the rubber needs certain time to return to its its deformation and best friction coefficient.

 

[jbriggs444]

In order for something to move, even just a little bit, requires kinetic energy.

However, in order for something to exist in a state of stress, no energy whatsoever is required.

 

[jaketodd]

However, in order for something to exist in a state of stress, no energy whatsoever is required.

Energy is required to create the state of stress. And I assume that something in a state of stress is "hotter" than one that is not.

 

[jbriggs444]

Energy is required to create the state of stress. And I assume that something in a state of stress is "hotter" than one that is not.

How much energy is required to support a book on a table with a tilt?

And no, temperature has little to do with stress. Both warm and cold tables can support books.

 

[jaketodd]

I think equilibrium is a key concept here. Two or more objects that have totally equalized their forces will not exert any additional heat to one another.

 

[jbriggs444]

I think equilibrium is a key concept here. Two or more objects that have totally equalized their forces will not exert any additional heat to one another.

We are way off topic here.

Heat is a flow of disordered energy. Heat flows based on the second law of thermodynamics. If the heat flow would result in an increase of entropy, then it spontaneously flows. If not, it won't. This gives rise to a definition of thermodynamic temperature as the ratio between the incremental change in energy $\delta E$ divided by the incremental change in entropy $\delta S$.

Heat will flow from a body with a high ratio $\frac{\delta E}{\delta S}$ to a body with a low ratio. This increases entropy. A flow the other way would reduce entropy and is forbidden by the second law. This behavior is exactly how we expect "temperature" to behave. Heat flows from hot objects to cold objects.

For a typical substance under typical conditions, the ratio $\frac{\delta E}{\delta S}$ corresponds closely to the average kinetic energy per degree of freedom in the molecules making up the substance. As a result, many students, teachers and textbooks explain temperature in terms of kinetic energy in this way -- as kinetic energy per particle.

In exotic conditions, the kinetic energy definition ceases to make much sense and negative temperatures are actually possible.

None of this has anything to do with a stress within a body or with any static frictional force between bodies.

But yes, equilibrium is a key concept. The concept of temperature is only formally defined for systems or regions that are at equilibrium. It is approximately defined for systems or regions that are approximately at equilibrium.

And yes, heat will not spontaneously flow between two objects that have equalized their temperatures.

 

[jaketodd]

"negative temperatures"
Sounds intriguing, tell me about that!

--

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

If they exist, maybe they keep the universe from decaying into total state of equilibrium/entropy.

--

And on a comedic note:

 

[jbriggs444]

"negative temperatures"
Sounds intriguing, tell me about that!

As I had suggested previously, temperature and thermodynamics are pretty much off topic In a thread that is nominally about friction. But you are the thread starter here so away we go...

If you simply Google "negative temperature", you can find this article among others.

Recall the thermodynamic definition of temperature: $\frac{\delta E}{\delta S}$. For almost any object we are accustomed to, this ratio is positive. The more energy you add to a system, the larger its state space becomes. It has more entropy.

Some exotic systems are such that their state space is eventually reduced as more and more energy is added. In this situation, the ratio $\frac{\delta E}{\delta S}$ is negative. This corresponds to a negative temperature.

If energy leaves such a system, that system's entropy increases. If that lost energy enters another system at a positive temperature, that system has its entropy increase as well. In accordance with the second law of thermodynamics, this means that heat will flow spontaneously from a body with a negative temperature to one with a positive temperature -- a negative temperature is "hotter" than all positive temperatures.

It is sometimes more convenient to use "inverse temperatures" like $\frac{1}{T} = \frac{\delta S}{\delta E}$. The inverse temperature scale behaves consistently with respect to heat flow. Heat always flows from lower inverse temperature to higher inverse temperature.

[Corrections welcome from anyone who has actually taken a course in thermodynamics where this stuff is discussed. What I think I know, I've learned from hanging out here and Googling for background]As far as white holes go and a potential rescue of the universe from an eventual heat death, that sounds like wishful thinking layered on top of speculation. Let us not go there.

 

[hutchphd]

Static and dynamic "friction" are very different. They are grouped together in introductory courses mostly because they are each approximately proportional to the normal force at an interface and provide, therefore, an easy way to formulate introductory problems. I think them overemphasized.
I would distinguish them microscopically as follows : the static friction interactions are time reversible whereas the dynamic friction interactions are not. This is a function of energetically available degrees of freedom and can be cast in a thermodynamic framework if desired.

 

[jaketodd]

As I had suggested previously, temperature and thermodynamics are pretty much off topic In a thread that is nominally about friction. But you are the thread starter here so away we go...

If you simply Google "negative temperature", you can find this article among others.

Recall the thermodynamic definition of temperature: $\frac{\delta E}{\delta S}$. For almost any object we are accustomed to, this ratio is positive. The more energy you add to a system, the larger its state space becomes. It has more entropy.

Some exotic systems are such that their state space is eventually reduced as more and more energy is added. In this situation, the ratio $\frac{\delta E}{\delta S}$ is negative. This corresponds to a negative temperature.

If energy leaves such a system, that system's entropy increases. If that lost energy enters another system at a positive temperature, that system has its entropy increase as well. In accordance with the second law of thermodynamics, this means that heat will flow spontaneously from a body with a negative temperature to one with a positive temperature -- a negative temperature is "hotter" than all positive temperatures.

It is sometimes more convenient to use "inverse temperatures" like $\frac{1}{T} = \frac{\delta S}{\delta E}$. The inverse temperature scale behaves consistently with respect to heat flow. Heat always flows from lower inverse temperature to higher inverse temperature.

[Corrections welcome from anyone who has actually taken a course in thermodynamics where this stuff is discussed. What I think I know, I've learned from hanging out here and Googling for background]As far as white holes go and a potential rescue of the universe from an eventual heat death, that sounds like wishful thinking layered on top of speculation. Let us not go there.

Is negative temperature related at all to potential energy, for lack of a better term? Like e=mc^2. Splitting an atom, like in a nuclear weapon, converts mass to pure energy, I believe mostly in the form of photons. And so I am guessing the entropy of what's left of the atoms increases, since there's less mass/energy there after the reaction.

 

[jbriggs444]

Is negative temperature related at all to potential energy, for lack of a better term?

No.

I am guessing the entropy of what's left of the atoms increases, since there's less mass/energy there after the reaction.

No. Regardless of the sort of interaction, including nuclear weapons, both energy and mass are conserved.

Most people are already comfortable with the notion of energy conservation. Even in a nuclear fission reaction, energy is conserved. Prior to the reaction, energy was present in the form of mass of the fissioning element. After the reaction, energy is present in the form of the mass of the fragments, their kinetic energy, and any resulting radiation.

But if energy is conserved, it follows that "mass" is conserved as well. Because "mass" in its modern usage is taken to mean "invariant mass". This, in turn, is defined via $E=mc^2$ as the total energy of the system in its center of momentum frame divided by $c^2$.

We ignore "relativistic mass" as a relic of understandings whose time has passed. When we use the word "mass", we mean invariant mass.

Note that although invariant mass is conserved, it is not additive. The mass of a system is not the sum of the masses of its components.

This has nothing to do with static friction and also nothing to do with negative temperatures. Are we done with this thread yet?

 

[jaketodd]

Sounds like you are. Anyone else... can you think of other things that impact the friction coefficients?

 

[Frabjous]

Sounds like you are. Anyone else... can you think of other things that impact the friction coefficients?

fine structure constant

 

[Frabjous]

Sounds like you are. Anyone else... can you think of other things that impact the friction coefficients?

As a practical matter, relative velocity, although it is not a mechanism per se.

 

[jaketodd]

As a practical matter, relative velocity, although it is not a mechanism per se.

You mean like momentum?

 

[Frabjous]

You mean like momentum?

No, relative velocity between the surfaces. There are plenty of measurements of different friction coefficients at different velocities for a variety of systems.

 

[hutchphd]

If you really want to dive into this subject, you should study the fluctuation dissipation theorem. This should take a few weeks or longer.

 

[jaketodd]

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?

 

[Frabjous]

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?

 

[jaketodd]

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.

 

[Frabjous]

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.