What is energy

  • Thread starter Gale
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  • #76
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
 
  • #77
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1,275
Strangely, it was your very spe-
cificity (particle) that made it
so unclear to me. An oxymoron
Shakespeare would have enjoyed:

"Oh vague specificity!"
 
Last edited:
  • #78
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
 
  • #79
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1,275
Pete,

I refered 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 sence I am not
aware of.

-Zoob
 
  • #80
pmb
Originally posted by zoobyshoe
Pete,

I refered 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 sence 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 refering 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
 
  • #81
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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
 
  • #82
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 existance 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
 
  • #83
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1,275
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
 
  • #84
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
 
  • #85
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1,275
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
 
  • #86
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
 
  • #87
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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
 
  • #88
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
 
  • #89
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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
 
  • #90
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
 
  • #91
49
0
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 existance 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
 
  • #92
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 existance 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
 
  • #93
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0
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
 
  • #94
FZ+
1,561
2
Umm... I don't really understand the question, but I'll harzard an response anyways. :wink:

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.
 
  • #95
49
0
Thank you "FZ+" that is exactly what I wanted to hear. Sorry that my question was vague.
 

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