# Gravitational energy

1. Dec 20, 2008

### Naty1

I made a note of the above quote a while ago and don't recall the source. I did not understand it then and in coming across it again I still don't...Can anyone explain?? Does this not completely ignore gravitational potential energy?

2. Dec 20, 2008

### HallsofIvy

Staff Emeritus
First you will have to explain what is meant by "gravitational energy". An object has kinetic energy because of its motion and potential energy because of its position. Both can, of course, be given to an object by gravity but i have never seen the term "gravitational energy" before.

3. Dec 20, 2008

### Crazy Tosser

Maybe they meant that a fast-moving object's mass increases, and thus it creates a bigger gravitational curvature.

4. Dec 20, 2008

### George Jones

Staff Emeritus
Is

http://arxiv.org/abs/gr-qc/9909014

related to what you have in mind?

5. Dec 20, 2008

### Naty1

George...Thanks for the reference, ....the first line

is much more clearly worded...now that I can understand!!!

When I just checked Wikipedia for 'mass' to see if that was my original source I found an interesting "distinction":

Now I get the potential distinction between inertial and gravitational mass, but the "active" versus "passive" is a new one on me...sounds like splitting hairs.....they don't do this for electromagnetic charge to my knowledge....so why here?? They could be merely proportional??

6. Dec 20, 2008

### HallsofIvy

Staff Emeritus
No, in order that all objects fall at the same rate the two "kinds" of mass must be exactly the same. They don't make that distinction for electric charge because the there is no "passive charge"- it is still inertial mass that determines how a charged body "reacts" to the electromagnetic force.

7. Dec 22, 2008

### Gnosis

If by "fall" you are referring to "free-fall" within a gravitational field, the "kinds" of mass do not have to be the same in the least.

For instance, a 1 kg lead ball and a 1 kg aluminum ball will free-fall at precisely the same rates (in a vacuum) just as a feather and a hammer will free-fall at identical rates in a vacuum (as demonstrated by astronaut David Scott while standing on the surface of the moon). Clearly, the feather and the screwdriver are entirely different in every manner except having a structure composed of atoms, but even the types of atoms are different and yet they free-fall at identical rates.

Would you please clarify what you meant?