# Source of potential energy of body made in space falling

1. Sep 15, 2015

### NotASmurf

Hey all, question about gravitational potential energy. If an object begins on ground then gains height when it falls then energy it consumes to fall down was the potential created by moving upwards, if an object was created in space, then moves gravitationally towards a celestial body where does it get the energy from? Does it use mass-energy? Any help appreciated.

2. Sep 15, 2015

### BvU

It gets the energy from that celestial body. And vice versa. If you want to separate them again, you'll need to put back in that same energy !

3. Sep 15, 2015

### NotASmurf

White is finite right. So can anything ever have 0 energy due to it all being used in circumstances like this and then gravity wouldn't apply to the scenario in my question?

4. Sep 15, 2015

### BvU

I'm afraid I can't follow. The energy you can pick up from falling towards a celestial body is finite, yes. After all, you bump into the thing at a certain painful moment. But it's not so that the celestial body gets tired from attracting falling objects. It just gets heavier.

5. Sep 15, 2015

### NotASmurf

Sorry i'm bad at phrasing today, what I meant was that object has E=mgh, and it gains that from the energy it uses to increase h, assuming it started on the ground with zero relative potential, then when it falls it uses that energy. But my point was that if two objects where created in a vacuum next to one another with no kinetic energy or potential since they haven't 'climbed' anywhere they still move towards each other, my question was what energy was converted to the kinetic energy

6. Sep 15, 2015

### BvU

Good question ! In fact so fundamental that I'll have to pass it to the theoreticians ! @Orodruin , @mfb ? Something with the energy of the gravitational field or so. But perhaps there is a simpler answer and I'm overlooking it. Glad I responded, now I also get an alert if some genuine wizard has a good answer !

But I can still comment on the first part: for convenience and by convention we define the potential energy from gravity to be zero at infinite separation. It's just a convention, because you can't feel potential energy. Forces, etcetera, anything noticeable depends on energy differences .

7. Sep 15, 2015

### Orodruin

Staff Emeritus
First of all, you really cannot create something out of nothing. There are conservation laws that prohibit this.

If you create something by somehow adding the appropriate amount of energy to the system, it will take more energy to create the object at a higher potential than it would to create it at a lower potential.

8. Sep 15, 2015

### NotASmurf

Alright, but in the beginning (after all the first epochs and everything solidified) and body A was far away from body B (the scenario you mentioned where the energy was effectively zero) and it moved towards body B by virtue of kinetic energy which was not converted from potential, then it stopped above a planet, then it becomes attracted to it, initially when it was created it wouldn't take more energy to produce by virtue of potential as the distance would be large enough for it to be negligible, but then it would still fall towards the planet.

9. Sep 15, 2015

### Orodruin

Staff Emeritus
Yes it would take more energy. The potential for two close objects is negative.

10. Sep 15, 2015

### NotASmurf

In last message I meant if they were far away