arfa's musings on Elastic surfaces

[James R]

James, can you please explain why momentum is conserved in an inelastic collision. if we get past that, I'll see how I'm feeling.

Sure. The momentum of any system is conserved as long as no net external force acts on the system. In an inelastic collision, if we include both colliding objects in the "system", then total momentum will be conserved. Of course, we're making the impulsive collision approximation there, as you'll no doubt be aware.

Your turn. Why do you say momentum is not conserved in an inelastic collision?

This place, this forum, is now one where the sarge comes along and demands to know what the hell you're talking about.

Is it not legitimate to ask you questions about your musings?

Perhaps you should start a blog. You could moderate it yourself and eliminate all inconvenient and irksome questions.

For fuck's sake, James, I know you know what an elastic material is, also that it has a surface, made of the same material. Don't be such a goddam dick.

I have my ideas about what might be meant by a term such as "elastic material", but I'm interested in what you mean by that. For example, above you seem to be saying that water is an elastic material, and wood. So, I'm interested in what features of those things make them elastic. More generally, I'd like to see you definition of what makes something elastic? So far, I gather from you that anything made of atoms is elastic. Since every "material" is made of atoms, that means everything is elastic. Right? Why do we need the term "inelastic" then?

 

[James R]

Oh, and that's the second strike with the "dick" namecalling. Want to try your luck for a third time?

 

[arfa brane]

More generally, I'd like to see you definition of what makes something elastic? So far, I gather from you that anything made of atoms is elastic. Since every "material" is made of atoms, that means everything is elastic. Right? Why do we need the term "inelastic" then?

You know how when you hear sound, it's because of waves, pressure waves, in the air, right?
Well, these waves propagate in an elastic medium--the air. I mean, they just do.

An elastic wave is exactly the kind of thing you can make a spring generate, and that is again exactly the kind of wave you get in guitar strings when you pluck them. Shake a spring with a moderate spring constant, and it will vibrate or shudder, depending if it's hanging or suspended somehow, or lying on another surface, same deal with a length of rope. Elastic waves propagate in elastic materials. Including air and water.

Honestly, this is all undergrad physics stuff. I can find it in a textbook that's nearly 50 years old.

 

[arfa brane]

I gather from you that anything made of atoms is elastic.

Well that must follow if an atom is not a solid little ball of matter, or not an inelastic 'field' of matter.

Why do we need the term "inelastic" then?

It's a contrast function . . .? What is an inelastic collision if total momentum is conserved? Don't objects that collide so a noise is heard, lose a bit of energy thereby?

I mean, I thought that was the deal and explained what an inelastic collision is, one that does not conserve the total momentum, some is absorbed by making a sound, for instance.

 

[exchemist]

This thread is becoming embarrassing to read.

Poor Arfa.

I suppose it can happen to any of us, eventually.

 

[arfa brane]

No look, this place is becoming embarrasing.

A completely elastic interaction in which total momentum is conserved, is for example a pair of fixed magnets on a frictionless surface; or magnetic pucks on an air table. You arrange a pair of these to exchange momentum, right? But the sum doesn't change, right? And, they don't need to actually touch each other (right kids?)

If you use ordinary pucks that interact so they literally collide, other shit happens involving noise, kinetic energy is lost.

So if you want to know why we use the word inelastic, why we need it, young James me lad and all the other kids watching, we need it because sometimes we build things that collapse. Total momentum is not conserved, at least, not locally.

 

[arfa brane]

Still waiting. I'm still waiting. For that example of a surface that is completely rigid.

Just one. You know, one where I won't be able to make it ring like a bell, say.

 

[arfa brane]

Let's review this idea of elastic and inelastic, in the context of a "collision".

If you've done high school physics and the curriculum included air tracks and linear momentum experiments, the difference between elastic and inelastic is whether two 'carts' are separate before and after, or separate before and stuck together after. Or at least, that's one way to spot the difference.

And if you do enough collisions and measure the results, you should get a good approximation to Newton's law of censervation of momentum; in the lab frame.

But you have to consider that it's only true to first order; your experiments probably didn't account for all those other interactions--the "microcollisions", now did it? Newtonian mechanics is where you find mechanical friction; saying it's the "reason" you need oil in a car engine, is true to first order.

I'll try to nail this to some kind of door. If you focus on what a pair of carts on an air track is doing, and not on what the track is doing to each cart (you assume it's a frictionless surface, don't you just?), you get a system which you can also assume is like an isolated system of particles--not subject to external forces, even though, on closer inspection that is not true.

It isn't because of the interaction between streams of air and the surfaces--the undersides--of the carts, in your frame of reference. This means there is s kind of, surface, a material one, that supports each cart with what we might call an elastic force -kx, equal and opposed to a force mg, which is then also an elastic force.

The claim that no external forces are acting is patently false, but kind of true if you assume it is, "locally". Then you get to repeat the experiments, tally up some kind of average and see if you and Isaac are in agreement.

 

[arfa brane]

And I don't know what does it, but people who like to think they know all about it, usually also try to tell you why you don't know.

"You don't know what the word elastic means. You don't because you don't know what its inverse, or converse means.
"What is the meaning of inelastic? How long do I need to spend understanding the meaning of surface, and collision, to reach your level?

What do you mean, a surface doesn't need to be solid? How can a surface be inelastic?
I think the answer to the last question there, is if it does not respond in any way to any kind of interaction, aka collision, with other matter.
It must not vibrate, there must be no way to deform it so a mark is left--no scratches or scuffs. What could this thing be physically? Maybe it's just an idea.

So, yeah, maybe that means all surfaces are elastic, so that means you get an elastic force -kx in Newtonian mechanics.

 

[arfa brane]

Obviously "you guys" didn't think all that critically about those physics experiments. Maybe you aren't the kind of dudes who can DIY an experiment with springs and observe a few things.

Like, to me it's obvious there's a weight in equilibrium--no net forces are acting--at the end of a vertically suspended spring. I have to accept that the spring is made of material with mass too. But I now know that this equilibrium in terms of forces is the same as it would be if I placed the weight on a 'solid' metal block, and k would have the same physical (elastic) units.

"No net forces acting" is something I can say is approximately true, because I'm hanging a spring from a stand in my backyard, there's a bit of breeze, the earth isn't 'inert' under my feet, etc. I need to ignore it all as not contributing significantly. Another almost true assumption.

So I must conclude that in terms of surfaces, the spring and the metal block 'generate' the same elastic force. Thanks, Isaac.

 

[DaveC426913]

<aside>
I am of two minds. On one hand my instinct is to engage in the discussion and analyze the scenarios Arfa puts forth.

On the other hand, my experience has been that there is too much unprocessed rage and violent name-calling to wade through to try engaging. Arfa's no fool - he must know that this is counter-productive to his ostensible goal of discussion. Why post? Is it like throwing grain on the ground to lure wild game so you can shoot them? Thats not engagement; that's predation. I'm mystified what I could have done to get on this shit list.

Anyway, I know I'm gonna catch a-lickin' for this, so I better git a move on...

 

[origin]

On one hand my instinct is to engage in the discussion and analyze the scenarios Arfa puts forth.

I'm not sure what his problem is, but I do know taking him off of ignore is a problem that is easily rectified...'click'.

 

[arfa brane]

Arfa's little problem is he is well aware of the challenges.
Challenging, by itself, is a challenge. People really don't like being challenged. especially when it means they have to think about whether they do have "the right ideas".

I think everyone has an innate sense of gravity, but struggles to understand what elastic means. Gravity is an elastic force? But it isn't a force, it's the curvature of spacetime, you mean spacetime is elastic?

Well, yeah, that is kinda what Al meant. You think about elastic in Newtonian terms, and it's hard not to.

 

[arfa brane]

This is embarrasing I have to say. One of the things I recall being told in my first year of tertiary study, and I accepted at the time it was pertinent advice, was that we undergraduates shouldn't just accept what we were being told; it was ok to question it.

Questioning what lecturers and textbooks tell you, is called learning. At some point the questions change--you don't need to spend time re-examining stuff you've gone over and are pretty sure about. You move on.

But maybe at some point you start discussing what you think you learned about whatever, and that turns into a root-canal exercise in brushing teeth.
Clearly, being able to question is less important than being convincing.

 

[Kristoffer]

Do you actually know how painful a rootcanal is?
How does this minor questioning compare to that?
When people ask you about your thoughts give an answer and not a snide remark.

Some of us might benefit from a less stuck up answer.
Or we might not.

 

[arfa brane]

FFS

 

[exchemist]

<aside>
I am of two minds. On one hand my instinct is to engage in the discussion and analyze the scenarios Arfa puts forth.

On the other hand, my experience has been that there is too much unprocessed rage and violent name-calling to wade through to try engaging. Arfa's no fool - he must know that this is counter-productive to his ostensible goal of discussion. Why post? Is it like throwing grain on the ground to lure wild game so you can shoot them? Thats not engagement; that's predation. I'm mystified what I could have done to get on this shit list.

Anyway, I know I'm gonna catch a-lickin' for this, so I better git a move on...

I now suspect he is elderly and slipping into a form of dementia. There were signs of it on the thread about photons and energy and it has now got worse - a lot worse. The terms are all there but popping out almost randomly, in a jumbled sort of stream of consciousness that makes no sense from the point of view of discussing science. I've decided not to try to engage him any more. It will antagonise him, frustrate me and nobody will be much the wiser as a result. So he's gone on Ignore.

We've a few in this sort of state: nebel, Quantum Quack, and, I suspect, Write4U, seem to me to be at various stages along the same journey. Arfa unfortunately tends to swear rather a lot. He may be as frustrated by his loss of grip on his thoughts as his readers. Anyway, that's my hypothesis.

 

[arfa brane]

No, you will need to explain when you last threw a pebble into an unfrozen pond and it rebounded off the surface elastically.

Where does a supposedly educated person get that from?

Why wouldn't that wind someone up? Why wouldn't that someone think they're dealing with a bit of an idiot, who seems to not understand what an elastic rebound is? Who comes across as never having been in a physics lab?

I now suspect he is elderly and slipping into a form of dementia.

Oddly, that's what I think about you and various others here who come across as all-knowing. Or just a bunch of know-alls.

But then you start off with things like, water isn't an elastic surface. It "has" an elastic surface, but that doesn't mean water is elastic.

But, that is what it means. That is exactly what it means you physics ignorami.
What winds me up is the thin veneer of knowledgeability that some here try to twist your arm into accepting is as good as it gets. Bollocks. You should be able to admit you don't know something, that maybe someone can tell you something you didn't know.

But by all means, keep telling yourself that if you can't bounce a pebble off it, it ain't elastic.
Dementia creeps up on you like that, I've heard.

And, since it seems there are people here, yourself included, who think they know of a material that doesn't have an elastic surface, I guess I have to accept those people are already demented--it's too late.

Given the lack of supporting evidence; the complete absence of a single example so far. That elephant hasn't left the building. I see the demented few are ignoring this.

 

[arfa brane]

Ok here's a curveball.

Liquids generally have a viscosity. Liquid He3 is not a classical fluid (yeah, sorry that 3 should be a superscript). I was informed there is no pressure at the bottom of a beaker full of it, and that's one reason it climbs up the sides, flowing upwards.

But what happens to surface tension, in that case? And if you had a big enough or tall enough container, what about the gravitational pressure at the bottom? In short, what happens to the elastic 'collisions' between molecules, and what happens to the elastic 'interaction' with the downwards g and total mass?

And what happens if we freeze it (even though we can't)?

Notice how, just one example opens up a whole lot of questions about friction--how does a liquid flow up the sides of a glass beaker? Is it like osmosis? Is it because of a slight temperature gradient? Is it something we only see with He3

 

[arfa brane]

To understand, you know too soon

You're going to need to look at the Lagrangian/Hamiltonian dynamics. You need a microscale equation; a positive is that degrees of freedom are constrained by the Bose-Einstein statistics. The beaker, however, is not so constrained.

Is there darkness at the break of noon?

You think it might mean adjusting your ideas of 'flow'; there must be a bulk flow that directs the fluid to stay in the beaker, the part climbing the sides (up the walls) sees a potential barrier.

The barrier is something to do with a gap between Maxwell-Boltzmann, and Bose-Einstein 'energies'.
I need to see if I can get back, to where I once belonged.
Maybe I do need some of that California grass.

Oh but I nearly forgot, a 'frozen' state would certainly indicate that the mass is now your usual inert mass, and subject to Newtonian physics (so it could be a pendulum weight, f.i.).
But it's a Fermi liquid with Newtonian mass (a 'bulk flow').