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Q & A: fundamentals of energy

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Most recent answer: 10/22/2007
Q:
For the past few months I have been studying physics. We are just getting to energy and I was not satisfied with the definitions for energy we were given. They all seemed circular to me. So I thought about the units. OK, a mass accelerating over a distance, sounds like work and work yields energy. Then came power. So, I tried the same thing and it comes out to be like jerks (m/s^3 I think) of a mass across a distance. Iím wondering if you can explain if this makes sense on some other level than the units (electricity and watts). I donít know much about the nature of energy. So, I decided to ask here. But, I have another question: Einstein said that matter is just energy slowed down (E=mc^2). If that is so then what is the fundamental unit in physics, because from what we know about physics I would think it should be a joule (other units being based off of it i.e. N = J/m). So really Iím asking the same question from two different ends. Can the units for power, and other energy-derived units, be explained conceptually? Should the units for energy be fundamental and others based off of it? Now, granted I am only a High school physics student so I may have misunderstood some concepts. Yet, Iíve been getting rather bored with the pace and depth of work in physics and am doing a bit of research on my own. Anyways, thanks again.
- Jeff Fenoli (age 17)
Kirkland, WA, USA
A:
Jeff- I hope youíre planning to be a physicist, or at least a philosopher. Those are deep questions.

First, on the question of what energy "is": youíre right that all standard definitions are circular. I donít want to get in too far over my head, but at some level thatís unavoidable. We canít always define each thing in terms of something else, without end. At some point we have to stop and ask whether our ideas hang together consistently (no bad circles) and can be interpreted in some way that allows comparison with observations. Thereís a wonderful chapter in R. Feynmanís little book "The Character of Physical Law" about the expanding definition of energy needed to preserve the law of conservation of energy. I think youíll love that chapter, and probably the whole book.

As for the most fundamental units: Joules are just put together out of some systematic units that were thrown together a couple hundred years ago, I think in a rationalization project following the French revolution. These standard international units fit together in a nice rational way, but the whole set is arbitrary. When physicists try to put together fundamental units based on properties of nature, we use the speed of light, Planck's constant, and Newton's gravitational constant G to set the fundamental units. These "Planck" units don't correspond to any standard practical system. A Joule is only something very roughly around a ten-billionth of one of these units. I can do that calculation more carefully if you want.

Mike W.

leh

(published on 10/22/2007)

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