[The Champ (OP)]

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?

Thanks in advance.

[MOD EDIT - PLEASE PHRASE YOUR POST TITLES AS QUESTIONS. THANKS. CHRIS]

[Geezer]

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.

[yor_on]

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.

[simplified]

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?

Thanks in advance.

Energy is ability to overcome resistance. Work is a victory of quantity of resistance.

[yor_on]

Heh

Energy is reverse engineered inertia.
Yep ..

Ahem again.

[Geezer]

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.

[simplified]

Heh

Energy is reverse engineered inertia.
Yep ..

Ahem again.

[Geezer]

Heh

Energy is reverse engineered inertia.
Yep ..

Ahem again.

[Bored chemist]

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.

[Melvin900]

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.

[Farsight]

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.

[simplified]

Voltage of an electric current shows energy of  electric photons. Force of an electric current shows quantity(amount) of electric photons.

[yor_on]

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?

[Geezer]

Yes, yes, and maybe.

[JP]

Yes, yes, and maybe.

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

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...

[Geezer]

Yes, yes, and maybe.

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

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.

[JP]

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). 

[Geezer]

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".

[JP]

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. 

[yor_on]

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.