# What is Energy?

1. Mar 16, 2007

### 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

2. Mar 16, 2007

### neutrino

3. Mar 16, 2007

### americanforest

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

4. Mar 17, 2007

### 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.

5. Mar 17, 2007

### pallidin

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

6. Mar 17, 2007

### Behrooz

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!

7. Mar 17, 2007

### americanforest

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.

Last edited: Mar 17, 2007
8. Mar 17, 2007

### Staff: Mentor

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

9. Mar 17, 2007

### americanforest

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.

10. Mar 19, 2007

### 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?

11. Mar 19, 2007

### 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.

Last edited: Mar 19, 2007
12. Mar 19, 2007

### 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.

13. Mar 19, 2007

### Jacquesl

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

Last edited: Mar 19, 2007
14. Mar 19, 2007

### haiha

15. Mar 19, 2007

### 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

Last edited: Mar 19, 2007
16. Mar 19, 2007

### 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

17. Mar 19, 2007

### Mk

Yeah, but, energy does not warp spacetime, mass does.
The Casimir effect does not happen because of gravity at all...?
No, it doesn't.

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

Last edited: Mar 19, 2007
18. Mar 19, 2007

### Mentz114

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.

Last edited: Mar 19, 2007
19. Mar 20, 2007

### Jacquesl

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.

If think you have your stuff mixed-up.

20. Mar 21, 2007

### 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.