Question about conservation of energy

[gib65]

I've been reading a paper by someone who thinks gravity violates the law of conservation of energy, and I can't quite find the flaw in his reasoning. He says that because the pulling force of gravity never ceases and that it doesn't seem to get the energy to apply this pulling force from anywhere external to itself, it represents a source of contiuously generated energy that comes out of nothing. It would be analogous to someone spinning a rock tied to a string around in circles forever without ever consuming energy in anyway in order to keep up the energy required to spin this rock.

 

I've often wondered about this myself. When I took physics in my first year at university, I asked my professor about this apearant paradox between gravity and the law of conservation of energy. He spouted off the same argument I heard before about potential energy. That is, in order for something to exhaust kinetic energy when falling under the influence of gravity, it must have consumed energy in order to be lifted to the level from which it fell. At that level it stores this energy as potential, and when it falls it gets converted to kinetic. Thus no energy is introduced from a void, it is only converted from one form to another. This argument can be applied to similar phenomena such as swinging pendulums and the orbit of bodies around more massive bodies.

 

I understand this, but there are other thought experiments that I've done that can't be explained by this reasoning. One, for instance, is when you have a small body floating along in space far, far away from any massive body exerting gravitational influence. albeit if the small body and larger body both coexist in the same universe, they would have to exert SOME gravitational influence on each other, but let's just say this influence is minute since they are so far apart. Now, suppose the smaller body is in motion towards the larger body, and although not under its gravitational influence by that much, it will become more and more influenced as it gets closer. And as it gets closer it begins to speed up. Eventually it will be close enough for its velocity to be markedly much more than it used to be when it was extremely far away. Now, I suppose that one could explain the seemingly spontaneous introduction of more and more kinetic energy into the smaller body as really just a gradual transformation of potential energy into kinetic energy, but that presupposes that the small body had this potential energy from the very begining. Well, maybe it did. But then where did it get THAT energy from? Furthermore, what do we say about another small body travelling with the same initial velocity in the same direction, but starting from further away? By the time it gets to the larger body, it should be travelling even faster than the first small body. Do we just say it had MORE potential energy initially than the first small body? Ultimately, there is no end to how much potential energy we could presume existed in any body's initial state in order to make up for the appearance of kinetic energy once it falls under the influence of gravity. It just seems too ad hoc to me.

 

I'm sure there is a reasonable explanation to my inquiry. Can anyone explain this?

[chadn]

I understand this, but there are other thought experiments that I've done that can't be explained by this reasoning. One, for instance, is when you have a small body floating along in space far, far away from any massive body exerting gravitational influence. albeit if the small body and larger body both coexist in the same universe, they would have to exert SOME gravitational influence on each other, but let's just say this influence is minute since they are so far apart. Now, suppose the smaller body is in motion towards the larger body, and although not under its gravitational influence by that much, it will become more and more influenced as it gets closer. And as it gets closer it begins to speed up. Eventually it will be close enough for its velocity to be markedly much more than it used to be when it was extremely far away. Now, I suppose that one could explain the seemingly spontaneous introduction of more and more kinetic energy into the smaller body as really just a gradual transformation of potential energy into kinetic energy, but that presupposes that the small body had this potential energy from the very begining. Well, maybe it did. But then where did it get THAT energy from? Furthermore, what do we say about another small body travelling with the same initial velocity in the same direction, but starting from further away? By the time it gets to the larger body, it should be travelling even faster than the first small body. Do we just say it had MORE potential energy initially than the first small body? Ultimately, there is no end to how much potential energy we could presume existed in any body's initial state in order to make up for the appearance of kinetic energy once it falls under the influence of gravity. It just seems too ad hoc to me.

 

The only way I can think of to solve this is to take into consideration the Big Bang. The Big Bang and the expansion of the universe results in all matter and energy being sepperated, right? So say you have two bodies, the big bang spreads out all the matter in the universe, which is equivalent to me lifting a rock, Im sepperating the rock from the earth, thus increasing its potential energy. But this all is a total guess on my part.

[swansont]

We just had this discussion. Energy doesn't get used up, so there is no need for a source.