View Single Post
May2-05, 08:49 PM
Sci Advisor
PF Gold
cepheid's Avatar
P: 5,196
Quote Quote by michael879
well actually, Im pretty sure mass and energy are basically the same thing (E=mc^2), but any way I get what your saying.
*Sigh*...yes, of course. But how do we harness that energy? The point I was trying to make is that we can only convert mass directly to energy in nuclear reactors. You need to blow up nuclei, or fuse particles together to form them. For example, if I remember right, two hydrogen nuclei and two neutrons when considered separately have a total mass greater than that of a helium nucleus, despite the fact that a helium nucleus consists of just those very same four particles, bound together! So where did the extra mass go? It was converted to energy in the process of fusion. That's what happens in the sun (well, to simplify things).

What I was saying is that plain simple everyday MOTION in a gravitational field is does not involve the conversion of mass to energy (and I'm not sure where you got the idea that it ought to). It involves the interchange of energy between only two forms: kinetic and potential. We generally call this total "energy of motion" (when gravitational fields are involved) mechanical energy. In a conservative field, in the absence of other forces (non conservative forces such as friction), mechanical energy is conserved. We say that gravity is a conservative force. The energy is not "lost" (note the quotes), i.e. converted to other non - mechanical forms such as heat, light, etc.

Quote Quote by michael879
However, what if the ball was never put there. What if the ball has never been on earth, it started on the other side of the universe. What about when it starts to accelerate towards earth. why would it have potential energy in this case?
Consider an analogy: I want to assemble a system of electric charges. The system includes two unlike charges separated by a distance r. In order to separate them, I need to do work, because there is an attractive force trying to keep them together. So are you surprised that when I take away whatever is keeping them separated, they shoot back together? Do you ask, where did the energy come from? No, because you saw me put in a lot of work to painstakingly assemble this system. The mere existence of this system of two charges and their associated fields, separated by a distance, means that there is certain amount of energy stored in it, equal to that work. We usually associate this energy "with the electric field", because knowing the strength of the total electric field of this system of charges, we can calculate the electrostatic energy. What if I want to move my charges really far apart? Your first impulse would be to say that I'd have to do a hell of a lot of work to separate them this huge distance. But remember that the attractive force between them diminishes, the farther away they get. So if the second charge starts out* really far away in the first place, is there some potential at that far away point (essentially, "at infinity") due to the field of the first charge? Yes. Is it small enough to be considered essentially zero? Yes. Will the second charge accelerate towards the first if they are separated by this vast distance? No. But the potential due to the Earth's/source charge's field at intermediate points between the earth and the ball is higher, so if it somehow ends up at those intermediate points (closer to the earth), its potential energy will increase.

*I know what you're going to ask about, that's why I starred it. How might the two charges "start out" that far apart in the first place? Well, consider what you are asking me. We were talking about assembling a system of masses (or charges). Then what did you do? You went and extended that "system" to include...


And now you are asking me: Where did the energy come from to "assemble" this system?!?!? I.e., why does matter exist, and why are objects in their current locations/how did they get there/what is their entire history since the beginning of time? Sorry, not qualified to answer that. If you're satisfied with "big bang", then fine.