is friction purely due to particle inertia

[J.C.MacSwell]

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?

 

I'm grasping, but...

 

Think of a closed container of water sitting at ambient temperature...essentially nothing happening. It is at some pressure and thereby exerting forces on the container. Where is the friction?

 

Now open it up and drag your hand through the water...friction!

 

I know this is obvious to you, but just trying to understand what you are thinking...

 

...my only thought is that if you invoke the second law of thermodynamics, the "you can't break even" part of "you can't win, you can't break even, and you can't get out of the game..." then you may be claiming that a "perfectly frictionless" process is impossible.

 

Outside of that I don't see where you are coming from though you seem to be repeating the same claim.

[lemur]

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.

I don't understand your reasoning process. Without an example of how an object can pass through a fluid without collisions among the particles of the fluid occurring, then I don't see how there can be no inertial resistance to displacement and therefore friction. I think if there was a concrete scenario that defied this logic, I could understand it but I just can't think of one.

 

 

Think of a closed container of water sitting at ambient temperature...essentially nothing happening. It is at some pressure and thereby exerting forces on the container. Where is the friction?

 

Now open it up and drag your hand through the water...friction!

 

I know this is obvious to you, but just trying to understand what you are thinking...

I think friction results from dissipation of a pressure concentration (wave) through the fluid, as Swansont implied talking about dissipative force. Well, maybe that's not exactly right - more like friction occurs in the building up of the pressure-concentration before it dissipates as a wave. If the displaced medium was perfectly inelastic, I would think it would displace without waves, e.g. like pushing on a solid object where force-transfer between all the particles of the object somehow occurs without any movement of the particles relative to each other. But because the force of motion isn't immediately resisted uniformly by all particles, pressure has to build up among the ones most immediately affected by a push. I guess then as the motion dissipates throughout the substance, the amount of force received and resisted by the inertia of subsequent particles decreases, like a ripple in smooth water fading as it dissipates.

 

Without some inertia resisting the force of the wave/ripple, though, there would be no ripple, correct, because the raised water of the ripple has to be held up from the front by some force? This sort of goes back to the aether issue, though that wasn't my intention, because I don't see how a frictionless fluid could be a medium for waves since there would be no inertia to hold the wave energy from immediately dissipating.

 

 

[J.C.MacSwell]

I don't understand your reasoning process. Without an example of how an object can pass through a fluid without collisions among the particles of the fluid occurring, then I don't see how there can be no inertial resistance to displacement and therefore friction. I think if there was a concrete scenario that defied this logic, I could understand it but I just can't think of one.

 

 

 

I think friction results from dissipation of a pressure concentration (wave) through the fluid, as Swansont implied talking about dissipative force. Well, maybe that's not exactly right - more like friction occurs in the building up of the pressure-concentration before it dissipates as a wave. If the displaced medium was perfectly inelastic, I would think it would displace without waves, e.g. like pushing on a solid object where force-transfer between all the particles of the object somehow occurs without any movement of the particles relative to each other. But because the force of motion isn't immediately resisted uniformly by all particles, pressure has to build up among the ones most immediately affected by a push. I guess then as the motion dissipates throughout the substance, the amount of force received and resisted by the inertia of subsequent particles decreases, like a ripple in smooth water fading as it dissipates.

 

Without some inertia resisting the force of the wave/ripple, though, there would be no ripple, correct, because the raised water of the ripple has to be held up from the front by some force? This sort of goes back to the aether issue, though that wasn't my intention, because I don't see how a frictionless fluid could be a medium for waves since there would be no inertia to hold the wave energy from immediately dissipating.

 

Pressure wave losses are not the same as friction losses. In the example I used in post 5 for a rounded body moving through a idealized frictionless fluid there are no surface effects or gravitational waves (think ideal submarine at depth). If there was (same submarine at surface) there would be drag even for a frictionless fluid. I do not know how that would work a superfluid. I suspect at the boundaries (including the surface of an object moving through it) of the fluid the collision rules described could not apply.

(I don't understand that part, or how these collisions can work...I'm better with billiard balls!)

Edited May 27, 2011 by J.C.MacSwell

[swansont]

I don't understand your reasoning process. Without an example of how an object can pass through a fluid without collisions among the particles of the fluid occurring, then I don't see how there can be no inertial resistance to displacement and therefore friction. I think if there was a concrete scenario that defied this logic, I could understand it but I just can't think of one.

 

The only example of a frictionless fluid is a frictionless fluid. Your demand of "without collisions" is a problem, since nobody claims that there are no collisions. The collisions just don't allow a loss of kinetic energy.

[lemur]

Pressure wave losses are not the same as friction losses. In the example I used in post 5 for a rounded body moving through a idealized frictionless fluid there are no surface effects or gravitational waves (think ideal submarine at depth). If there was (same submarine at surface) there would be drag even for a frictionless fluid. I do not know how that would work a superfluid. I suspect at the boundaries (including the surface of an object moving through it) of the fluid the collision rules described could not apply.

(I don't understand that part, or how these collisions can work...I'm better with billiard balls!)

Forget about collisions for a moment and just think about waves. How can a wave form except by resistance in the transfer of force through a substance? If you push on a substance and it creates a bulge, whatever is on the other side of the bulge is resisting the force-transfer. I suppose you could say that the ability to form the bulge at all and the fact that it can move through the substance as a wave indicates friction among the particles. The only way I can see a fluid being frictionless is if the particles don't displace each other in any way, e.g. if they just pass each other without force-transfer. But even if that occurred, the particles displaced by the object would still have to accelerate so there would be a very small amount of (resistance) force that would be exerted on the object pushing them (equal and opposite reaction). If that's not friction, what is it?

 

 

 

[swansont]

If there is no loss of kinetic energy, it's not friction. Use another word. You can't redefine it to suit your needs.

 

An ideal spring will oscillate forever if there are no losses. Obviously, there is a force. A spring force is not friction.

With gravity, an object can return to the same position (height) if it moves along a closed path and there are no losses. Gravity is not friction.

 

You can't use friction interchangeably with force.

[lemur]

If there is no loss of kinetic energy, it's not friction. Use another word. You can't redefine it to suit your needs.

What do you mean by kinetic energy loss? Energy isn't created or destroyed - it just changes form.

 

An ideal spring will oscillate forever if there are no losses. Obviously, there is a force. A spring force is not friction.

With gravity, an object can return to the same position (height) if it moves along a closed path and there are no losses. Gravity is not friction.

Every time a spring changes direction, it has to decelerate and re-accelerate, doesn't it? An object in orbit may be a perpetual motion machine in that it is an object in motion tending to stay in motion, but how can an oscillating spring remain perpetually in motion when its motion has to keep stopping and starting in a forceful way? Doesn't that require work?

 

You can't use friction interchangeably with force.

Not even force of resistance incurred by motion?

 

 

[swansont]

What do you mean by kinetic energy loss? Energy isn't created or destroyed - it just changes form.

 

Kinetic energy is only one form of energy. Friction requires kinetic energy be changed into another form of energy that leaves the system.

 

Every time a spring changes direction, it has to decelerate and re-accelerate, doesn't it? An object in orbit may be a perpetual motion machine in that it is an object in motion tending to stay in motion, but how can an oscillating spring remain perpetually in motion when its motion has to keep stopping and starting in a forceful way? Doesn't that require work?

 

The force is position dependent, which allows you to use the concept of potential energy (stored energy). The amount of work done (or stored as potential energy) in one half of a cycle is returned in the other half.

 

Not even force of resistance incurred by motion?

 

Not unless it dissipates the kinetic energy.

[lemur]

Kinetic energy is only one form of energy. Friction requires kinetic energy be changed into another form of energy that leaves the system.

 

 

 

The force is position dependent, which allows you to use the concept of potential energy (stored energy). The amount of work done (or stored as potential energy) in one half of a cycle is returned in the other half.

 

 

 

Not unless it dissipates the kinetic energy.

I don't see what dissipation has to do with it. If an object is pushing through a fluid, the resistance of the fluid to the object's force is friction regardless of what happens to the energy that gets transferred to the fluid later, no?

[J.C.MacSwell]

I don't see what dissipation has to do with it. If an object is pushing through a fluid, the resistance of the fluid to the object's force is friction regardless of what happens to the energy that gets transferred to the fluid later, no?

 

Form (pressure) drag is not friction. Wave resistance is not friction. Both resist the objects motion.

[lemur]

Form (pressure) drag is not friction. Wave resistance is not friction. Both resist the objects motion.

when is object-motion-resistance different than friction?

[swansont]

I don't see what dissipation has to do with it. If an object is pushing through a fluid, the resistance of the fluid to the object's force is friction regardless of what happens to the energy that gets transferred to the fluid later, no?

 

No. Dissipation has everything to do with it. Friction requires a loss of kinetic energy. If that energy is returned, it's not friction. Systems with friction are not reversible.

[J.C.MacSwell]

when is object-motion-resistance different than friction?

 

What is considered frictional resistance or drag is somewhat arbitrarily defined, but the force of friction is a dissipating force brought about by movement in shear.

 

So if you drag something along the ground that is friction. If you look at it on a different scale their are impacts involved that you may not consider completely friction in isolation, but it all gets lumped in as friction.

 

In hydrodynamics of ships, skin friction is the shear force effect on the surface of the ship.

 

Pressure on the front of the ship that displaces the water upwards creates a wave. This energy generally gets dissipated and lost but is not considered friction (no shear losss or viscous effects, at least not immediately). The pressure also displaces water down and to the sides. Some of that energy gets returned to the ship. The portion that is lost is form drag. Friction can be involved in the dissipation of some of that energy but to the ship that is not considered frictional drag.

 

If you drive into the back of a bus and metal crumples on impact internal friction in the metal is involved (the metal heats up), but for you that is not friction, but if you drive and scrape along the side of the bus thats friction to you even while you are transferring some of your momentum and kinetic energy to the bus.

 

So it is not always clearly defined but it is clearly not everything that is a resisting force to motion.

[lemur]

I don't see the functionality of differentiating between friction and other kinds of motion-resistant force. Brake pads heat up because the KE of the vehicle's momentum is getting transferred into them, creating waves of (heat) energy in the pad. If the same vehicle's brakes fail and it plunges into water, the momentum that would have heated up the brake pads and the tires with friction would be lost as waves in the water. Yes, for those waves to be formed, the vehicle can't pass through them frictionlessly, but for any fluid displacement to occur, there has to be resistance, and that resistance will translate into waves rolling away from the pushing object. Those waves may be waves in a fluid medium, but they are still waves of energy just like the waves of energy that manifest as heat in a brake pad or a stopping tire, aren't they?

[swansont]

I don't see the functionality of differentiating between friction and other kinds of motion-resistant force.

 

Physicists do.

[J.C.MacSwell]

I don't see the functionality of differentiating between friction and other kinds of motion-resistant force. Brake pads heat up because the KE of the vehicle's momentum is getting transferred into them, creating waves of (heat) energy in the pad. If the same vehicle's brakes fail and it plunges into water, the momentum that would have heated up the brake pads and the tires with friction would be lost as waves in the water. Yes, for those waves to be formed, the vehicle can't pass through them frictionlessly, but for any fluid displacement to occur, there has to be resistance, and that resistance will translate into waves rolling away from the pushing object. Those waves may be waves in a fluid medium, but they are still waves of energy just like the waves of energy that manifest as heat in a brake pad or a stopping tire, aren't they?

 

As long as you are not designing anything I might use or come across, I'm fine with that.

[lemur]

Physicists do.

My response comparing heat caused by friction with waves caused in fluid was reasonable and you respond by asserting that physicists don't care because they're physicists?

[swansont]

My response comparing heat caused by friction with waves caused in fluid was reasonable and you respond by asserting that physicists don't care because they're physicists?

More specifically, they don't care because they do physics. You said you don't see the functionality of the definition, which is probably because you don't do physics. There is little functionality in a definition that is so broad that it would include pretty much any force that could be found in a problem. A physicist understands that the presence of friction automatically excludes the system from being a reversible process. That contains information — the entropy doesn't change for a reversible process, and you can define potentials if the work done is path-independent, but your definition would remove that.