Understanding Gravitational Energy in Motion

[Naty1]

A mass has gravitational energy because of its motion.

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?

 

[HallsofIvy]

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.

 

[Crazy Tosser]

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

 

[George Jones]

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?

Is

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

related to what you have in mind?

 

[Naty1]

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

According to the general theory of relativity, kinetic energy contributes to gravitational mass...

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":

Passive gravitational mass is a measure of the strength of an object's interaction with a gravitational field. Within the same gravitational field, an object with a smaller passive gravitational mass experiences a smaller force than an object with a larger passive gravitational mass.
Active gravitational mass is a measure of the strength of the gravitational field due to a particular object. For example, the gravitational field that one experiences on the Moon is weaker than that of the Earth because the Moon has less active gravitational mass.
Although inertial mass, passive gravitational mass and active gravitational mass are conceptually distinct, no experiment has ever unambiguously demonstrated any difference between them. Newton's third law implies that active and passive gravitational mass must always be identical (or at least proportional), but the classical theory offers no compelling reason why the gravitational mass has to equal the inertial mass.

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??

 

[HallsofIvy]

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.

 

[Gnosis]

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.

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?