# The Naked Scientists Forum

### Author Topic: Why is energy hard to define?  (Read 18767 times)

#### Pmb

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##### Why is energy hard to define?
« Reply #50 on: 24/07/2010 04:29:37 »
I forgot to place the derivation of the exact derivation of a uniform gravitational field from the classical point of view. This has the benefit of going one step further of using the weak field approximation from GR.
http://home.comcast.net/~peter.m.brown/gr/grav_cavity.htm

#### Bored chemist

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##### Why is energy hard to define?
« Reply #51 on: 24/07/2010 16:19:47 »
This bit
"If the particle is between 0 and one and not moving"
means you have a particle which has zero momentum (because it's not moving)- and the uncertainty on that momentum is also zero.
However you also specify it's position as being between zero and one.
That's a breach of the uncertainty principle; if you know the velocity absolutely then you don't know the position at all.

#### yor_on

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##### Why is energy hard to define?
« Reply #52 on: 28/07/2010 10:26:09 »
Energy is the capacity to do work.

The concept of energy is abstract and therefore not as easy to define as the concepts of mass and volume. One definition of energy, as what Bored chemist said, is the capacity to do work.

There are two principal forms of energy: potential and kinetic.

Potential energy is stored energy.

Kinetic energy is the energy of motion.

Hope this helps.

Potential energy is not stored energy, it's exactly the type of energy it states itself to be as far as I understands it. Potential = The inherent capacity for coming into being.

You can at the same time depending on your choice of observation have a multitude of potential 'energy's stored' in a object versus Earth, the moon etc.

Kinetic Energy is defined by the mass and speed/velocity of an object but as with potential energy it will only become in f.ex a collision. That is, I don't think you can measure it solely with a instrument even though we can count on it?

==

As for what energy really really is :)
Tell me, I want to know that too.

Knowing that space expands and that we constantly gets more vacuum, containing more intrinsic energy, scrambles me brain, not that that is such a feat :)

#### Pmb

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##### Why is energy hard to define?
« Reply #53 on: 31/07/2010 07:12:21 »
Does anybody hear have acccess to a library? I'd love to read the following article

Energy is the capacity to do work-or is it? by Nancy Hicks, The Physcs Teacher, Volume 21, Issue 8, pp. 529-530 (1983).

The abstract to this paper reads
Quote
Abstract - Discusses the definition of energy found in many high school and college level physics texts, the conflict of that definition with the second law of thermodynamics, and the relation to the concept of the conservation of energy. Suggests not using a definition of energy and provides some alternative strategies.
If I'm able to get a hold of this article is there anybody who'd like to read it? If so then either PM me or E-mail me and I'd be happy to share it.

Best wishes

Pete
« Last Edit: 31/07/2010 11:15:49 by Pmb »

#### Bored chemist

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##### Why is energy hard to define?
« Reply #54 on: 31/07/2010 15:45:00 »
I would like to see the article.

I'd also like to see whether or not Pmb accepts that the system he suggested earlier is a breach of the uncertainty principle.

#### Pmb

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• Posts: 1838
• Physicist
##### Why is energy hard to define?
« Reply #55 on: 31/07/2010 21:59:09 »
I would like to see the article.

I'd also like to see whether or not Pmb accepts that the system he suggested earlier is a breach of the uncertainty principle.
No system is in breach of the uncertaint principle. I've already stated that before in another thread.
« Last Edit: 31/07/2010 22:04:27 by Pmb »

#### The Naked Scientists Forum

##### Why is energy hard to define?
« Reply #55 on: 31/07/2010 21:59:09 »