is friction purely due to particle inertia

[lemur]

Before the aether thread was closed, the question emerged for me of whether such a thing as a "frictionless superfluid" is actually possible. My reasoning is that it isn't because particles have inertia and thus resist momentum-exchanges during collisions. Therefore any fluid/gas consisting of particles with mass and thus inertia should have friction. Am I forgetting something? Also, in practice how much fluid/gas friction is due to particle-inertia and how much is due to other things, such as electrostatic force among the particles or maybe the shape of particles causing viscosity. I guess a lot would depend on the particular substance in question.

[imatfaal]

Lemur

 

There is superfluidity - which is a strange state of matter that seems to allow the substance to behave unusually (it has zero viscosity and complete thermal conductivity)

 

You can read up here http://en.wikipedia.org/wiki/Superfluid

and here http://hyperphysics.phy-astr.gsu.edu/hbase/lhel.html

 

Nice video

 

In a way this can be called frictionless - the atoms (or now molecules) do not hinder the progress of their fellows - this is what leads to the zero viscosity. I don't know if a foreign body would be frictionless - although my feeling is not

[swansont]

In simple elastic collisions there is no friction, because there is nowhere for the energy to go. In most situations the collisions are not elastic — there is somewhere else for the kinetic energy to go. There's also a semantic point that friction is generally considered a bulk property, but that's partly because you need multi-particle systems to have these dissipative pathways. Frictionless fluids are in a state where you have removed those other pathways for the energy loss.

[lemur]

Maybe such fluids can be frictionless up to a certain point, but think about what has to happen to particles as something moves through them. If there is momentum transfer between the object pushing through the fluid and the particles of the fluid, those particles are going to run into other particles, etc. Each collision of two particles with mass requires a corresponding deceleration and acceleration, which is force that adds up to friction. So AT SOME LEVEL any substance with mass has to have potential friction unless the particles can somehow miss each other and not collide during fluid displacement, right?

[J.C.MacSwell]

Maybe such fluids can be frictionless up to a certain point, but think about what has to happen to particles as something moves through them. If there is momentum transfer between the object pushing through the fluid and the particles of the fluid, those particles are going to run into other particles, etc. Each collision of two particles with mass requires a corresponding deceleration and acceleration, which is force that adds up to friction. So AT SOME LEVEL any substance with mass has to have potential friction unless the particles can somehow miss each other and not collide during fluid displacement, right?

 

 

In a theoretically frictionless (inviscid) fluid there are no shear forces.

 

For, say, a rounded and symmetric body moving through a frictionless fluid it would have no net drag.

 

Although the fluid would be displaced and therefore accelerated at the leading portion of the body, the fluid would transfer that energy back to the object on the trailing portion of the body.

 

With friction there is an irreversible process and this cannot happen. Not only would the shear stresses in the fluid be transferred as drag forces along the sides of the body, but form drag would also be created from the differences in pressure that would result.

[swansont]

Maybe such fluids can be frictionless up to a certain point, but think about what has to happen to particles as something moves through them. If there is momentum transfer between the object pushing through the fluid and the particles of the fluid, those particles are going to run into other particles, etc. Each collision of two particles with mass requires a corresponding deceleration and acceleration, which is force that adds up to friction. So AT SOME LEVEL any substance with mass has to have potential friction unless the particles can somehow miss each other and not collide during fluid displacement, right?

 

In a quantum system you have the possibility of being in a single energy state, and a collision does not get you into another allowed state, so there can be no energy transfer. That's where you see superfluid behavior. You could still have momentum exchange, but it would be in direction only, not magnitude.

[lemur]

In a quantum system you have the possibility of being in a single energy state, and a collision does not get you into another allowed state, so there can be no energy transfer. That's where you see superfluid behavior. You could still have momentum exchange, but it would be in direction only, not magnitude.

I don't understand. Electrons in a conductor can theoretically alternate without losing energy to resistance up to a certain point, but at some point the electrons cannot behave as inertia-less anymore, right, so they emit energy as photons (maybe I'm mixing too many things here). Just at the most simplistic level of photons vs. particles with mass, photons can reflect without decelerating and accelerating but can electrons or any other particles with mass?

[swansont]

I don't understand. Electrons in a conductor can theoretically alternate without losing energy to resistance up to a certain point, but at some point the electrons cannot behave as inertia-less anymore, right, so they emit energy as photons (maybe I'm mixing too many things here). Just at the most simplistic level of photons vs. particles with mass, photons can reflect without decelerating and accelerating but can electrons or any other particles with mass?

 

Electrons are superconducting when they have formed bosonic pairs (Cooper pairs), which gives rise to a quantum effect. At a low-enough temperature, these pairs are in the ground state of a confining potential and cannot lose energy. I don't think you would ever describe them as inertia-less.

[lemur]

This thread is accruing informative replies, but I am still lacking a clear sense of how particle-inertia relates to fluid/gas friction. How can there be collisions involving particles with mass that don't involve acceleration/deceleration and thus force-resistance? Motion-change always involves force unless it is photons, right? Objects in motion tend to stay in motion unless acted upon by outside force and actions have equal and opposite reactions, correct?

[J.C.MacSwell]

This thread is accruing informative replies, but I am still lacking a clear sense of how particle-inertia relates to fluid/gas friction. How can there be collisions involving particles with mass that don't involve acceleration/deceleration and thus force-resistance? Motion-change always involves force unless it is photons, right? Objects in motion tend to stay in motion unless acted upon by outside force and actions have equal and opposite reactions, correct?

 

Motion changes for photons requires forces as well. Not sure what is being described where there is no energy transfer. In a normal elastic collision there is still energy transfer, it's just that it is reversible.

[swansont]

An object can change momentum without changing the magnitude of its momentum. It's a vector.

[J.C.MacSwell]

An object can change momentum without changing the magnitude of its momentum. It's a vector.

 

...and this is all that happens in a superfluid?...every collision is not just elastic, but there is no net energy transfer in each collision? (this seems statistically unlikely, does it not? or is there some mechanism that makes it happen?)

 

Assuming I have this right, that could only be with respect to one inertial frame only, correct? The magnitude of the momentum must have changed when measured in others.

Edited May 25, 2011 by J.C.MacSwell

[swansont]

...and this is all that happens in a superfluid?...every collision is not just elastic, but there is no net energy transfer in each collision? (this seems statistically unlikely, does it not? or is there some mechanism that makes it happen?)

 

Assuming I have this right, that could only be with respect to one inertial frame only, correct? The magnitude of the momentum must have changed when measured in others.

 

As far as I understand it. You have a Bose condensate, putting all of the particles in question in the ground state of the confining potential. You do have the possibility of a collision putting the particle in a higher energy state, but then it's not part of the condensate anymore. These are bosons, not billiard balls, so they can exist at the same point, meaning the particles can just not scatter if such an interaction required they end up in a state that isn't allowed, similar to a photon that doesn't interact with an atom when it doesn't have the right energy.

 

Yes, all of this is in one frame. A transform into another frame changes the values.

[lemur]

An object can change momentum without changing the magnitude of its momentum. It's a vector.

If an object has mass, changing direction without changing speed/momentum would require deceleration and re-acceleration, wouldn't it?

[swansont]

If an object has mass, changing direction without changing speed/momentum would require deceleration and re-acceleration, wouldn't it?

 

Acceleration is likewise a vector quantity. You can accelerate without changing speed or energy; you do it every time you move in a circular path. Also, this is a quantum system, so all the rules of QM apply, including [math]\Delta{E}\Delta{t}>\hbar[/math]

[lemur]

Acceleration is likewise a vector quantity. You can accelerate without changing speed or energy; you do it every time you move in a circular path. Also, this is a quantum system, so all the rules of QM apply, including [math]\Delta{E}\Delta{t}>\hbar[/math]

Ok, but acceleration whether linear or circular still constitutes force, which would amount to friction right?

[swansont]

Ok, but acceleration whether linear or circular still constitutes force, which would amount to friction right?

 

No. Friction is a dissipative force. Neither a conservative force (e.g. gravity) nor one which does no work (circular motion) is dissipative.

[lemur]

No. Friction is a dissipative force. Neither a conservative force (e.g. gravity) nor one which does no work (circular motion) is dissipative.

In what sense does circular motion relate to friction?

[swansont]

In what sense does circular motion relate to friction?

 

It's an example of a force that does no work.

[lemur]

It's an example of a force that does no work.

I guess I'll just continue tentatively with the view that friction is at least partially due to inertia and that no substance with mass can be totally frictionless in all situations because that would mean no acceleration/deceleration of the displaced particles in their motion. I can't apply the information that's been posted on this thread to come to any other conclusion. Sorry if this makes me thick headed.

 

 

 

[swansont]

I guess I'll just continue tentatively with the view that friction is at least partially due to inertia and that no substance with mass can be totally frictionless in all situations because that would mean no acceleration/deceleration of the displaced particles in their motion. I can't apply the information that's been posted on this thread to come to any other conclusion. Sorry if this makes me thick headed.

 

Nobody claimed it would be true under all conditions. But frictionless superfluids do exist, and friction is not the inherent result of objects having mass.

 

It's one thing to say you don't understand. That's life. Denying the existence of something because you don't understand treads on the path of crackpottery; it's the fallacy of argument from personal incredulity.

[lemur]

Nobody claimed it would be true under all conditions. But frictionless superfluids do exist, and friction is not the inherent result of objects having mass.

 

It's one thing to say you don't understand. That's life. Denying the existence of something because you don't understand treads on the path of crackpottery; it's the fallacy of argument from personal incredulity.

Actually it's the other way around. It is unreasonable to reject logical conclusions unless information is found to reasonably undermine them. I said "tentatively" because I am not excluding the possibility that I'll end up rejecting this conclusion with reasonable cause. I just haven't heard anything that causes me to think that inertia isn't a factor in fluid-displacement that would necessitate friction. If any pushing of matter with mass takes place, there has to be acceleration of the displaced particles, which requires force between the objects pushing against one another. I haven't read anything that shows this to be flawed reasoning yet.

[J.C.MacSwell]

I guess I'll just continue tentatively with the view that friction is at least partially due to inertia and that no substance with mass can be totally frictionless in all situations because that would mean no acceleration/deceleration of the displaced particles in their motion. I can't apply the information that's been posted on this thread to come to any other conclusion. Sorry if this makes me thick headed.

 

Why would it mean that? Unless I'm getting it wrong, a very restricted form of acceleration/displacement of the particles is being described...but not no acceleration or displacement.

[lemur]

Why would it mean that? Unless I'm getting it wrong, a very restricted form of acceleration/displacement of the particles is being described...but not no acceleration or displacement.

If any acceleration is involved, then there would be force at play, right? So if there's force, then it can't be frictionless, can it?

 

 

 

[swansont]

If any acceleration is involved, then there would be force at play, right? So if there's force, then it can't be frictionless, can it?

 

No! All forces are friction is a false statement. Friction is a dissipative force. If the force is not dissipative, it is not friction. Seems I've said that before.

 

In stricter physics terminology, friction (in frames where the object is the moving entity) involves work being negative, regardless of the direction of motion. If negative work is followed by positive work of the same amount, (or vice-versa), or no work at all is done, then no net energy is transferred. Those forces are not in the category of friction.