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Gravity in relation to mass & energy

by Denton
Tags: energy, gravity, mass, relation
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Denton
#1
Jul29-08, 06:36 AM
P: 114
I was thinking, a particle a certain distance from another would have a certain potential energy. Now the potential energy must be in the form of extra mass if im not mistaken (as its not kinetic, however does kinetic energy increase mass?) so therefore an object is at its maximum mass at infinity and at its lowest at 0.

I can then conclude that the at the pre universe, the 'singularity' or whatever you call it would have had less mass than the current universe. Does this mean that the early universe had less energy than the present day?
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Denton
#2
Jul31-08, 07:00 AM
P: 114
Was this too hard, or was i way off? My reasoning comes from the fact that you add energy to an atom in an excited state, its mass increases by E / c^2. Following this im concluding that GPE is stored in mass also. (disregard my previous post asking if kinetic energy increases mass, ive blatantly overlooked special relativity :)

If this is correct then what i stated must be true, the pre big bang had less mass than it does today.
lightarrow
#3
Jul31-08, 07:28 AM
P: 1,521
Quote Quote by Denton View Post
I was thinking, a particle a certain distance from another would have a certain potential energy. Now the potential energy must be in the form of extra mass if im not mistaken (as its not kinetic, however does kinetic energy increase mass?) so therefore an object is at its maximum mass at infinity and at its lowest at 0.

I can then conclude that the at the pre universe, the 'singularity' or whatever you call it would have had less mass than the current universe. Does this mean that the early universe had less energy than the present day?
When two particles approach themselves from infinity, the system of the two particles looses mass, in the form of energy radiated away. Where do this energy go? Of course it can't escape the universe, so the total energy of the universe doesn't change (at least, not for this reason).

Denton
#4
Jul31-08, 08:25 AM
P: 114
Gravity in relation to mass & energy

As gravitational waves?
Ich
#5
Jul31-08, 10:13 AM
Sci Advisor
P: 1,910
GPE is negative stored mass. Kinetic Energy is positive. If the Objects are free floating, both parts change simultanely, conserving the total energy. If you bring the particles to rest wrt each other, you gain energy (from KE) in form of explosions, heat, or something usable. Radiate it away, and the mass of the System decreases. So, objects at rest in a gravitational field have less mass.
Mentz114
#6
Jul31-08, 11:04 AM
PF Gold
P: 4,087
Denton:
Now the potential energy must be in the form of extra mass if I'm not mistaken
I've never heard that before. Where did you get it ?

Ich:
So, objects at rest in a gravitational field have less mass.
Does this mean as something accelerates in a gravitational field it loses (inertial?) mass ?
lightarrow
#7
Jul31-08, 03:40 PM
P: 1,521
Quote Quote by Denton View Post
As gravitational waves?
I don't see others possibilities.
cryptic
#8
Jul31-08, 04:17 PM
P: 34
Quote Quote by lightarrow View Post
I don't see others possibilities.
Does it mean the planets lose continuously their mass because of orbital motion (acceleration)?
Denton
#9
Jul31-08, 11:10 PM
P: 114
Does it mean the planets lose continuously their mass because of orbital motion (acceleration)?
Well they're not changing their GPE.

GPE is negative stored mass.
Im not sure how to take this, are you saying this purely mathematical or realistic?
Ich
#10
Aug1-08, 02:07 AM
Sci Advisor
P: 1,910
Quote Quote by Mentz114
Does this mean as something accelerates in a gravitational field it loses (inertial?) mass ?
I'd say it loses rest mass and gains KE. Its total energy is conserved. This should be true at least in weak fields, when calculated in an inertial frame. Unfortunately, the people that would point out all the caveats are no longer around.


Quote Quote by Denton
Quote Quote by Ich
GPE is negative stored mass.
Im not sure how to take this, are you saying this purely mathematical or realistic?
Ok, if you bring two masses together, you gain energy. You can carry the energy away, and what's left ist a system with less energy (also less mass) than before. That deficit is called binding energy, which is negative. So I'm actually talking about mass defect here, not strictly potential energy.
cryptic
#11
Aug1-08, 09:33 AM
P: 34
Quote Quote by Ich View Post
I'd say it loses rest mass and gains KE. Its total energy is conserved.
...
But what kind of mass is it? The number of particles does not change . Have particles less mass if they come nearer and why?
snoopies622
#12
Aug1-08, 10:41 AM
P: 611
I asked a question similar to this months ago.

http://www.physicsforums.com/showthread.php?t=226703

It went something like, if I lift an object does its mass increase due to increased potential energy? There was disagreement about whether a change in GPE was stored in the object, in both the object and the Earth, or "in the field", which puzzled me because I didn't know how an increase in energy -- which should also be an increase in mass -- could appear in a field and not concentrated in a particular location. Then someone posted a link to another thread

http://www.physicsforums.com/showthread.php?t=57502

which mentioned that in GR mass/energy is not even defined in a consistent way, or it depends on your state of motion, Killing vectors, etc...and it quickly went over my head.

You might find these threads helpful. My impression is that there is no simple answer to this kind of question.
lightarrow
#13
Aug1-08, 12:38 PM
P: 1,521
Quote Quote by cryptic View Post
But what kind of mass is it? The number of particles does not change . Have particles less mass if they come nearer and why?
No, their mass don't change. The mass is not only in the particles but in the field too.
MeJennifer
#14
Aug1-08, 12:51 PM
P: 2,043
Quote Quote by lightarrow View Post
No, their mass don't change. The mass is not only in the particles but in the field too.
I think I know what you are getting at but how do you rationalize that this position holds up to the fact that what you call 'the field' is on the other side of the Einstein field equations?
cryptic
#15
Aug1-08, 01:06 PM
P: 34
Quote Quote by lightarrow View Post
No, their mass don't change. The mass is not only in the particles but in the field too.
This is true only in case of electromagnetic field, because em-field disappears completly if partikels move very close together whereas gravitational field does not change.
lightarrow
#16
Aug1-08, 03:22 PM
P: 1,521
Quote Quote by MeJennifer View Post
I think I know what you are getting at but how do you rationalize that this position holds up to the fact that what you call 'the field' is on the other side of the Einstein field equations?
Sorry, could you please explain better what you mean?
cryptic
#17
Aug2-08, 12:43 PM
P: 34
Quote Quote by snoopies622 View Post
...

It went something like, if I lift an object does its mass increase due to increased potential energy?
...
We can use object's mass (heat) to increase its GPE. In this case system has less mass - mass is converted to GPE - but if object falls GPE is converted back to mass (heat).
cryptic
#18
Aug5-08, 08:20 AM
P: 34
Quote Quote by lightarrow View Post
No, their mass don't change. The mass is not only in the particles but in the field too.
You are right, gravitational field has mass too. And the question remains, how is this mass induced in "empty" space.


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