What is Energy?

[Lote-Tree]

Hello All,

I am new here.

By joining this forum I hope to improve my knowledge of science.

At school we have been taught that "Energy" can neither be created or destroyed and that Energy and Matter are equivalent.


So my first question is:

What is Energy?


Thank you all.

Lote-Tree

[neutrino]

Hi Lote-Tree, welcome to PF!

Here you go: what is energy?! (http://www.physicsforums.com/showthread.php?t=153105)

[americanforest]

Simplest answer: Energy is the ability of something to do work.

[tim_lou]

just a constant in system with time symmetry.

Actually, even without time symmetry, we can make up things like heat (Q) so that E is still a constant.

[pallidin]

Energy is a very broad term.
In essence, it is some type of influence which alters the stasis of an event.

[Behrooz]

your question has no answer!
i have asked this before u and i found no good answwer.
all we no is:energy is a quantity that can not be seen and has no mass.
when an object has energy it either can do work or has heat(wich is a kind of energy)
but what is the reason for wich an object has energy?
i dont know!

[americanforest]

all we no is:energy is a quantity that can not be seen and has no mass.


Isn't it true that objects with sources of potential energy have a greater mass and rest energy? ie. a compressed spring weighs more than a non-compressed spring.

This is the reason you can't push something over the speed of light, it's mass becomes infinite because of it's energy and to get it past the speed of light would require an infinite force.

[Astronuc]

Isn't it true that objects with sources of potential energy have a greater mass and rest energy? ie. a compressed spring weighs more than a non-compressed spring. No. A compressed or tensioned spring does not change mass. Weight is proportional mass via the local acceleration due to gravity.

[americanforest]

No. A compressed or tensioned spring does not change mass. Weight is proportional mass via the local acceleration due to gravity.

A hot potato weighs more than a a cold potato; a compressed spring weighs more than a relaxed spring....whenever you weigh an object, you are measuring not only the masses of its constituent parts, but all of their interaction energies as well.

In inelastic collisions kinetic energy goes to rest energy. On macroscopic scales the particels original rest energy is just so much bigger than it's changes that the change is just insignificant. Maybe you just misunderstood me the first time.

[Lote-Tree]

Hello All,

Thank you for replies.

I have studied them but I am not yet come to any fuller understanding.

If Energy can't exists on its own ie you can't put pure energy in a glass - then what does it mean you can't neither create or destroy energy?

[pixel01]

In my thinking, mass and energy all have the same roots. I've read in some physics books about string theory. The strings themselves have almost no mass (or very small), but when they have energy and oscillate with specific patterns, they have mass. I like this explanation because you can imagine you can not hold a bicycle wheel by holding its axis, but if you rotate the wheel, you can hold it for a while. So with some spinning energy, the wheel seems to have more inertia.
That's just what i am thinking.

[Mentz114]

A compressed spring weighs more than when it is uncompressed. If this were not so I could make a 'perpetuum mobile'. I will post details if anyone is curious.

[Jacquesl]

A compressed spring weighs more than when it is uncompressed

Huhh? I don’t think so man

Than I must weigh more when I’m sitting then when I’m standing


But I like your example Mentz114

[haiha]

A compressed spring weighs more than when it is uncompressed. If this were not so I could make a 'perpetuum mobile'. I will post details if anyone is curious.

It sounds strange to me and I must doubt about this. Very please to see the details.

[Mentz114]

Jaque and Haiha - before I explain, I need to establish one thing.

Do you agree that energy gravitates ? This is not taken into account in Newtonian theory, but the Einstein theory of gravity tells us that energy gravitates as if it has mass \frac{E}{c^2} (approximately).

If you accept this without argument I will post the demonstration. Also it will help if you look at this, especially the diagram of the plates.

http://en.wikipedia.org/wiki/Casimir_effect

[Jacquesl]

It’s an interesting example you’ve made I must tell you.

If I look at E=M x C^2 , I get confused with your example, but it’s a good brainteaser for me
I can’t argue with you because I’m not an expert on physics but I like people like you how wants to try new stuff, but please post your data on a 'perpetuum mobile'. I will like a build one; I hope I don’t receive another infraction point for these statements

[Mk]

Jaque and Haiha - before I explain, I need to establish one thing.

Do you agree that energy gravitates ? This is not taken into account in Newtonian theory, but the Einstein theory of gravity tells us that energy gravitates as if it has mass \frac{E}{c^2} (approximately).
Yeah, but, energy does not warp spacetime, mass does.

If you accept this without argument I will post the demonstration. Also it will help if you look at this, especially the diagram of the plates.

http://en.wikipedia.org/wiki/Casimir_effect

The Casimir effect does not happen because of gravity at all...? :confused:
A compressed spring weighs more than when it is uncompressed.No, it doesn't.

I suggest everybody in the thread reads the original answer, it'll help.
http://www.physicsforums.com/showthread.php?t=153105

Remember, energy is non existent, it is a bookkeeping device.

[Mentz114]

Yeah, but, energy does not warp spacetime, mass does

Energy does indeed warp space-time. If you think it does not please quote an authoritative source. The Einstein field equations read

R_{\mu\nu} - \frac{1}{2}Rg_{\mu\nu} = \kappa T_{\mu\nu}

The quantity on the right is the stress-energy-momentum tensor which includes mass, energy momentum and pressure. Given T, one solves for the metric g.

A spring weighs more when compressed and has greater inertial mass. I'll post the demonstration in a few hours.

Jacques, it is impossible to build a perpetuum mobile.

[Jacquesl]

A compressed spring weighs more than when it is uncompressed. If this were not so I could make a 'perpetuum mobile'. I will post details if anyone is curious.

So a compressed spring weights the same as an uncompressed spring, so where is your perpetuum mobile?
That’s basic knowledge about that spring example man.


Jacques, it is impossible to build a perpetuum mobile.

If think you have your stuff mixed-up.

[Fletcher]

Energy isn't a thing. It's a helpful mathmatical device. You define a system and use conservation of energy to say stuff doesn't just happen without reason.

[Lote-Tree]

Energy isn't a thing. It's a helpful mathmatical device. You define a system and use conservation of energy to say stuff doesn't just happen without reason.

In response to Define Energy I have been given this response:


The conserved quantity associated with the temporal invariance of the laws of physics.


Does that finally define what Energy is?
Or is it another attempt to define the undefinable?

[Mk]

Once more I say:
I suggest everybody in the thread reads the original answer, it'll help.
http://www.physicsforums.com/showthread.php?t=153105

Remember, energy is non existent, it is a bookkeeping device.

And can somebody tell me if Mentz knows what he is talking about? You never posted your demonstrations and references you said you would.

[Ki Man]

energy is the ability to affect mass

[thiotimoline]

energy is the ability to affect mass

Energy is like a universal property to show the amount of mass in a body, in relation to c^2, thus E=mc^2. In order to explain the warping of spacetime, it's necessary to use m instead of E. It's just like using the particile theory instead of wave theory to explain photoelectric effect. In different situations, use different explanations.

[Mentz114]

Mk, have a look at this.

http://www.physicsforums.com/showthread.php?t=162106

As you can see, no interest. I guess people think it's crackpot. Maybe it is.