# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: The Champ on 30/08/2010 16:23:08

Title: What are "energy" and "work" ?
Post by: The Champ on 30/08/2010 16:23:08
I can't get the actual concept or meaning of various forms of energies . Actually what is energy ? My textbook defines energy as capacity to do work.Most of the websites define energy in the same way. But i can't understand any thing from that definition. I also want to know what is work.
Actually, why do we need these quantities?

[MOD EDIT - PLEASE PHRASE YOUR POST TITLES AS QUESTIONS. THANKS. CHRIS]
Title: Re: What are "energy" and "work" ?
Post by: Geezer on 30/08/2010 18:16:46
Energy "can" do work, but it can also be dissipated and do nothing useful.

For example, a given mass of fuel can be converted into thermal energy (heat) by burning it, but it won't do any work. However, if you burn it in an engine you can get it to do some work, although a significant amount of waste heat will also be produced in the process.

Work and energy are expressed in the same units, so you might prefer to think of work as mechanical energy or useful energy.
Title: Re: What are "energy" and "work" ?
Post by: yor_on on 07/09/2010 19:53:44
The simple definition of work is work = force x distance, like you hanging up a painting, depending on the distance and weight you will exert more or less force to do it, and when done you have indeed 'done work'. But assume that this is a painting heavier that what you can move. You try and try but it won't budge at all. Well, no matter how much force you have expended in trying there will be no 'work done' from that exertion :).  So it's not the answer to what energy is, more a generalization describing how energy transforms into something else, and as all of this craves what we call 'energy' it have a direct coupling to that.

As for what exactly energy is? I don't know, as far as I can guess it just might be the kaaa, or kiii, or whatever primordial essence there might be empowering and powering SpaceTime. We can't isolate, and lift some shimmering essence saying 'this is it' it but we can see its transformations.
Title: Re: What are "energy" and "work" ?
Post by: simplified on 08/09/2010 16:56:07
I can't get the actual concept or meaning of various forms of energies . Actually what is energy ? My textbook defines energy as capacity to do work.Most of the websites define energy in the same way. But i can't understand any thing from that definition. I also want to know what is work.
Actually, why do we need these quantities?

Energy is ability to overcome resistance. Work is a victory of quantity of resistance.
Title: What are "energy" and "work" ?
Post by: yor_on on 08/09/2010 22:15:52
Heh :)

Energy is reverse engineered inertia.
Yep ..

Ahem again.
Title: What are "energy" and "work" ?
Post by: Geezer on 09/09/2010 01:54:06
It takes a bit of energy to do some work.

I like to conserve my energy, so I try to avoid work as much as possible.
Title: What are "energy" and "work" ?
Post by: simplified on 13/09/2010 15:05:53
Heh :)

Energy is reverse engineered inertia.
Yep ..

Ahem again.
Title: What are "energy" and "work" ?
Post by: Geezer on 13/09/2010 21:20:33
Heh :)

Energy is reverse engineered inertia.
Yep ..

Ahem again.
Title: What are "energy" and "work" ?
Post by: Bored chemist on 25/09/2010 15:25:04
In believe in an all-encompassing universal medium that fills the entire space.

Therefore I would define ‘work’ as magnitude of distortions in universal medium about a matter body and define ‘energy’ as the stress produced in universal medium due to work, existing in any region.

You might believe that and define it so but I doubt anyone else would. Such ideas should be posted in the "New Theories" section of the site.
Title: What are "energy" and "work" ?
Post by: Melvin900 on 27/09/2010 13:13:39
Energy is the capacity of a system to do work. That system may be a jet, carrying hundreds of passengers across the ocean and Work is the application of a force over a distance. Work is equal to the product of the force and the distance through which it produces movement.
Title: What are "energy" and "work" ?
Post by: Farsight on 27/09/2010 13:40:54
I can't get the actual concept or meaning of various forms of energies . Actually what is energy? My textbook defines energy as capacity to do work.
That definition describes what it does, not what it is. As for what it really is, at the fundamental level: energy is a volume of stressed space.

That might sound unusual, but think about a steel spring. Steel is typically an alloy of iron and carbon, and the energy stored in a compressed steel spring isn't in the iron atoms or the carbon atoms. It's in the bonds between them, in the space between them. It's stored in what we call the electromagnetic field, but it is in the space. For nuclear energy the bonds involve the strong force, but it's the same principle. It always ends up like this, and this is why Einstein talked about stress-energy when he was talking about space and gravitational fields.
Title: What are "energy" and "work" ?
Post by: simplified on 29/09/2010 13:32:45
Voltage of an electric current shows energy of  electric photons. Force of an electric current shows quantity(amount) of electric photons.
Title: What are "energy" and "work" ?
Post by: yor_on on 29/09/2010 18:59:41
Simple question.

Assume that you walk across a room, from one wall to the other.

Was that 'work'?

Ok, as you walked you had your monocle balanced over your nose, to see that wall approaching so much clearer.

Was that work?
==

Tell me what you think :)
==

And lastly.

A book falls off the table.

Was that work?
Title: What are "energy" and "work" ?
Post by: Geezer on 29/09/2010 21:33:02
Yes, yes, and maybe.
Title: What are "energy" and "work" ?
Post by: JP on 30/09/2010 05:53:32
Yes, yes, and maybe.

Just to be different: maybe, maybe, maybe.   [:P]

I sense a tangent coming about biological work vs. physics work, air resistance, friction and the importance of specifying work done by what force on what object.  Not that I would complicate the thread by suggesting such a tangent...
Title: What are "energy" and "work" ?
Post by: Geezer on 30/09/2010 05:58:38
Yes, yes, and maybe.

Just to be different: maybe, maybe, maybe.   [:P]

I sense a tangent coming about biological work vs. physics work, air resistance, friction and the importance of specifying work done by what force on what object.  Not that I would complicate the thread by suggesting such a tangent...

Not to be pedantic or anything, but you left out various acoustic waves, accelerated dust particles and probably a few more things I can't think of right now.
Title: What are "energy" and "work" ?
Post by: JP on 30/09/2010 08:15:51
None of it matters anyway, since the uncertainty principle means we can't know for sure if work's been done!  :p
(No, I'm not being serious.)

But to show its tricky, in the first two cases, the kinetic energy of yourself (and the monocle) hasn't changed, but you've clearly expended energy in walking.  (I'm assuming that you were thinking of the case where you were already walking.)

In the last case, the book's kinetic energy has changed, so I think its clearer to see that work was done on the book (by gravity).
Title: What are "energy" and "work" ?
Post by: Geezer on 30/09/2010 08:29:10
I agree with the first two. The geezer sort of did something useful, so we might say that work was done.

The third one is a bit more tricky. Potential energy was lost, but it was also converted into kinetic energy, and that was dissipated in a less than useful manner, so, it's not clear that work was actually done.

Also, some may argue that gravity ain't really a force at all, so if there was no force etc., etc.

I suspect "work" is a slightly outmoded concept. That's why I prefer something like "mechanical energy".

Title: What are "energy" and "work" ?
Post by: JP on 30/09/2010 08:42:43
Yeah--usually physics textbooks phrase the question along the lines of "was net work done on ______?"  In that case, there was no net work done on the person or the monocle, since their kinetic energy remained constant.  If you phrase it as "was any work done on ____?" then it's yes, especially since walking isn't terribly efficient and requires constant pushing.
Title: What are "energy" and "work" ?
Post by: yor_on on 30/09/2010 17:53:09
Ahh, from my view, yea, no, and ?
Work uses the definition of displacement I found out.
In the first case you can easily see the 'displacement'. In the second there are no displacement to be seen. In the third there is a displacement as the book travels a certain amount in the direction of the 'force' of gravity..

So the first was simple, all 'forces' pointing to the same direction.

The second becomes a little tricky, as we could imagine the guy also putting on his monocle f.ex. But according to what I understand the 'force' have to be in the direction of the overall displacement to be considered a such.

An easier example would be a waiter lifting a tray above his head, to then go to serve. As we are discussing the tray here then it would have no displacement as the force upon it was directed in a right angle to the general 'displacement'. This one you really need to think a little about to see what it means.

We are discussing two forces here, the one lifting the tray at a angle to the one making the overall displacement. Ignore the lifting btw.. Just consider the force applied, by holding the tray above his head. Then it becomes a little more understandable.

The third one is like JP said, Yes.. but it hurts my head thinking of gravity as a 'force' as the book expends no energy moving, and neither it seems does the Earth?

To me its just geodesics and the way matter adapt to them..

==
In a way everything seems to come back to 'displacements', like the difference between a 'speed' and a velocity. When you bounce a light-corn between two mirrors it will have its speed at 'c' but its velocity will be limited to the distance between the mirrors. And it's displacement will depend on if you count it 'odd or even' when counting the bounces. If there only is two bounces you will find its displacement to be zero. With three bounces the displacement will be the same as its velocity, all as I understands it.
Title: What are "energy" and "work" ?
Post by: yor_on on 30/09/2010 19:46:42
So to understand the definition of 'work' we need to understand the geometry of the 'forces' applied on a object. Take a dog on a leash, the force can either be seen as diagonal following the leash, or as two forces (leashes if you like), one going | vertically the other going -- horizontally, like this |_ . How would the dog be able to differ those two situations, no peeping though :)?

"If the work done by the waiter on the tray were to be calculated, then the results would be 0. Regardless of the magnitude of the force and displacement, F*d*cosine 90 degrees is 0 (since the cosine of 90 degrees is 0). A vertical force can never cause a horizontal displacement; thus, a vertical force does not do work on a horizontally displaced object!!"

So 'work' seems very much a question about geometries if this is correct.

Title: What are "energy" and "work" ?
Post by: JP on 01/10/2010 08:35:42
Work is useful because its a way of keeping track of kinetic energy and potential energy in a system where very little is lost due to friction, air resistance, etc. and you're not charging up any fields over time.  In that system, the only way you can add/subtract energy from an object is as a result of moving it.    A force that only acts perpendicular to the motion of an object won't move it.  It will only change its direction of motion.  Therefore, it can't add kinetic or potential energy to it, so it doesn't do work.
Title: What are "energy" and "work" ?
Post by: JP on 01/10/2010 08:36:50
The third one is like JP said, Yes.. but it hurts my head thinking of gravity as a 'force' as the book expends no energy moving, and neither it seems does the Earth?

To me its just geodesics and the way matter adapt to them..

In the Newtonian view of things, the book spends its gravitational potential energy to fall to the ground the amount of potential energy it loses is equal to mass (m) times gravitational acceleration (g) times height of the book above the ground (h), E=mgh, and this gets converted into kinetic energy just before impact.  When the book hits the ground, the energy is dissipated into the earth and as sound, heat, etc.

The total work done on the book right before it hits the ground is the force acting it times the height it fell.  The force on the book is just its weight, which is given by mg.  So the total work is Force time height or mgh, which is the same as the kinetic energy right before it hits the ground.
Title: What are "energy" and "work" ?
Post by: Geezer on 01/10/2010 08:54:32

The third one is like JP said, Yes.. but it hurts my head thinking of gravity as a 'force' as the book expends no energy moving, and neither it seems does the Earth?

I already have a sore head, so how much worse can it get?

When the book falls, the capacity to do work of the Earth/book system has been reduced. Not only that, but some of the energy that previously existed in the system, went down the entropy drain.
Title: What are "energy" and "work" ?
Post by: yor_on on 01/10/2010 14:00:18
Yes Sires, me think we gotten close here to the mystery of the spheres.
So, as our esteemed colleague points out, that's one point more for the entropy dragon. Now me friends, is that correct?

That with no energy expended by any of the opponents, according to the geodesic definition, we get an 'energy released' that also transforms into 'work done' and thereby take us forever closer to that entropic conformity?

As my master once said. "What's a clap with one hand'?
Well, I can tell you :)

Just ask and I'll show you..
Title: What are "energy" and "work" ?
Post by: lightarrow on 01/10/2010 16:19:07
Energy is ability to overcome resistance. Work is a victory of quantity of resistance.
Define "ability", define "overcome" and define "resistance".
Title: What are "energy" and "work" ?
Post by: lightarrow on 01/10/2010 16:26:27
Energy is the capacity of a system to do work.
This kind of definition is often found in books, but it doesn't say anything; with that "definition" everything has the capacity of doing work and the in-capacity to do it.

Take two mass point: one stationary and the other moving at velocity v. Which has the capacity of doing work? The first? The other has not this capacity? Ok, the two mass point are electrically charged with charge +Q, and the stationary one is fixed spatially in his position. Now which of the two "has the capacity of doing work" on the other?

Furthermore, what can do work is not energy, and not even a system which has energy: what can do work are *forces* (or fields).
Title: What are "energy" and "work" ?
Post by: simplified on 02/10/2010 12:11:00
I don't understand energy.What pushes away a photon from light source? Repellent fields or blast wave?
Title: What are "energy" and "work" ?
Post by: yor_on on 02/10/2010 16:17:09
I think that depends on how you choose to define 'forces' myself. If you expect there to be forces as f.ex that 'blast wave' then, under the assumption that a photon is a propagating entity, there will have to be some sort of explanation to why it 'moves'. If you on the other side consider 'forces' to be something that we use, lacking a better/simpler description for the phenomena, and perhaps also wonder about how those 'photons' can 'move' without acceleration, as well as how we ever are going to prove them to be a 'source' more than in a indirect way.. Then you've got me? I don't know either.. The idea of them not 'moving' makes it even weirder :)
Title: What are "energy" and "work" ?
Post by: simplified on 02/10/2010 16:58:30
energy of blast = energy of recoil + energy of photon ?
Title: What are "energy" and "work" ?
Post by: lightarrow on 02/10/2010 18:38:25
I don't understand energy.What pushes away a photon from light source?
Its speed  [:)].  Why do you think it needs something to push it? Photons are massless...
Title: What are "energy" and "work" ?
Post by: yor_on on 02/10/2010 20:47:02
Yeah, it seems to fall back to what mass is?
And why there is that difference, mass needs an acceleration, a mass-less particle is just assumed to 'move' by itself, as if it the idea of an action-reaction have no relevance to it?

And it fits with all experiments we have made up today too?
It's phreakinly weird that one :)

And we use the 'photon frames' invariance (as seen from all frames thought up) as a proof as the universe actually 'compress' itself to our own 'frame of reference'. And that's really impressive if it is correct as we then seem to 'contract' all energy there is, adapting it to our motion/relative mass/momentum.

That is, if you agree with me on it being a real occurrence, distances contracting, as well as time dilation existing for real.
==

And btw: For this one it doesn't matter if you look at it as a 'time dilation' only, or a 'length contraction. You are free to exchange those two I think? To look at it only as a time dilation only f.ex won't change that fact.

When observing the moving twin you can follow him traveling in your super telescope, at no time losing sight of him, but he will still be younger than his twin on Earth. And as you think of it, putting away your telescope, there is only one explanation available. Somehow his motion changed his time, making it slower.

And if you translate that into distance you will find that he must had a shorter journey than the one you thought yourself to measure, relative you. If you exchange that 'slower time', relative you, into his 'distance made', then that distance had to have been shorter for him. And if you ask him he will agree, from his frame the distance actually was shorter although his 'intrinsic frame of times arrow' ::)) never changed for him.

Very weird.
Title: What are "energy" and "work" ?
Post by: yor_on on 02/10/2010 21:26:39
But if you look at as a length contraction only?
Why would he need to have aged slower?

That falls back to how we measure a distance, we do it using 'times arrow'.
If you want to assume that time is invariant, never changing, and at the same time introduce only a length contraction you are contradicting yourself. The only way we can define a 'distance' is using a clock and, at least, two 'frames of reference'. Your own relative what you measure. Without using that clock distance won't exist, much in the same way as it doesn't seem to exist for our 'photon'.

Or, can you see any other way to define 'distance'?
Title: What are "energy" and "work" ?
Post by: yor_on on 03/10/2010 00:41:55
Although you might be able to argue that time is invariant, if seen from your own frame of reference. Meaning that the twin traveling never experienced any 'slowing' of his time, making his clock the 'universal one'. Against that we have the twin staying at home who then, with as much right, could argue that it was his time that was the 'invariant and universal.'. But they don't fit together, do they?

That's why we use 'frames of reference'.

But it still makes a very strange truth in that your clock, for you, will be the universal one, no matter where you are. Just as mine will make mine 'universal invariant time'. And, that only when being together, as defined by being 'at rest' relative each other, will we ever share that 'universal time', and then only if we also happen to have the exact same mass, it seems to me? And as I said before, as you can change your point of view, observing your own atoms instead, and then define them different 'frames of reference' according to their mass and motion relative each other, how the he* do I define a 'frame of reference'? That one gives me a headache.
===

Maybe I'm looking at it from the wrong point of view?

Maybe there are no 'frames of reference'? We think there are but we use clocks to define them right? And those clocks all ticks differently depending on our 'frame of reference'. And that goes for distances too. So? Is there something wrong in our conception of time? And is there something wrong in our conception of 'distance'. Use the 'forbidden' frame of the photon, and ask yourself what that frame sees? We do use it as we refer to its timelesness as the explanation to why it can keep its 'intrinsic energy' no matter how far it travels. Or do you have a better explanation for that? Length contraction perhaps :) And what would that do SpaceTimes 'geometry'?

Without a clock you can't have a distance.
And that one I'm fairly sure on.
Title: What are "energy" and "work" ?
Post by: Ron Hughes on 03/10/2010 04:39:34
yor, when was the last time that someone shot a beam of light from here then jumped a billion light years away from Earth and waited to check the validity of the statement that light does not lose energy? I don't know if it does or not. It seems to me that if light lost energy at a rate of say 10^-20Hz/light yr we would never know it. If there is proof that would sway me I would be interested.
Title: What are "energy" and "work" ?
Post by: simplified on 03/10/2010 06:17:02
I don't understand energy.What pushes away a photon from light source?
Its speed  [:)].  Why do you think it needs something to push it? Photons are massless...
Effect of recoil reduces energy of a photon. Therefore.
Title: What are "energy" and "work" ?
Post by: yor_on on 03/10/2010 07:13:22
Yeah Ron, I've started to wonder anew :)

Considering that if a photon would be proofed to have even the slightest of masses it would have to be inside 'fermion time' so to speak.. I've reopened Ethos thread for those questions, and I would be pleased if we started with why mainstream physics consider it to be intrinsically time less, and then take it from there..

I know why I consider it to be so, but I'm not sure of how much of that is just my own conjecture and how much is actual proofs, mathematical or not. And it sure have a relevance.
Title: What are "energy" and "work" ?
Post by: lightarrow on 03/10/2010 12:00:18
I don't understand energy.What pushes away a photon from light source?
Its speed  [:)].  Why do you think it needs something to push it? Photons are massless...
Effect of recoil reduces energy of a photon. Therefore.
And if my laser gun doesn't recoil at all, since its mass >> photon's momentum/c ?  The photon shouldn't be shoot away? I don't understand your reasoning.
Title: What are "energy" and "work" ?
Post by: simplified on 03/10/2010 15:19:26
I don't understand energy.What pushes away a photon from light source?
Its speed  [:)].  Why do you think it needs something to push it? Photons are massless...
Effect of recoil reduces energy of a photon. Therefore.
And if my laser gun doesn't recoil at all, since its mass >> photon's momentum/c ?  The photon shouldn't be shoot away? I don't understand your reasoning.
Recoil reduces energy of a photon. Your laser gun has no recoil. Therefore energy of your photons is not reduced.
Title: What are "energy" and "work" ?
Post by: Ron Hughes on 03/10/2010 17:58:36
yor, take a photon with E = fh and calculate it's mass from m = fh/C^2. The equation is derived from  fh = E = mC^2. As you can see we could claim mathematically that all photons have mass.
Title: What are "energy" and "work" ?
Post by: yor_on on 03/10/2010 19:34:03

I'm already 'arguing' as good as I know there, which doesn't say much ::))
And I'm sure you will get better responses to your equation there as I hope the 'heavy artillery' will roll out to define what that elusive light does and does not :)

Heh.
And ahem :)
Title: What are "energy" and "work" ?
Post by: yor_on on 03/10/2010 19:39:44
Sim have you seen this one?
Measuring the Recoil of Photons. (http://www.photonics4life.eu/P4L/News/Little-Nudges-from-Light-Measuring-the-Recoil-of-Photons) That one seems to say that there is a measurable recoil?

But if there is no acceleration? You could use it as a proof of that a photon have a 'source' that it 'propagates' from, if correct too, it seems? But how does something without a intrinsic 'time frame' leave a recoil? That one I'm not sure how to see at all.
==

And yeah, Ron, I agree, so much for discussing 'photons' at the 'correct' thread.
Ah well :)
Title: What are "energy" and "work" ?
Post by: syhprum on 03/10/2010 20:12:32
I always understood that a photon gun radiating 300MW had a recoil of 1 Newton, hence the photon drive spaceships beloved by sci fi authours.
Title: What are "energy" and "work" ?
Post by: yor_on on 03/10/2010 20:19:27
That is when 'bouncing' isn't it?
I never thought of it in any other way when reading about Enterprises warpdrive?
Hmm shouldn't the correct sentence be?

"Beam me up Scotty, but hey, watch out for that dam*d recoil please.."

That way it's acceptable for me, but if we consider one photon, just 'materializing'?
Title: What are "energy" and "work" ?
Post by: JP on 04/10/2010 02:44:15
But if there is no acceleration? You could use it as a proof of that a photon have a 'source' that it 'propagates' from, if correct too, it seems? But how does something without a intrinsic 'time frame' leave a recoil? That one I'm not sure how to see at all.

I think the problem is that you're trying to apply classical mental pictures to non-classical processes.  With photons you're dealing with particles (quantum mechanics) that are moving fast (relativity), so classical intuition might not hold.  In this case, it doesn't because the photon has no rest frame (relativity), plus the photon at some point is created by some process (quantum mechanics).  This all together means you don't have to accelerate the photon because it moves at the speed of light always, including when it gets created.  There is no acceleration.

Even quantum mechanics has to obey some familiar rules, though.  You can use conservation of momentum to determine recoil if you know the momentum of the photon.
Title: What are "energy" and "work" ?
Post by: yor_on on 04/10/2010 15:03:40
So the momentum will be 'ring/sphere' formed at its 'materialization' then?
Or how it is thought to work, I always thought of momentum as a 'force' well, at least pointing in the direction of the photons velocity, but it seems hard to state that the recoil lies before the photon?

Maybe that's more correct way to look at it thinking of it, as something evenly distributed? But to talk about a recoil from something just 'coming into existence' should then also mean that what we call that recoil take place everywhere at that photon it seems. The easiest way i can imagine a recoil in this way is if there would be some sort of 'sticky elasticity' involved in its materialization, like boundaries letting go. Does that make sense?
Title: What are "energy" and "work" ?
Post by: Ron Hughes on 04/10/2010 20:39:56
If the equation above is correct ( and we know it is ) then what ever emits the photon must have a recoil.
Title: What are "energy" and "work" ?
Post by: JP on 05/10/2010 03:32:10
So the momentum will be 'ring/sphere' formed at its 'materialization' then?
Or how it is thought to work, I always thought of momentum as a 'force' well, at least pointing in the direction of the photons velocity, but it seems hard to state that the recoil lies before the photon?

Momentum is a vector, so its like an arrow pointing in the direction of the photon's motion.  Momentum of the photon source is a vector pointing in the direction of its motion.  If the source was initially stationary, its initial arrow was zero length.   Since momentum is conserved, after emitting a photon the arrow of the photon plus the arrow of the source have to add up to zero length (meaning that if you align them tip-to-tail and follow them, you end up where you started after traversing both arrows).

In Newtonian mechanics, change in momentum means that something changed its velocity.  This means acceleration, which by Newton's second law means there was a force.  However, the creation of photons isn't Newtonian mechanics, so the photon doesn't have to experience a force or accelerate.
Title: What are "energy" and "work" ?
Post by: yor_on on 06/10/2010 18:46:07
"If the source was initially stationary, its initial arrow was zero length. Since momentum is conserved, after emitting a photon the arrow of the photon plus the arrow of the source have to add up to zero length (meaning that if you align them tip-to-tail and follow them, you end up where you started after traversing both arrows)."

Makes perfect sense, except for one thing. A photon can't be seen as 'stationary', can it?
Or do we allow it a 'instant' inside our arrow, where it is 'stationary' before it starts to move, without accelerating?
Title: What are "energy" and "work" ?
Post by: Ron Hughes on 06/10/2010 22:38:20
A photon or wave ( http://www.thenakedscientists.com/forum/index.php?topic=34333.0 ) can never be stationary, anytime an electron or proton is moved the change in it's field position will propagate away at C. A photon/wave is only created by the movement of charged particles. If there is another way please tell me.
Title: What are "energy" and "work" ?
Post by: JP on 07/10/2010 10:27:24
"If the source was initially stationary, its initial arrow was zero length. Since momentum is conserved, after emitting a photon the arrow of the photon plus the arrow of the source have to add up to zero length (meaning that if you align them tip-to-tail and follow them, you end up where you started after traversing both arrows)."

Makes perfect sense, except for one thing. A photon can't be seen as 'stationary', can it?
Or do we allow it a 'instant' inside our arrow, where it is 'stationary' before it starts to move, without accelerating?

You're perfectly right.  A photon's momentum is never zero, so its momentum can always be drawn as an arrow of non-zero length.  What's key here is that the photon didn't exist initially, so the total momentum was zero initially.  When the photon comes out of the source, in order for the total momentum to stay the same, the source has to recoil in the opposite direction of the photon.
Title: What are "energy" and "work" ?
Post by: Pmb on 07/10/2010 11:12:19
I can't get the actual concept or meaning of various forms of energies . Actually what is energy ? My textbook defines energy as capacity to do work.Most of the websites define energy in the same way. But i can't understand any thing from that definition. I also want to know what is work.
Actually, why do we need these quantities?

[MOD EDIT - PLEASE PHRASE YOUR POST TITLES AS QUESTIONS. THANKS. CHRIS]
We've discussed this subject in the recent past so you might want to look up that debate. I created a web page on my website at

http://home.comcast.net/~peter.m.brown/mech/what_is_energy.htm

to support the answer that nobody knows what energy is. There is an e-book online which discusses it too. See "What is Energy?, by Dave Broyles. Basically it' what Richard Feynman said (See my web page for reference for this quote and Feynman's reasoning for it)
Quote
It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. However, there are formulas for calculating some numerical quantity, and we add it all together it gives “28” -  always the same number. It is an abstract thing in that it does not tell us the mechanism or the reasons for the various formulas.
In the former discussion I presented some arguments which I believe render the "Energy is the ability to do work" meaningless.
Title: What are "energy" and "work" ?
Post by: Ron Hughes on 07/10/2010 15:39:36
It's funny how someone can post a logical irrefutable fact and it means nothing to the discussion underway.
Title: What are "energy" and "work" ?
Post by: Pmb on 07/10/2010 22:02:19
It's funny how someone can post a logical irrefutable fact and it means nothing to the discussion underway.
I posted a response to the comment what is energy?What are you talking about?
Title: What are "energy" and "work" ?
Post by: Ron Hughes on 07/10/2010 23:43:41
Pmb, that wasn't pointed at you. Energies lowest common denominator is radiation.Any other explanation requires the use of matter's momentum which is used every day to do work and matter is composed of radiation, E = mC^2.
Title: What are "energy" and "work" ?
Post by: lightarrow on 08/10/2010 12:19:14
Recoil reduces energy of a photon. Your laser gun has no recoil. Therefore energy of your photons is not reduced.
Ok, but what this has to do with what pushes away a photon from light source?
Title: What are "energy" and "work" ?
Post by: lightarrow on 08/10/2010 12:20:49
yor, take a photon with E = fh and calculate it's mass from m = fh/C^2. The equation is derived from  fh = E = mC^2. As you can see we could claim mathematically that all photons have mass.
Photons are massless, and that's all, no possibility of opinions here.
Title: What are "energy" and "work" ?
Post by: lightarrow on 08/10/2010 12:26:23
and matter is composed of radiation, E = mC^2.
Title: What are "energy" and "work" ?
Post by: Ron Hughes on 08/10/2010 17:29:09
Then the equation must be wrong? I agree that photons are massless, the equation only shows the energy required to produce that photon.
Title: What are "energy" and "work" ?
Post by: lightarrow on 08/10/2010 19:20:39
Then the equation must be wrong? I agree that photons are massless, the equation only shows the energy required to produce that photon.
That equation is wrong for photons. The correct one, valid for all particles and regardless if stationary or moving (in SR) is this:

E2 = (mc2)2 + (cp)2

p = momentum.

For light, in classical EM, E = cp. If you apply it to photons (certainly you will agree on the fact this is allowed) you find:

(cp)2 = (mc2)2 + (cp)2

=> 0 = (mc2)2

so m = 0.
Title: What are "energy" and "work" ?
Post by: simplified on 09/10/2010 06:19:02
Recoil reduces energy of a photon. Your laser gun has no recoil. Therefore energy of your photons is not reduced.
Ok, but what this has to do with what pushes away a photon from light source?
Radiator loses thermal energy because  photon steals it.
Title: What are "energy" and "work" ?
Post by: simplified on 09/10/2010 07:24:11
Let's consider analogy to a lightning. A cloud beats  ground by negative charge, in the answer the ground beats the cloud by positive charge.
When a particle and an antiparticle meet, they beat  radiator by gravitational charge. In the answer the radiator beats them by charge of energy.
Title: What are "energy" and "work" ?
Post by: lightarrow on 09/10/2010 20:18:23
Let's consider analogy to a lightning. A cloud beats  ground by negative charge, in the answer the ground beats the cloud by positive charge.
When a particle and an antiparticle meet, they beat  radiator by gravitational charge. In the answer the radiator beats them by charge of energy.
hmmm...maybe you should change your nickname from "simplified" to "complicated"  [:)]
Title: What are "energy" and "work" ?
Post by: simplified on 10/10/2010 09:30:21
Let's consider analogy to a lightning. A cloud beats  ground by negative charge, in the answer the ground beats the cloud by positive charge.
When a particle and an antiparticle meet, they beat  radiator by gravitational charge. In the answer the radiator beats them by charge of energy.
hmmm...maybe you should change your nickname from "simplified" to "complicated"  [:)]
Then maybe you  a ruddy critic with superficial judgement [::)]
Title: What are "energy" and "work" ?
Post by: lightarrow on 10/10/2010 12:31:31
Let's consider analogy to a lightning. A cloud beats  ground by negative charge, in the answer the ground beats the cloud by positive charge.
When a particle and an antiparticle meet, they beat  radiator by gravitational charge. In the answer the radiator beats them by charge of energy.
hmmm...maybe you should change your nickname from "simplified" to "complicated"  [:)]
Then maybe you  a ruddy critic with superficial judgement [::)]
Ok, I accept your critic. But then can you please explain me what is that you intended? I sincerely have not understood it ( it can be me, of course).
Title: What are "energy" and "work" ?
Post by: simplified on 17/10/2010 07:21:13
And so if a mass is a neutralized energy then energy is an activated mass  [:P]
Photon has 100% of activated mass and 0% of usual mass.Usual object has usual mass and activated mass = (Lorentz's coefficient-1)* usual mass .
Usual mass isn't relative thing. Activated mass is just relative thing.
I only can not understand that why photons take and mass of Sun and energy of Sun? They must to take only mass,because mass turned into their energy.
Title: What are "energy" and "work" ?
Post by: lightarrow on 17/10/2010 13:42:32
And so if a mass is a neutralized energy then energy is an activated mass  [:P]
Photon has 100% of activated mass and 0% of usual mass.Usual object has usual mass and activated mass = (Lorentz's coefficient-1)* usual mass .
Usual mass isn't relative thing. Activated mass is just relative thing.
I only can not understand that why photons take and mass of Sun and energy of Sun? They must to take only mass,because mass turned into their energy.
I'm sorry to tell you, but if you talk of "neutralized energy" or "activated mass", you are not talking of physics.
Title: What are "energy" and "work" ?
Post by: simplified on 17/10/2010 15:10:38
And so if a mass is a neutralized energy then energy is an activated mass  [:P]
Photon has 100% of activated mass and 0% of usual mass.Usual object has usual mass and activated mass = (Lorentz's coefficient-1)* usual mass .
Usual mass isn't relative thing. Activated mass is just relative thing.
I only can not understand that why photons take and mass of Sun and energy of Sun? They must to take only mass,because mass turned into their energy.
I'm sorry to tell you, but if you talk of "neutralized energy" or "activated mass", you are not talking of physics.
And so if a mass is a neutralized energy then energy is an activated mass  [:P]
Photon has 100% of activated mass and 0% of usual mass.Usual object has usual mass and activated mass = (Lorentz's coefficient-1)* usual mass .
Usual mass isn't relative thing. Activated mass is just relative thing.
I only can not understand that why photons take and mass of Sun and energy of Sun? They must to take only mass,because mass turned into their energy.
I'm sorry to tell you, but if you talk of "neutralized energy" or "activated mass", you are not talking of physics.
Title: What are "energy" and "work" ?
Post by: Bill S on 17/10/2010 18:59:50
Quote from: yor_on
A vertical force can never cause a horizontal displacement; thus, a vertical force does not do work on a horizontally displaced object!!

Imagine a situation in which you are standing under an object which is on rails, such that it will move only horizontally.  The underside of this object slopes.  You use a pole to apply vertical force against that slope, and the object responds by moving horizontally. Is this not a vertical force causing horizontal displacement?
Title: What are "energy" and "work" ?
Post by: Geezer on 17/10/2010 21:11:17
Quote from: yor_on
A vertical force can never cause a horizontal displacement; thus, a vertical force does not do work on a horizontally displaced object!!

Imagine a situation in which you are standing under an object which is on rails, such that it will move only horizontally.  The underside of this object slopes.  You use a pole to apply vertical force against that slope, and the object responds by moving horizontally. Is this not a vertical force causing horizontal displacement?

Only if you prevent from rotating where you are holding it, but then you are producing a horizontal force. If you allow the pole to pivot freely where you are holding it, it will just tilt over as it goes up the incline and no force will be imparted to the vehicle at all.
Title: What are "energy" and "work" ?
Post by: lightarrow on 18/10/2010 18:20:24
Ok, how many angels can stay on a pin's head?
If you can't answer then I say 12.
Is this science?
If you use *scientific*, *precisely defined* terms, you will have answers. If you don't use, you can't have, but not because I don't have answers or because I don't want to answer, but because your question is completely  *undefined*; I don't have the least idea of what you want to ask or if it has a real meaning. Is it more clear now?
Title: What are "energy" and "work" ?
Post by: Bill S on 18/10/2010 23:48:19
Thanks Geezer. I tried thinking of a situation in which the poll is constrained to move only vertically, but I guess whatever was constraining it would be converting vertical to horizontal force; right?

What about orbital motion, though. E.g. does the Earth's gravitational attraction not act perpendicular to the direction of motion of the moon?
Title: What are "energy" and "work" ?
Post by: Geezer on 19/10/2010 01:56:22
Thanks Geezer. I tried thinking of a situation in which the poll is constrained to move only vertically, but I guess whatever was constraining it would be converting vertical to horizontal force; right?

What about orbital motion, though. E.g. does the Earth's gravitational attraction not act perpendicular to the direction of motion of the moon?

Well, sort of. You are transferring the horizontal force at the top of the pole to a horizontal force at your wrist.

The Earth does not make the Moon move relative to the Earth. The gravitational force between the Moon and the Earth just maintains the Moon in an orbit around the Earth. If that force suddenly ceased, the Moon would keep on moving in a straight(ish) line.
Title: What are "energy" and "work" ?
Post by: Bill S on 19/10/2010 20:49:17
Quote from: Geezer
The gravitational force between the Moon and the Earth just maintains the Moon in an orbit around the Earth.

Here we seem to be treating gravity as a force, in spite of G R. This raises the question: If a force is holding the moon in orbit, is it doing any work?
Title: What are "energy" and "work" ?
Post by: Geezer on 19/10/2010 21:02:32
Yes, that's the Newtonian view. It still works quite well though!

In Newtonian mechanics there is a force, but as the force acts perpendicular to the motion, it's not doing any work.
Title: What are "energy" and "work" ?
Post by: simplified on 20/10/2010 03:51:59
Then the equation must be wrong? I agree that photons are massless, the equation only shows the energy required to produce that photon.
That equation is wrong for photons. The correct one, valid for all particles and regardless if stationary or moving (in SR) is this:

E2 = (mc2)2 + (cp)2

p = momentum.

For light, in classical EM, E = cp. If you apply it to photons (certainly you will agree on the fact this is allowed) you find:

(cp)2 = (mc2)2 + (cp)2

=> 0 = (mc2)2

so m = 0.
This is  pompous and complicated formula because you do not use correct scientific terms. [;D]

E=m'c²

p=(m+m')* v

E - energy for any object
p - momentum for any object
m - usual mass,this mass has gravitation field
m'- activated mass(mass turned into energy),this mass has not gravitation field
c - light speed
v - speed of object
[:P]
Title: What are "energy" and "work" ?
Post by: Geezer on 20/10/2010 04:15:55
WOAHH!!

Simplified,

Please try to be be careful with your translation (I know it's not easy) but "pompous" is probably not what you really meant to say. "Pompous" is a big insult in English.

It means "you are just saying something, but it means nothing."

Title: What are "energy" and "work" ?
Post by: lightarrow on 20/10/2010 15:44:55
E=m'c²

p=(m+m')* v

E - energy for any object
p - momentum for any object
m - usual mass,this mass has gravitation field
m'- activated mass(mass turned into energy),this mass has not gravitation field
c - light speed
v - speed of object
nonsense.
Title: What are "energy" and "work" ?
Post by: simplified on 20/10/2010 17:30:32
E=m'c²

p=(m+m')* v

E - energy for any object
p - momentum for any object
m - usual mass,this mass has gravitation field
m'- activated mass(mass turned into energy),this mass has not gravitation field
c - light speed
v - speed of object
nonsense.
Your definition is a law for dark people.
Title: What are "energy" and "work" ?
Post by: peppercorn on 20/10/2010 17:52:59
Your definition is a law for dark people.
[???] [???] [???]

What? ...Are you saying this a law for people from the African sub-continent??? [:D]

[To clarify for anyone who might have misunderstood - my intention above is to gently ridicule the poster's English NOT make a racist comment.  Thanks!]
Title: What are "energy" and "work" ?
Post by: simplified on 20/10/2010 18:06:36

What? ...Are you saying this a law for people from the African sub-continent??? [:D]
[/quote]Can you disprove the formulae?
Title: What are "energy" and "work" ?
Post by: peppercorn on 20/10/2010 18:19:41
Can you disprove the formulae?

I wasn't aware that the responsibility was on me to disprove it!
Title: What are "energy" and "work" ?
Post by: imatfaal on 21/10/2010 13:48:49
Sorry - but is this forum now using  "for dark people" as a metaphor for bad science and ignorance.  FFS I thought there were rules about racism.  I have reported it to the moderators and I hope they do something about it.

Matthew
Title: What are "energy" and "work" ?
Post by: BenV on 21/10/2010 13:57:13
Simplified - I have sent you a private message.  Please respond to me as soon as you receive it.

In the meantime, this thread will be locked.

This thread is now unlocked to give Simplified the opportunity to clarify his meaning.
Title: What are "energy" and "work" ?
Post by: simplified on 21/10/2010 18:57:19
Sorry - but is this forum now using  "for dark people" as a metaphor for bad science and ignorance.  FFS I thought there were rules about racism.  I have reported it to the moderators and I hope they do something about it.

Matthew
I never thought of a literal origin of this expression.I am so sorry.
Title: What are "energy" and "work" ?
Post by: imatfaal on 21/10/2010 19:36:27
We all say things in heat of an argument that we might not have fully thought out - the fact that large portions of the world suffer because of prejudice means that I tend to be over-sensitive when I come across the language of bigotry in an arena such as this great forum.  Your last post reads true and with that apology I personally hope that the forum can go back to the cut-and-thrust of academic debate and avoid disharmony .  Regards  Matthew Newell - Imatfaal

ps come to think of it the forum probably has a rule against profanity - and if the derivation of FFS isnt profane then nothing is!
Title: What are "energy" and "work" ?
Post by: Geezer on 22/10/2010 05:58:04
"What we've got here is failure to communicate"

Perhaps, due to a translation problem, Simplified meant "people in the dark"
Title: What are "energy" and "work" ?
Post by: simplified on 22/10/2010 14:42:50
"What we've got here is failure to communicate"

Perhaps, due to a translation problem, Simplified meant "people in the dark"
People without light ideas.
Title: What are "energy" and "work" ?
Post by: lightarrow on 22/10/2010 17:41:05
"What we've got here is failure to communicate"

Perhaps, due to a translation problem, Simplified meant "people in the dark"
People without light ideas.
Then is appropriate for those people who write "smoky", "unclear", non-scientific things, with the aim to make other believe they are scientific.
Title: What are "energy" and "work" ?
Post by: BenV on 22/10/2010 17:59:37
The off topic/misunderstanding posts are shrunk to allow easier reading of the actual science discussion - hopefully folks can pick it up from here...
Title: What are "energy" and "work" ?
Post by: simplified on 23/10/2010 04:28:14
If  activated mass does not exist, then communication of object with the past exists. It slows down time of object. [8D]
Title: What are "energy" and "work" ?
Post by: simplified on 23/10/2010 04:34:34
Energy is communication of object with the past .
Title: What are "energy" and "work" ?
Post by: Bill S on 27/10/2010 18:15:40
Quote from: simplified
Energy is communication of object with the past

Please explain, or change your name from "simplified", it doesn't suit you. [:P]
Title: What are "energy" and "work" ?
Post by: simplified on 30/10/2010 17:26:55
Then the equation must be wrong? I agree that photons are massless, the equation only shows the energy required to produce that photon.
That equation is wrong for photons. The correct one, valid for all particles and regardless if stationary or moving (in SR) is this:

E2 = (mc2)2 + (cp)2

p = momentum.

For light, in classical EM, E = cp. If you apply it to photons (certainly you will agree on the fact this is allowed) you find:

(cp)2 = (mc2)2 + (cp)2

=> 0 = (mc2)2

so m = 0.
Well. But which units I should use in this formula?
Title: What are "energy" and "work" ?
Post by: simplified on 30/10/2010 17:38:45
Units of measurement
Title: What are "energy" and "work" ?
Post by: lightarrow on 30/10/2010 22:56:31
Then the equation must be wrong? I agree that photons are massless, the equation only shows the energy required to produce that photon.
That equation is wrong for photons. The correct one, valid for all particles and regardless if stationary or moving (in SR) is this:

E2 = (mc2)2 + (cp)2

p = momentum.

For light, in classical EM, E = cp. If you apply it to photons (certainly you will agree on the fact this is allowed) you find:

(cp)2 = (mc2)2 + (cp)2

=> 0 = (mc2)2

so m = 0.
Well. But which units I should use in this formula?
It's a strange question. The measurement's units are:
kg for mass,
Joule for energy,
m*s-1 for c
kg*m*s-1 for p.
Joule = N*m = kg*m2*s-2
Title: What are "energy" and "work" ?
Post by: simplified on 31/10/2010 05:14:04
Object travels with speed of 1 m/s.Mass of the object=1kg
I want to calculate energy of this object.
E ≈ mv²/2 = 0,5 J
E² = (mc²)² + (cp)²

E²=(1*299792458²)²+(299792458*1*1)²=8,077608713*10³³

E=√(8,077608713*10³³)=89875517873681764,5 J  [:o]

I don't understand where is my mistake? [:(]

Title: What are "energy" and "work" ?
Post by: Geezer on 31/10/2010 05:52:15
I'm not sure how to explain this properly, but please try to remember that English may not be first language of some of our posters. Translation can be a tricky business, so please try to make sure there are no misunderstandings because of a translation problem.

Thanks!
Title: What are "energy" and "work" ?
Post by: lightarrow on 31/10/2010 10:23:19
Object travels with speed of 1 m/s.Mass of the object=1kg
I want to calculate energy of this object.
E ≈ mv²/2 = 0,5 J
Wrong. What you have written is *not* the energy E. With the term "energy" in relativity and in particle physics, we intend "total energy". What you have written is *just* kinetic energy. E = kinetic energy + mc2 (in absence of a potential or other kinds of energy).
(Let's say is a language problem, as Geezer said... [:)])
Title: What are "energy" and "work" ?
Post by: simplified on 31/10/2010 15:25:10
Object travels with speed of 1 m/s.Mass of the object=1kg
I want to calculate energy of this object.
E ≈ mv²/2 = 0,5 J
Wrong. What you have written is *not* the energy E. With the term "energy" in relativity and in particle physics, we intend "total energy". What you have written is *just* kinetic energy. E = kinetic energy + mc2 (in absence of a potential or other kinds of energy).
(Let's say is a language problem, as Geezer said... [:)])
Thank you very much.
Title: What are "energy" and "work" ?
Post by: yor_on on 02/12/2010 19:58:56
It's a pleasure reading you all.
Thank God (or someone else? All depending..) That TNS exist :)
Title: Re: What are "energy" and "work" ?
Post by: RubyPerkins on 11/12/2017 10:43:44
Thanks sharing the concept of work and energy.
Title: Re: What are "energy" and "work" ?
Post by: yor_on on 03/03/2018 02:14:41
Is energy time?
Time expended?

No

If not, what came first?
Title: Re: What are "energy" and "work" ?
Post by: PmbPhy on 20/12/2018 20:23:38
I can't get the actual concept or meaning of various forms of energies . Actually what is energy ?
See - http://www.newenglandphysics.org/physics_world/cm/what_is_energy.htm

My textbook defines energy as capacity to do work.
In my opinion that's a poor definition of energy. It omits the defining property that energy is always conserved.

I also want to know what is work.
If the force is constant and the object on what you're pushing moves in a straight line then the work is the product of force times distance. In general its the integral of force times dl where l is the infinitesimal distance moved.

Actually, why do we need these quantities?
Because it makes things simpler.
Title: Re: What are "energy" and "work" ?
Post by: PmbPhy on 20/12/2018 20:29:13
For example, a given mass of fuel can be converted into thermal energy (heat) by burning it, but it won't do any work.
Heat can be used to do work. One needs only to transfer the thermal energy to a gas which will increase its pressure which can them be used to cause an expansion which can do work.

Work and energy are expressed in the same units, so you might prefer to think of work as mechanical energy or useful energy.
EM radiation is not mechanical energy but can be used to do work.
Title: Re: What are "energy" and "work" ?
Post by: Bill S on 23/12/2018 20:40:33
Quote from: Pete
In my opinion that's a poor definition of energy. It omits the defining property that energy is always conserved.

Is energy conserved in an expanding universe?
Title: Re: What are "energy" and "work" ?
Post by: jeffreyH on 23/12/2018 22:04:52
An expanding universe changes the value of the energy of photons travelling between distant objects. I would say that this requires a dark energy field that contains it's own energy. In that respect you could argue that energy is conserved. There is a transfer of energy from the photon to the field. It could also be viewed as a simple Doppler shift due to the recessional acceleration of the sources. Acceleration does require a force and this does suggest a field. Is this the same as the vacuum? I have no idea.
Title: Re: What are "energy" and "work" ?
Post by: alancalverd on 24/12/2018 00:14:06
Momentum of a photon = hf/c. Momentum is conserved, so every photon source exhibits recoil.
Title: Re: What are "energy" and "work" ?
Post by: yor_on on 31/12/2018 20:54:40
you're right Alan. And that is the weirdest part, 'momentum is conserved'. So a 'photon recoil' is needed for this to be correct, and that makes the recoil a result of conservation laws. So different from the idea of 'action and reaction' in where something 'shoots out' a photon, accelerating it to the limit 'c'. There is no acceleration existing here.