What is Energy?

[pmb]

Originally posted by jeff
pmb,
This thread was about answering the question, "what is energy?". I want to emphasize that my answer that "energy is what gravity couples to" is not my opinion, but is in fact the correct answer.

I disagree. I do not believe that is the correct answer. I think that is your opion of the correct answer. No physicist has ever figured out what energy *is*. We only know some properties of energy. How did you arrive at this conclusion that this is the correct aswer?


Please note: I asked you to explain what you mean by "Energy couples to gravity" etc. You didn't answer.

And please note that I did not claim that given a non-vanishing energy-momentum tensor that there is no gravitational field generated or that given a gravitational filed that it didn't effect particles both with and without rest mass.

Let me clarify - Consider the energy-momentum tensor for a beam of light. The components are a function of the E and B field in a given frame of referance. Since these components do not vanish there is a gravitational field generated.

Did you think that anything I said here implied otherwise?

If you know of a deeper way to understand energy in the context of modern physical theories of fundamental interactions, I'd like to here about it.

And you will. I wrote a first draft on an article on this. I sent it to a relativist that I know and a astrophysicist that I know. They thought that it was well though out and didn't contain any erroneous arguements etc. The general consensus was that its a good paper. But I've been waiting for Rindler to come back from a trip he is on. He told me that since this is subject that he's interested in that he wanted to discuss it.

In the mean time I've decided to make some major modifications since I've had some new insights. The paper should be done in a few weeks. However even if it gets published it will be a long time from now. I'll be sending it overseas. I want to get Max Jammer's input on this subject because he's leading authority on the concept of mass.

If/when it gets accepted for published then I'll put it on line for all to read. Otherwise I'd only e-mail it and then only after I modify it (which should be done by the weeks end).

So if you want to see it at that time I'd be happy to e-mail it. I'm always open to constructive criticism.

Pete

 

[pmb]

Originally posted by zoobyshoe
Dear pmb,

Thank you for taking the time to
compose an explanation.

Unfortunatly I got stuck on the
first paragraph. The equation looks simple enough, but as soon as I observe that for E to be an
integral of motion the solution
must be zero, I also realize the
solution will never be zero unless dt=zero.

What I'm saying is that the only
division problems I can concieve
of whose solutions are zero are
those in which something is divided by zero. That is as far
as my algebra goes.

As a result I was unable to follow
you into the rest of your explan-
ation.

-Zoob

My appologies again. It's difficult to describe oneself if nothing is know about the person who is doing the reading. Do you know what conservation of energy is? It means that the total energy of a closed system is constat - i.e. it doesn't change as time passes. The dE/dt = 0 means that dE = 0 since dt cannot eual zero. The dE means "change in"


Think of it in these terms. Let

K = Kinetic Energy
V = Potential Energy

E = Total Mechanical Energy = K + V

Think of a ball hanging from he ceiling by a spring. Now put the ball down a bit and let it go. It will start to oscillate up and down right? When you first let it go the speed it zero. Then it starts to accelerate upwards. The gravitational potential energy thus increases. The spring is less tense now so that the potential energy from the spring is decreasing. Eventually the ball will start to slow down, the gravitational energy will come to a maximum and the spring will become compressed and the ball will stop. All that is left is potential energy, since there is no kinetic energy left. However the sum E = K + V has never changed

K is a form of energy
V is a form of energy
E = energy


Pete

 

[zoobyshoe]

Pete,

That was actually very helpful.

I think I'm getting there. Now
I'm having a problem with the
change in time always having to
equal zero.

Any insights for me?

-Zoob

 

[pmb]

Originally posted by zoobyshoe
Pete,

That was actually very helpful.

I think I'm getting there. Now
I'm having a problem with the
change in time always having to
equal zero.

Any insights for me?

-Zoob

I'm trying to understand where the confusion lies. You wrote

The equation looks simple enough, but as soon as I observe that for to be an integral of motion the solution must be zero, I also realize the solution will never be zero unless dt=zero.

Look at it like this. Let E = E(t) = Energy as a function of time.

Constant in time means nothing more and nothing less the exactly the following.

Define E_o = E(t = 0) = E(0). Then

E(0) = E_o
E(1) = E_o
E(2) = E_o
E(3) = E_o
E(4) = E_o
E(5) = E_o
E(6) = E_o
E(7) = E_o
E(8) = E_o
E(9) = E_o
...
E(t) = E_o

The equation dE/dt = 0 means nothing more and nothing less the exactly the following.

dE/dt = [E(t+T) - E(t)]/dt as dt --> 0

But notice that
E(t) = 0
E(any time) = E(t+T) = 0

Plug this in above

dE/dt = [0 - 0]/dt = 0

Pete

 

[zoobyshoe]

Pete,

Thanks for all your effort.

I GOT it that time!

-Zoob

 

[wimms]

Originally posted by jeff
Energy is just the gravitational analogue of electric and color charge.
For example, energy is often defined in terms of motion or dynamics. But the motion of systems through spacetime are determined by their interaction with the gravitational field out of which spacetime is actually made: How can a system avoid violating conservation of energy-momentum as it moves (evolves) through spacetime without gravity telling it about the local spacetime geometry?

wow, so any energy is basically field of gravity?

 

[pmb]

Originally posted by Gale17
What is energy, really? i know what my teacher says and all, but does someone have a better definition, or a real comprehension?

See

www.geocities.com/physics_world/mech/what_is_energy.htm

Pete

 

[zoobyshoe]

Pete,

If you asked "What is money?"
and someone replied "Money is
bookkeeping." Would you find
that to be a satisfactory
answer?

-Zoob

 

[pmb]

Originally posted by zoobyshoe
Pete,

If you asked "What is money?"
and someone replied "Money is
bookkeeping." Would you find
that to be a satisfactory
answer?

-Zoob

Nope. That's not an answer I'd give at all. If you said "What is a bank account" then you'd be close.

If I said "energy is bookeeping" then I don't recall saying exactly that - but the bookeeping part is at the heart of what Energy is - it's a constant - if it decreases in one place then it increases in another place - it's a constant.

What is it you think energy is anyway? And did you read the article I posted by Feynman? Do you disagree with Feynman? Do you think I said something different than Feynman?

Pete

 

[zoobyshoe]

Pete,

I posed the question in response
to this line from the link you
provided for Gale:

"Therefore energy is meaningful only as a simple bookkeeping
device."

Despite the fact this statement
is correct Gale was trying to
find a meaningful definition or
description of that which is
being accounted for in this
bookkeeping.

Your statement: "...the bookeeping
is at the heart of what Energy
is-it's a constant..." wouldn't
have drawn that question from me

I did read the Feynman when you
posted it. I had actually read
"Six Easy Pieces" about a month
ago. I agreed with him when I
read it the first time and also
when I re-read it in your post.
I don't think you said anything
different than he did.

I think that what is essential
to forming a conception of energy
that comprises it's various forms
is relative motion, (including,
of course, potential relative
motion). I haven't been able to
think of a form of energy where
this wouldn't apply. This is an
effective concept for me.(And I'm
not married to it. If you see pro-
blems with it I'd be interested
to know what they are.)

Alain, I think it was, came up
with something she found useful,
so the thread has kind of outlived
it's purpose. I found it peculiar
that most were missing the point
of the kind and quality of the
information she was asking for.
-Zoob

 

[jeff]

Originally posted by pmb
explain what you mean by "Energy couples to gravity"

It just means that energy is a source for gravity, like electric charge is a souce for the electromagnetic field. For example, in the QED lagrangian there will be an interaction term of the form A^μJ_μ in which the gauge field A^μ is the electromagnetic potential representing the particle that mediates the interaction - the photon - and J^μ =eψ^†γ⁰γ^μψ is the source term, the electric current consisting of the incoming and outgoing electron field operators with the momenta of the associated states differing by that of the exchanged photons to which they couple. We refer to this interaction term as the electromagnetic coupling, with the coupling constant, the electric charge e, giving it's strength. Gravitational couplings are given as multiplication by √(-g) of the matter lagrangian together with the action of the covariant derivative compatible with the metric g on the various fields in it.

Originally posted by pmb
Feynman summarizes his comments about energy:

"It is important to realize that in physics today, we have no knowledge of what energy is."

I agree with this. In fact I feel that way about everything, not just energy. Consider my initial post to this thread (you can check that I haven't altered it):

Originally posted by jeff
Among the most profound results of GR is a fundamental definition of energy and momentum in terms of what gravity couples to, namely the stress-energy tensor T^μν, defined as the variation of the matter action S_M with respect to the metric g_μν (holding the coordinates fixed): T^μν(x) = -(2/√(-g))δS_M/δg_μν(x), with energy defined as E = P⁰ ≡ ∫d³x√(-g)T⁰⁰(x) and momentum as Pⁱ ≡ ∫d³x√(-g)T⁰ⁱ(x).

Notice that I've italicized "definition". I'm not posting what energy is. In other words, I'm not interested in it's ontological status, as is clear from another (unaltered) post of mine in this thread:

Originally posted by jeff
Now, I'm not going to debate the ontological status of energy with either of you. My point about gravity and energy is that whatever conventions with respect to energy one might use in treating a system cannot be used when it's coupling to gravity is taken into account.

Notice that I've italicized "conventions".

{However, I also posted

Originally posted by jeff
This thread was about answering the question, "what is energy?". I want to emphasize that my answer that "energy is what gravity couples to"...

which seems to be at variance with the above, but really, I only used "is" to match the "is" in the original question, which seemed appropriate because I was just reminding you what the origin of the thread was in case after all the discussion you'd forgotten, though I knew you probably hadn't.}

Originally posted by pmb
Since you can transform gravity away...

In general curved spacetimes the principle of equivalence - which is a property peculiar to the gravitational interaction - allows the effect of gravity to be transformed away at any single spacetime point by setting up an inertial reference frame at that point. However, such reference frames will not remain inertial beyond that point, which expresses the fact that one cannot truly transform the gravitational field away. After all, the manner in which the Earth goes about it's business of curving space by exchanging gravitons shouldn't be affected by one's choice of reference frame.

Originally posted by pmb
One of the main reasons I don't like what youv'e defined is that it doesn't tell me what energy is.

I gather from this that your bothered more than I am about the status quo referred to by Feynman about no one knowing what energy is. My view - which is the most widely held one among scientists - is that theoretical terms derive their meaning from the theories in which they're couched: they're "theory-laden".

From this perspective, the question is, with respect to which one of our theories should energy be defined? For example, with respect to which theory should electric charge be defined? One choice that makes a lot of sense is QED.

Similarly, GR is an awfully good choice for defining energy. Conservation of energy-momentum is simply an expression of the constraints placed on the evolution of physical systems by spacetime geometry, and this is achieved by the particular way gravity couples to all forms of energy as described by GR. GR is our theory of energy, just like QED is our theory of electric charge.

I don't see how you can improve upon the definition of energy derived from GR unless you've secretly invented a better theory with respect to which energy is somehow defined differently.

 

[Jonathan]

You all don't get it. Gale's question seems really more philisophical and needs an answer like that. I think you've all lost her because you are beatting around the bush. So, the fact is, no one knows what energy is. In Newtonian physics, energy is defined as being the ability to do work, and work is defined as the dissipation of energy. Does anyone else see the problem? Energy is defined by one of the greatest minds of science as being the ability to dissipate itself. That's like saying that since people can catch fish, they can be defined as being the ability to catch a fish. Maybe Gale's onto something, maybe energy = God.

 

[jeff]

Originally posted by Jonathan
You all don't get it. Gale's question seems really more philisophical and needs an answer like that. I think you've all lost her because you are beatting around the bush. So, the fact is, no one knows what energy is. In Newtonian physics, energy is defined as being the ability to do work, and work is defined as the dissipation of energy. Does anyone else see the problem? Energy is defined by one of the greatest minds of science as being the ability to dissipate itself. That's like saying that since people can catch fish, they can be defined as being the ability to catch a fish. Maybe Gale's onto something, maybe energy = God.

Oh. In that case, never mind. And now for a completely inappropriate smiley: I really love this little guy.

 

[Tyger]

After all of this I still believe that the proper view of Energy is just that it is proportional to the time rate of change of quantum mechanical phase. Anything that is proportional to the time rate of change of quantum mechanical phase is some kind of energy.

As far as rest energy, it just means that the phase of system is changing even when it is at rest.

 

[pmb]

Originally posted by jeff

In general curved spacetimes the principle of equivalence - which is a property peculiar to the gravitational interaction - allows the effect of gravity to be transformed away at any single spacetime point by setting up an inertial reference frame at that point. However, such reference frames will not remain inertial beyond that point, which expresses the fact that one cannot truly transform the gravitational field away.

You were right in this first part. You can always transform the gravitational field away. You just can't always transform tidal forces away.

Pete

 

[pmb]

Originally posted by Jonathan
In Newtonian physics, energy is defined as being the ability to do work, and work is defined as the dissipation of energy. Does anyone else see the problem? Energy is defined by one of the greatest minds of science as being the ability to dissipate itself. That's like saying that since people can catch fish, they can be defined as being the ability to catch a fish. Maybe Gale's onto something, maybe energy = God.

In Newtonian physics the so-called definition that you have is also unacceptable as a definition. And Newtonian never defined energy. In fact I don't think the concept evem existed in Newton's time

 

[Jonathan]

I have recently taken a physics class, and no matter what I said my dumb teacher would say that energy is defined as being the ability to do work. It should be noted though that I referred to this as Newtonian physics but I don't mean that it is what Newton thought exactly, but it is passed off that way today anyway.
What is Jeff talking about?: "oh. in that case nevermind...inappropriate smilie.."

 

[jeff]

Originally posted by pmb
You were right in this first part.

And the rest?

 

[pmb]

Originally posted by jeff
And the rest?

Regarding the rest I either had no comment on or had made my position clear. If I was unclear then I don't think I can explain it better than I have.

However regarding your so-called "definition" of energs "That which couples to gravity" - Problem with that, in the very least, is that when I asked for clarification on this 'coupling thing you answered that its the source, i.e. the energy-momentum tensor. But there is gravitational self energy which does not show up in that tensor.

Pete

 

[jeff]

Originally posted by pmb
...on this 'coupling thing...its the source, i.e. the energy-momentum tensor. But there is gravitational self energy which does not show up in that tensor.

The source is not the coupling, rather the gauge field couples to the source, i.e. the coupling is between the source and the gauge field. It's the term most commonly used in referring to interactions, especially fundamental ones.

I didn't mention the pure gravity sector of GR, represented by the lagrangian L_g = √(-g)R, because it doesn't contain the analog of the QED example I gave of the photon coupled to matter.

So pete, what precisely is your view of what energy "is"?

 

[JAL]

Originally posted by zoobyshoe
Think of it as a problem in inter
personal relativity. Each person
believes his perspective is the
pertinent one. The solution to
the problem at hand is dependent
on first figuring out where Gale
is and what's causing her pro-
blem in grasping the concept.
I sense people are impatient and
want to bulldoze over that crit-
ical part.

Agreed. Perspective is reality. In the end, the only way we have to understand anything is via our own concepts, our own paradigms. We each need to develop a way to generalize information so we can recognize patterns and form new concepts and paradigms using this generalization.

Some people (for whatever reason), chooses to generalize information pertaining to our physical world via the all encompassing God concept. That is fine but it doesn't provide a very sharp way to see and understand the world since every situation eventually boils down to that concept. In this regard, "Understanding" could I guess be defined as a sort of information categorization machanism through a set of self-consistent internal paradigm just like a theory is only a way to relate measurements between themselves through a set of self-consistent axioms.

I will leave you with a phylosophical question which pretty much explains my point of view in this regards: Assume we create a conscious program running on a computer that has the ability (like us) to learn and inspect it's "parts". The software would see machine instructions as it's basic constituent parts. What kind of concept and understanding do you think it could acquire/develop about this?

You see, without access to outside information, a introspecting system is limited to an internal representation of it's functionning. This representation may describe very well the functionning but it can never be sure about the actual implementation of it. Same for us, we can develop nice theories about the world but we can never "know" or "see" past this "epistemic cut". We can never know "What energy IS", we can only develop concepts about it.

It may very well be that we do have access to "outside information" but till then we'll have to bite the bullet and realize that there are limits to our knowledge and (in my humble opinion) it is best to accept those limits (without trying to push & expand them of course) than to make stuff up and say that "God did it".

 

[pmb]

Originally posted by jeff

So pete, what precisely is your view of what energy "is"?

As I said - I've already told you. I've even created an entire detailed web page on it and posted the link here - Nobody knows what energy is - that means I don't know either.

And you don't have to write another post to explain that you know what energy is and that the claim "Nobody knows" is wrong. You've already explained yourself. As I said - all of this has been said already - so at this point let's say we agree to disagree?

Pete

 

[jeff]

Originally posted by pmb
let's say we agree to disagree?

Agreed :



You disagree with my view that GR is the right theory with respect to which energy should be defined.


I disagree with your view that it's somehow not correct to say that "energy" is the source of gravity or that "energy" is what gravity couples to but it is correct to say that energy should be thought of as mass and this is what gravity couples to etc.

Edited out unnecessary personal comments [/color]
Integral[/color]

 

[pmb]

jeff - Please calm down. This attitude of yours is getting out of control. This insult you've just posted is not acceptable on this forum - this is a moderated forum.

This post has been directed to the attention of the moderator.

If you feel the need to carry on in this way then you can go to an unmoderated newsgroup such as sci.physics - I post there too. They don't mind it when people have this attitude

Pmb

 

[zoobyshoe]

Pete,

This question is for my edificat-
ion, not for Gale.

I went back and reread your arti-
and I'm having a great deal of
difficulty following your logic
in the paragraph about the par-
ticle and the spring.

The first thing I want to ask
is what is the signifigance of
speaking about a particle colliding with a spring? I would have expected someone to speak
of a marble or steel ball
because the image of compression
would be clearer. This makes me
wonder if you are using the word
"spring" in a special sence.

In addition it puts me on alert
that I may not be able to follow
you at all because I am aware
that particles are not tiny
balls of matter. They have their
own peculiar quantum behaviours.

I'll let you explain that before
continuing.
-Zoob

 

[pmb]

Originally posted by zoobyshoe
...
The first thing I want to ask
is what is the signifigance of
speaking about a particle colliding with a spring?

Perhaps I should elaborated on that. The so-called 'definition' that I was addressing was saying that "energy is the ability to do work" -= Kinetic enrergy is energy of motion. So how can something moving do work? "Work" is defined as "force" x "distance" . So I gave an example of how a moving body can apply a force over a distance - a body compressing a spring is doing work on the spring - perhaps a nice diagram will illustrate that.

I was thinking of something like a steel ball moving along with kinetic energy - it hits and compresses the spring. All the kinetic energy of the ball has gone - there is not potential energy is the spring.

Sorry if I was unclear. But I appreciate your mentioning it. I'll make the correction tomorrow - with a nice picture - I love making pictures .. :-)

Pete

 

[zoobyshoe]

Strangely, it was your very spe-
cificity (particle) that made it
so unclear to me. An oxymoron
Shakespeare would have enjoyed:

"Oh vague specificity!"

 

[pmb]

Originally posted by zoobyshoe
Strangely, it was your very spe-
cificity (particle) that made it
so unclear to me. An oxymoron
Shakespeare would have enjoyed:

"Oh vague specificity!"

Okay - I fixed it

Have a good night

Pete

 

[zoobyshoe]

Pete,

I referred back to your page after
your last post. The text still
refers to a moving "particle"
(emphasis on the word "particle")
colliding with a spring.

I still don't understand the
implications of your having
chosen this particular word,
instead of, for example, "steel
ball".

It makes a difference, in my
mind, because the particles that
might collide with a spring e.g.:
electron, photon, would almost
certainly not compress it,
unless, as I wondered before,
you are using the word "spring"
in some special sense I am not
aware of.

-Zoob

 

[pmb]

Originally posted by zoobyshoe
Pete,

I referred back to your page after
your last post. The text still
refers to a moving "particle"
(emphasis on the word "particle")
colliding with a spring.

I still don't understand the
implications of your having
chosen this particular word,
instead of, for example, "steel
ball".

It makes a difference, in my
mind, because the particles that
might collide with a spring e.g.:
electron, photon, would almost
certainly not compress it,
unless, as I wondered before,
you are using the word "spring"
in some special sense I am not
aware of.

-Zoob

I wrote it like that because its an important distinction in my paper. A point object has no internal stucture and thus the mass can be represented as m = gamma*m_o. However id the body has structure then the physics becomes more complex. Then the inertial mass is a tensor quaantity. I hoped to avoid touching on that in the page - especially until my paper reaches the Editor - and that will be a while.

I'm referring to the term "particle" as its used in classical mechanics. All the term "particle" means is that you're ignoring the internal stucture.

To see these complications consider what happens if the "object" is very mallable - like putty. Then the will be a deformation of the puddy when it hits the spring and I'd have to make the example equations more complicated by adding heat since I have to do work to distort the putty and that work goes into heat

Using "particle" gets rid of all these complications.


So keep in mind that when you're reading material on physics that the author does not mean sub atomic particles. He means "small" compared to the system and he can forget about the structure. You can think of the particle as a steel ball if you'd like.

Note that in classical celestial mechanics the Earth is treated as a particle! Yikes! :-)

Pete

 

[zoobyshoe]

Pete,
Excellent new information for
me in your last post. Thank you.

Before I continue, let me ask
if the terms "point object" and
"particle" are synonymous in the
arena of classical mechanics?

Zoob

 

[pmb]

Originally posted by zoobyshoe
Pete,
Excellent new information for
me in your last post. Thank you.

Before I continue, let me ask
if the terms "point object" and
"particle" are synonymous in the
arena of classical mechanics?

Zoob

I don't know. I don't think people make that level of distinction Zoob.

But thanks for the question. I think you've made a good point. Let me quote you a comment from a paper on relativity and objects etc. It's an important paper.

From "The Advantage of Teaching Relativity with Four-Vectors," Robert W. Brehme. Am. J. Phys. 36 (10), October 1968

To see how the concept of mass as a form of energy arises, we first define a structureless particle to be one for which the mass is constant during the existence of the particle. We next regard an object to be an agrgregate of these particles, moving within the boundary of the object.

However this sounds like something like a box containing an ideal gas, doesn't it? The author then defines the mass of the object as the magnitude of what's known as the 4-momentum. However if the boundary mentioned about if a rigid boundary then such a 4-vector isn't very meaningful in general.

However that is not the case. The author does not mean the bounday to be a rigid wall. What the boundary is is an imaginary surface which expands in such a manner as to contain all the particles. The particles are not confined at all and it's more like a gas expanding in empty space. That's what the author means by "object." So one has to be very careful with such terms.

Then the author speaks of the object as cooling which can't happen for this object. So this section is not very clear.

Pete

 

[zoobyshoe]

Pete,

Then let me limit my question:
Were you using the terms synony-
mously? (Your post 07-31-2003
06:28 A.M.) We were speaking
of the "particle" and you un-
expectedly brought in the term
"point object", and from the con-
text it seems to be interchangable
with "particle".

zoob

 

[pmb]

Originally posted by zoobyshoe
Pete,

Then let me limit my question:
Were you using the terms synony-
mously? (Your post 07-31-2003
06:28 A.M.) We were speaking
of the "particle" and you un-
expectedly brought in the term
"point object", and from the con-
text it seems to be interchangable
with "particle".

zoob

Yes. I used them to mean the same thing. But that's a very good question. The size of an object may not mean one can ignore it's internal structure. The weight of an object in a freaky-complex gravitational field may depend on the objects orientation in the field. And the size might not play a role. I tried some calculations/thought experiments on this. Turned out that if you have frame dragging effects in your frame of referance and you try to weigh an object which has particles inside moving faster in one direction than the other then the weight will depend on the orientation of the object in the field. So one has to be careful about using the term "passive gravitational mass" which is the mass you're weighing. I.e. it's the M in W = Mg or

"weight" = "mass" x "gravitational accleration"

But that's not really a practical concern - yet.

Pete

 

[zoobyshoe]

Pete,

I think, now, I grasp how you are
using the word "particle" well
enough to proceed to the sentence
in your paper that confused me
the most. This was:

"What part of the definition `ab-
ility to do work" tells us that
momentum is not kinetic energy?"

I don't understand why the def-
inition should be placed under
this obligation.

Given that the particle does have
kinetic energy, why are you requiring the definition to dis-
tinguish between its kinetic en-
ergy and its other qualities and
properties? It isn't clear to me
how it would change anything if
it did make the distinction.

Sorry it took so long for me to
get back to this thread. You are
really forcing me into some effort! Just needed a vacation.

-zoob

 

[pmb]

"What part of the definition `ab-
ility to do work" tells us that
momentum is not kinetic energy?"

I don't understand why the def-
inition should be placed under
this obligation.

Because a definition should uniquely identify a concept. From the definition "ability to do work" it doesn't tell us what it applies to. Does it mean energy or does it mean momentum - the definition does not allow you to determine which is which

Pete

 

[zoobyshoe]

Pete,

It seems to me that anyone with
a fair understanding of the mean-
ing of the word "momentum", and
of the term "kinetic energy" isn't
likely to confuse them, and won't
need "ability to do work" to
include a specific differentiation
between the two. They are not
similar enough to be used inter-
changably.

When we look at a moving object
in terms of it's momentum we are
not concerned with it's con-
commitant ability to do work.
Therefore "ability to do work"
is not required to distinguish
between the two to satisfy your
criteria for a definition - that
it should uniquly identify a con-
cept.

-zoob

 

[pmb]

Originally posted by zoobyshoe
Pete,

It seems to me that anyone with
a fair understanding of the mean-
ing of the word "momentum", and
of the term "kinetic energy" isn't
likely to confuse them, ..

Anyone with a fair understanding doesn't need a definition do they?

We're talking about a definition from which one can logically deduce various things. A poor definition would lead to ambigous results.

Tell me - Do you think "ability to do work" is suitable for a definition of energy?

pete

 

[zoobyshoe]

Pete,

I think that "ability to do work"
is sufficiently specific that
the ambiguity you're worried about
i.e.: someone wondering if it is
refering to momentum or kinetic
energy,is not a serious concern.

-zoob

 

[pmb]

Originally posted by zoobyshoe
Pete,

I think that "ability to do work"
is sufficiently specific that
the ambiguity you're worried about
i.e.: someone wondering if it is
refering to momentum or kinetic
energy,is not a serious concern.

-zoob

I think you missed my point - the point was that defining "energy" as "the ability to do work" is not meaningful enough to be a definition. What I mentioned regarding energy and momentum was an example to clarify that and was only mentioned as an example.

I doubt that you'd find a decent physics teacher that would say "the ability to do work" is a definition. Although many might write that for lack of something better. A survey through the more respectable physics texts will show that. E.g. "Newtonian Mechanics," A.P. French, Feynman lectures etc.


Pete

 

[bdkeenan00]

I have been corrected before in this forum so I'm not going try to explain anything due mostly to the fact that all I really know about physics is from the internet, but I just wanted to bring up the second law of thermodynamics. Even though energy may be just "book keeping" it is known that this quantity called energy tends to be go froming being concentrated to being spread out(as in the total disorder of a closed system increases over time) and this tells use that time has a physical existence and is not just made up by humans. So if I know what I'm talking about it would seem that this energy is related to the direction of time(macroscopically) and tends to spread out. I'm sure that someone else here could better explain what I'm saying or they could correct me if I'm wrong. or not

 

[pmb]

Originally posted by bdkeenan00
I have been corrected before in this forum so I'm not going try to explain anything due mostly to the fact that all I really know about physics is from the internet, but I just wanted to bring up the second law of thermodynamics. Even though energy may be just "book keeping" it is known that this quantity called energy tends to be go froming being concentrated to being spread out(as in the total disorder of a closed system increases over time) and this tells use that time has a physical existence and is not just made up by humans. So if I know what I'm talking about it would seem that this energy is related to the direction of time(macroscopically) and tends to spread out. I'm sure that someone else here could better explain what I'm saying or they could correct me if I'm wrong. or not

Its not energy that is doing that. The energy of the Earth going around the Sun is not being "spread out" at all. The notion you're really thinking of is entropy.

Pmb

 

[bdkeenan00]

Thank you for your reply. When I said the entropy increases I meant the entropy of the entire universe. So when you said that our solar system has a low entropy doesn't that mean that somewhere else there is even more disorder to cancel out those places where there is high order? If am wrong please correct me for my knowledge of Physics is limited. Thank you very much

 

[FZ+]

Umm... I don't really understand the question, but I'll harzard an response anyways.

If we have a local decrease in entropy (like say, in a fridge), then by the 1st law of thermodynamics, you must have an increase in entropy somewhere else. (like in the air at the back of the fridge) Furthermore, by the second law of thermodynamics, that increase in entropy must be larger that the first decrease in entropy.

But that applies to change in entropy.
Saying the solar system has low entropy means that by some scale, the solar system has less than "normal". In the abscence of some comparision, I am assume that something like "low relative to the average entropy of the universe" is meant. In this case, it's kinda obvious - if something exists that is less than the mean, then somewhere there must be stuff that are higher.

 

[bdkeenan00]

Thank you "FZ+" that is exactly what I wanted to hear. Sorry that my question was vague.