Why Does Sound Propagate?

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Waves are a powerful addiction...maybe we need the wave "patch"?

3v0 said:
Air is a compressible. The compressibility and rebound from compression are important in understanding sound in air.

I agree with skyhawk regarding the piston and air sensor.
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I've already commented on the compressibility of air in a previous post and I should probably go find it but, I'm lazy and it's easier to just write a new review so:

If you've ever read anything about supersonic flight, you will have seen comments about air being an incompressible fluid at subsonic speeds and a compressible one at supersonic speeds. That would seem to fly in the face of logic for anybody that owns an air compressor.

I had theorized (and didn't get any complaints...in fact, there were no comments about it at all) that air (or any gas) can't compress when the disturbance is subsonic because the pressure is able to move away from the point of disturbance faster than the disturber is moving. Of course, that can only happen when the air is not constrained to a fixed volume, such as an air tank.

Regarding the importance of compressibility to understanding sound in air, tha'ts pretty much what this thread is all about.

3v0 said:
We have to look at how the sound/compression (they are the same thing) moves as fast as it does.
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Like the Slinky and Newton's Cradle toys, this is something we could nit-pick the definitions of ad nauseum, ad infinitum...and ultimately get nowhere. So, we need to come to some sort of concensus about them. We are obviously not very close to being there...yet...but, I have high hopes of winning you over.

3v0 said:
Perhaps if you stopped telling us how wrong we are about waves and tried to understand the application of wave theory to sound you would be closer to understanding
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I'm not saying that what you are saying about waves is wrong. Just that talking about waves is the wrong venue when the subject is sound propagation.

When somebody can come up with a reasonable, conceptual expalnation of how longitudinal (or traverse) waves can act to propel a distrubance from a subsonic distruber, near instantly to Mach 1 and propagate it at Mach 1, then I'll be happy to consider waves and wave analysis to be the answer.
 
Bouncy Bouncy

3v0 said:
Use the piston and pressure sensor as described in skyhawks post. You can then find out how long it takes the sound/pressure to travel.

I predict that you will see a pressure change at a time set by the speed of sound.
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I agree. But, that's not what the video shows. It's true that iff you set up the right equipment to sense the ball position and resulting pressure at the ball and then corrolate it to the pressure at the sensor....blah, blah, blah....

I say, "blah" because, that's an extremely awkward way to measure the sopeed of sound.

The video shows the "bounciness" of compressed air quite well and not much more.
 

I said nothing about dropping a ball. This has almost nothing to do with the video except that it involves air. It is about a piston in a cylinder creating a rarefaction discontinuity.

Yes, I know what I am measuring. I am measuring pressure. The sensor measures the initial pressure until the discontinuity arrives, after which it measures a lower pressure. The size of the pressure drop is determined by the speed at which the piston is withdrawn.

The interesting thing about this senario is that prior to the passage of the discontinuity the bulk velocity of the air is zero. After the passage the bulk motion of the air is toward the piston while the disturbance is propagating away from the piston. This follows from conservation of mass and momentum.

I'm still not sure by what you mean by instant by instant since it is non-standard terminology. I'm guessing that you mean the concept of a function of time. Yes, I am looking at pressure and bulk velocity of the air as a function of time. And yes, I know what I'm doing. I've been doing problems like this for more than 30 years. I've even served as adjunct faculty in two large universities where I taught these methods to chemical engineering majors. I currently make a comfortable living modelling fluid flow. How about you?
 
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We are supposed to take anything you say as gold. Yet when we put forth an idea all you hear is blah blah blah.

You also like to twist things around. The above may be a awkward way to measure sound but that was not the intent. The intent was to show the pressure wave (which you claim does not exist) travels at the speed of sound.

Air is compressible! Taking the bit about "incompressible fluid at subsonic speeds" out of context does not change that fact.

crashsite said:
(air) can't compress when the disturbance is subsonic because the pressure is able to move away from the point of disturbance faster than the disturber is moving.
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Without compression there would be no pressure. You notion of pressure traveling indicates compression.

crashsite said:
I have high hopes of winning you over.
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Perhaps it is better to view this as a search for truth. Wanting to be right is not always productive.

crashsite said:
I'm not saying that what you are saying about waves is wrong. Just that talking about waves is the wrong venue when the subject is sound propagation.
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Then you have changed your mind. I seem to recall that you were saying sound waves in air did not exist.
I think we were close to this not too far back. The fact that the molecules act on each other at a distance. How far each molecule has to travel to significantly effect its neighbor. How density reduces the force applied to each molecule offsets density. That sort of thing.

3v0
 

That's not quite true. What is said is that in modelling flows below about .3 Mach, air can be treated as though it had constant density for the purpose of calculating forces and moments on an aircraft. How do I know this? I worked at the Manned Spacecraft Center/Johnson Space Center from 1972 to 1977 during design work on the Space Shuttle. I was involved in simulation of the Space Shuttle flight dynamics. I know about the tables of aerodynamic coeficients used for calculating forces and monents. Below .3 Mach the coeficients were treated as independent of Mach number. Above .3 Mach, one had to interpolate with respect to Mach number.
 
Professor Peabody

skyhawk said:
I said nothing about dropping a ball. This has almost nothing to do with the video except that it involves air. It is about a piston in a cylinder creating a rarefaction discontinuity.
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That's my bad. I was continuing to mentally reference the video and was envisioning the ball as the piston. An assumption I shouldn't have made; proving once again that when you dissect the word, "assume", it makes an ass out of u and me.


I must disagree with your last sentence. I think it might be more accurate to note that the pressure drop, at the sensor, follows that at the sensor side of the piston but, delayed by the time it takes for the effect to reach the sensor. Pretty much the same scenario as the inward excursion of a speaker cone and the time it takes for the sound to reach our ears.

The speed of the piston only affects the rate at which the pressure drops, not the magnitude (barring leakage around the seals, of course).

Another interesting thing to note (also common to the piston and speaker) is that, as the excursion takes place, there's not only a pressure change that gets propagated, but there's also a doppler effect.

There's lots of interesting things to consider. They just don't relate to the mechanism by which the propagation of the pressure change (or sonic wavefront or sound or whatever you'd want to call it in this scenario) takes place.


Sounds reasonable and, while it doesn't exactly describe the process of sound propagation it does provide some information that probably needs to be considered when pondering sound propagation. It's something I'll have to think about more before being able to expound on it.

skyhawk said:
I'm still not sure by what you mean by instant by instant since it is non-standard terminology. I'm guessing that you mean the concept of a function of time.
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I had commented that, like math, textual concepts of physics are, at best, a poor substitute for the real thing. But, if "sound" is propagated by the collisions of molecules then, an "instant" would mean the time it takes for one molecule, having just been bumped, to bump its neighbor. That definition is further compounded by the fact that an "instant" could be anything from picoseconds to perhaps years (SWAG) in deep space.


None of those things.
 
Defensibility

3v0 said:
We are supposed to take anything you say as gold.
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That would be a mistake. The things I write are merely the way I think they are at the time I write them. I often have changed my mind after being challenged. But, so far, I haven't seen a challenge that makes me want to change my mind about sound propagation.

I will stoutly and stubbornly defend my postion, as I expect others to do. But, I reamin open to changing it and I'd have a lot of respect for someone who can give me a better explanation of sound propagation that I have now.

3v0 said:
You also like to twist things around. The above may be a awkward way to measure sound but that was not the intent. The intent was to show the pressure wave (which you claim does not exist) travels at the speed of sound.
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Sometimes I do.

3v0 said:
Without compression there would be no pressure. You notion of pressure traveling indicates compression.
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At the risk of opening yet another can of worms, I have just about concluded that, if there is a pressure (or, indeed any force in the universe), it's energy and energy is heat. So, you can only have pressure when you have heat and you can only feel a force when there is more heat on one side than on the other (whether that heat differential is due to greater molecular activity or more molecules acting...or a combination).

That may seem overly simplistic but, maybe it's not.

3v0 said:
Then you have changed your mind. I seem to recall that you were saying sound waves in air did not exist.
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That's not true. I said that waves are merely an artifact of the sound propagation process. To "construct" the waves, you need to integrate the "instant-by-instant" events of sound propagation.

There! You forced me to say, "instant-by-instant" again!!!
 
Now you are talking high school freshman physical science and you have it wrong.

There are many forms of energy. In the Newtonian world they are either potential or kinetic. Heat or thermal energy is a form of kinetic energy. None of the potential energy forms are heat. Potential can be converted to heat. But that does not make potential energy heat or the same as heat.

You can raise the pressure in a sealed vessel by heating it.

You can also raise the pressure by cramming more molecules into it. You may generate some waste heat in the process but it is the mechanical energy which raises the pressure, the heat will flow away. The pressure remains.

The energy stored in compressed air is potential energy.

3v0
 
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Taking some heat

3v0 said:
Now you are talking high school freshman physical science and you have it wrong.
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I'm not completely convinced of that.

The physical sciences, as taught in schools, are designed to turn people into engineers not philosophers. That's why they are so heavy in math.

If I had better math skills I most likely would have been one of the flock of electrical engineers (baaaa). But, I don't have that knack so, I must at least tend toward the philosophical side of things. But, I don't consider myself a philosopher (at least not in the classical sense of pondering if a molecule may move simply because it wants to). But, I guess I am enough of a philosopher to wonder how things work in spite of how it's taught that they work.

An excellent example of that involves, sound propagation. I really do feel like I'm at least on the right track with this.


You are right if you say that is the way it's taught in high school physical science classes (and, by extension, college classes). But, I suggest that there is no such thing as "potential energy".

You can sigh and roll your eyes all you want....

The instant I realized that sound is propelled by heat, a whole bunch of things suddenly clicked into place. I only regret that it didn't happen when I was much younger. I have wasted a lifetime thinking about these things all wrong!

There's no potential energy. That compressed spring. That brick on the upper shelf. That drawn bow with a nocked arrow. They are all in as vigorous motion as when the spring is rebounding and the brick is falling and the arrow is being launched. The motion is simply at a scale that we can't see it so, we teach high school students that the motion is not there....it's just potentially there.

The energy is there in moving molecules. Those molecules are bouncing off each other in an essentially Newtonian way (action and reaction). What's more, they are doing it as vigorously when something is at rest as they are when it is in motion (maybe, even more so). I had started another thread awhile back linking the propagation of sound to heat and included some of these concepts. I even expalined how an airplane flies by heat. I don't know where that thread went but, it went off on a zany tangent, anyway (which is too bad since it had some good ideas, I thought).


Yep, that's definitely high school science.

But, you could also say that you can raise the pressure in a sealed vessel by making the molecules move more vigorously by adding heat to them instead of just noting that you are adding heat.

You could also say that when you add more molecules, and each molecule has some thermal energy, you are, in fact, adding more heat...even if you can't measure it directly with a thermometer.

Think of how the addition, subtraction and moving around of heat relates to the conservation of motion and energy.

I had broached a related subject a short while ago in this thread regarding EM waves. There is a linkage. One of the ways heat moves is radiently (both out of and into matter). If fact, radient heat also is a form of matter (think, e=mc^2)...or matter is a form of energy...however you want to think of it.

But, all this is getting away from the thread topic of sound propagation. Related but, not directly in line with.

It has to be noted that this post is a very big-picture view and is thus vulnerable to a lot of very valid nit-picking. But, I'm thinking that, as a core system, it may not be too bad.
 
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I do not see it. The brick is sitting on the shelf. Where is the heat that propels it downward.

We can look at how it got there. We used physical force to counteract gravity. F=MA.

On the way back down we get the force back by the same equation.

The brick is not warmed by lifting or falling execpt for heating due to the air. If we move it up real slow there is almost none. It does not count.

Whats up? If I move an object up do you think it gets warmer so it has the energy to come back down??

3v0
 
If there is no potential energy in a brick on a shelf and you drop the brick, where do you suppose the energy came from such that you don't violate the law of conservation of energy?

Heat?

Prove it.
 
You have to love this line of logic !

3iMaJ said:
If there is no potential energy in a brick on a shelf and you drop the brick, where do you suppose the energy came from such that you don't violate the law of conservation of energy?

Heat?

Prove it.
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We can have some fun with this. If heat in any object increases as it rises about the earth then the further away from the earth an object travels the hotter it will get.

If that were true how far would one have to travel up prior to burning.

Oh but maybe one would simply loose heat and that keeps us from burning up.

But if we lost the heat how can we get back down. Hey.... we just invented anti gravity !

I LOVE IT !

3v0
 
Thick as a Brick

3v0 said:
We can have some fun with this. If heat in any object increases as it rises about the earth then the further away from the earth an object travels the hotter it will get.
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I would normally try to quash something like this as being off the topic of sound propagation but, it gets right down to the underlying principle of it so, let's have at it.

I haven't come to grips with gravity (yet). It may be the one exception to the rule. The one thing in the universe that can apply a force without the use of heat. at this point, I can't say. But, also at this point, and as a starting point, let's say that's true. Gravity is simply an attractive force that doesn't use heat.

But, gravity must be acknowledged so let me take a quick, cursory stab at it. Gravity is not a pervasive, universal force. It is a force that is directly corrolated with mass. The more mass the more gravity. That makes gravity a local phenomenum. I don't know for sure but, I suspect that all the gravitational forces of all the masses in the universe cancel to zero, leaving only the local effects to be dealt with. Of cours, a "local effect" can be an entire galaxy.

Of course, we Earthlings are most familiar with the 1 g of gravity here. When we put a brick on the top shelf and say it has potential energy because we've moved it upward against some gravity and then if we drop it, the gravity pulls it back down (toward the center of the Earth). But, how does that relate to the rest of the universe. For example, we know that not only does the brick fall but the Earth rises up. How does that falling brick affect a brick knocked off a shelf on planet, Iciouerlkus, in the Andromeda Galaxy, by a careless Zinkulan...and how does his brick affect ours? Indeed, how do you know the Zinkulan is a "he" since neither the male or female has sex organs?

The upshot of all that is that I don't know how gravity fits into the energy or heat picture.

We had a discussion about the role of gravity in buoyancy but, did not get into the heat issue of it. As a quick review, the question was:

If you have a helium filled balloon in the weightless environment of the ISS, what direction will it float to? What about if it's a brick?

In a gravitational field and in air, the balloon will float upward. The reason given is that there is more air pressure pushing the balloon up than is pushing it down. But, what if the "pressure" is actually, heat. What if the reason the balloon rises is because there is more or more vigorous molecular movement pushing on the bottom of the balloon (Newtonian action and reaction)?

As the medium becomes more dense, the balloon becomes even more buoyant. There are either heavier molecules in the medium or more of them.

A brick, is acted on by the same forces as the balloon but, the brick is too small for it's mass and the heat in the air is insufficient to support it. The brick falls. But, even as the brick is falling, it interacts with more air molecules (and thus more heat) per unit time. Eventually, the brick will achieve a speed where there is an equilibrium between the air and the brick (it reaches terminal velocity).

If we change the medium by making it dense enough or by changing the material (changing the air to a pool of mercury, for example), the brick then becomes quite buoyant.

This is all very cursory so, I expect some "clarification" (read that, "flak").

About the comment of something getting hotter as you lift it in opposition to a gravitational field....We know, from experiment, that it doesn't. I've never thought about it so, at this point can't comment.
 
It goes like this....

You have denied the existence of potential energy.

You have stated the only energy is heat.

Thus by your school of physics when we lift a brick into the air, the energy we add to the brick is heat (no other kind). The brick gets hotter as it is pushed up. It has to get hotter in that we are not adding molecules to the brick, but we are adding heat.

As strange as this is the logic is soundly based on your understanding of physics.

EDIT: And it is all a bunch of hot air.

To change the existing views/laws that are physics you need to do better then declare them null and void. Only religion is allowed to ask us to "take it on faith". You refuse to take physics on faith but expect us to do so for your take on it.

You either need to vigorously prove your declarations (what is and is not) or accept the thinking of the scientific community. The accepted version of physics has been challenged many times to arrive at the refined version accepted by scientists.

If someone were to take the time to teach you science and math it would be possible to prove the scientific principals you will not take on faith. But you seem to lack basic training in the scientific method and most of what goes with it. The problem is not with science but your limited scientific education.

Sort of like asking a 4 year old to explain the engine in his dads car. It has an amusing aspect.

Thick as a brick, Indeed !

3v0
 
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Regroup

Okay, I rather deliberately went out toward the fringes to get the general thinking a little more outside the box. Now, let me come back and try to make some statements that I think we can all agree with.

  • Any object that is warmer than absolute zero has some thermal energy.

  • Even at a fairly low value, such as "room temperature", the object has a substantial amount of thermal energy. An easy way to get a feel for the relationship between the nominal thermal energy an object has just being in the room and the energy it gains from work being done on it is to stretch a rubber band. Measure the temperature of the band before and after stretching it. The difference is quite small.

    Actually, a fun little domonstration of the refrigeration principle can be done with a rubber band. It should be a wider one. Stretch the band out tight and hold it. Immediately place the band on your skin, just below your lower lip (an area very sensitive to temperature). You can feel that the band is slightly warm. Then, hold the band stretched for several seconds, in the air, for the temperature to normalize back toward room temperature. Quickly release the tension on the band and again put it against your skin. It will feel cooler.

    Of course, that experiment doesn't work if you have one of those, Frank Zappa patches on your face.

  • That experiment not only shows the refrigeration principle, it also shows how little effect the work done on the rubber band has relative to the total thermal energy in the band. That second part is the one to consider as to how it relates to sound propagation.

  • Almost all the heat energy in the medium that propels the sound is already there. The sound itself adds or subtracts only a small amount.

  • Thermal energy manifests itself (in matter) by making the atoms/molecules of the matter move. There is a direct relationship between the amount of thermal energy and the speed of the molecules. However, the speed the molecules move depends both on the amount of heat and the nature of the matter. It depends on whether it's a solid, liquid, gas or plasma. It also depends on the mass and arrangements of the molecules in the matter.

  • I think we can also agree that sound propagation gets more complex as one considers how it works in different materials so, it makes sense to limit the initial discussion to air only.
Does any of this give anyone heartburn?
 
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Sinning

Emphasis mine:


Ah, yes. The, scientific method. I have come up with an hypothesis to prove. But, I have commited the unpardonable sin of trying to prove the concept of it by logically showing how it works rather than by the method preferred by the scientific community of doing it with rigorous mathematics (hehe, I just love that term...makes me envision a 1920s Russian poster exhorting the proletariat to work).

Do you believe the guy(s) who wrote the article on sound and sound propagation, in Wiki, used the, scientific method when coming up with the explanations of how it all works?

Did he get into the heat aspects of it like I did? Did he come up with a reason why the speed of sound can remain constant even as density changes like I did? Did he postulate how longitudinal or traverse waves can propel sound through a medium at Mach 1 like I believe he can't? In fact, did he do anything more than parrot back a cursory verrsion of what he'd learned in his own physics education, followed by a spate of memorized equations? I mean, seriously...am I the only person in the world to wholm this is obvious?

I don't doubt that the scientific community has the knowledge of all the pieces needed to understand the concepts of sound propagation but, they are so hung up on waves that they can't seem to mentally get past them to apply the scientific method to get at the root concepts. At least I don't see much questioning of the Wiki version of it and that's been around for several years now. So, either the Wiki stuff is accurate and I just don't get it or the other way around.

So, anyway, until someone can come up with a good explanation for how longitudinal waves can explain the Mach 1 speed (I keep asking and noone even attempts it), I have to continue to believe that's not the answer.

Likewise, I don't expect anybody to accept what I say just because I say it so, I need to be espeicially diligent in going back to the most basic principles to build up my case. But, even when I'm back at molecules, everybodly still just wants to discuss waves.

BTW: I am also writing a treatise on the life of Moses. But, it's not going to be well received since there, as here, I am committing the unpardonable sin of following the wording of the King James Bible (and a little common sense) rather than parroting back the time-honored, hyped up version preached from church pulpits and revival tents.
 
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A Refrigerator Analogy


I've been thinking about your "heated brinck". You know, that is not a trivial question. A brick that has been pushed up to a higher altitude against gravity does have more energy than one that hasn't. The conventional high school physics explanation is that the brick has more "potential energy" and to write some equations around it an pronounce the problem "solved".

But, that brick does have more energy and that energy needs to be accounted for. Maybe some clues lie in a familiar analogy? I had mentioned a demonstration of the refrigeration principle using a rubber band. But, to be practical, a refrigerator needs to have a more continuous cycle.

Energy is added to the refrigerant (usually with a compressor but, sometimes with a flame). That pressurizes the refrigerant. The refrigerant is then run through a heat exchanger to get rid of some of the heat of compression.

But, even if the refrigerant is allowed to cool back to ambient temperature, it still has more energy in it than if it were not compressed. How do we account for that? Logically, we are pretty much forced to conclude that the refrigerant can have energy stored in it in some manner that we can't measure with a thermometer.

It's easy to say, "ah, ha! See! it's not heat!". Okay, so just what is that energy then?

So, we look to the molecules and say that the energy is in the pressure of having the molecules squeezed closer together and therefore are interacting with each other more forefully. Does that mean that they are bumping into each other harder...or does it mean that there are simply more bumping interactions going on because there are more molecules?

Lest I make too many assumptions, perhaps there's yet a completely different, unrelated-to-molecules explanation for the pressure of the refrigerant? I can't think of what it might be but, I'm open to other explanations for the pressure.

If we equate pressurizing a refrigerant to lifting a brick, we are confronted with similar questions. The brick on the top shelf has more energy than the one on the bottom shelf and, it took some energy to put it there. Anyone who has ever stacked bricks knows that it takes energy and that heat is involved (sweat is a clue).

But, just as the refrigerant absorbed heat from the compressor, does the brick absorb heat from being lifted? After all, we are adding energy to it and even if the brick does heat up, after it sits of the shelf for awhile, it cools back to ambient temperature again and, like the refrigerant, still has the extra energy.

Are we essentially in a situation like that of the rubber band where the amount of heat added is very small compared to the heat energy in the brick in the first place and so, we simply ignore it? Or is there some other physics involved?

In a refrigerator, the refrigerant is then routed through an expansion valve and and into another heat exchanger. As the refrigerant expands, it loses its pressure (energy) which is replaced by the heat energy in the immediate environment (the refrigerant absorbs heat from the environment).

When a brick falls (or even if it's just picked up and placed on a lower shelf), does it have a natural tendency to cool?

The thing I'm especially interested in is the property of having the extra energy without being warmer (as being measurable with a thermometer).

I'm sure I'll be told that this is all "well understood"...so, how about a couple of simple to understand answers?
 
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What is the actual question that you're trying to examine? There is a lot of rambling with out a real point made. Potential energy is a fine description of the energy contained in a brick on a book shelf. Certainly it has heat energy, but heat energy is not "ordered energy" IE you cant use the heat energy to do work. However, I can use the potential energy of the brick through a pulley system to lift another object off the ground, but good luck using the heat in the brick to do the same.
 
Where's the energy?

3iMaJ said:
What is the actual question that you're trying to examine?
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That's a good question. I don't really know the answer.

3iMaJ said:
There is a lot of rambling with out a real point made.
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I guess the closest I can say is that, user 3v0 asked about an object getting hotter as it's lifted up against gravity and I don't know the answer.

3iMaJ said:
Potential energy is a fine description of the energy contained in a brick on a book shelf.
Click to expand...

If you have three bricks, one on each level of shelving, the uppermost one has the most potential energy. If the three bricks, themselves, are examined, is that energy difference apparent in any measurable way? Does the energy only manifest itself by the way it acts when it moves (in other words, is it the environment that interacts with the brick as it falls or is lowered that makes the brick seem to have had more potential energy?


Unfortunately, perhaps, there's no shortage of nit picking that can be done with something like this. Can the heat in the brick convert some water to steam and if the steam does some work, is that the same as connecting the brick to the pulley? But, even there, does the upper shelf brick have the capacity to make the water hotter? Directly from its inherent heat or only if you sum it's heat and kinetic motion as it's lowered?
 
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