[Pmb]

Does Gravity do any work?  I think there is a school of thought that believes that Gravity does no work.  It appears to me that the concept may be in error.  I welcome comments.  Thanks.  Joe L. Ogan

I’m amazed at how much discussion there is on such a simple question. The answer to your question is yes. Gravity does work. And that is a result which both Newtonian Gravity and General Relativity (GR) give. If you’d like a textual statement of this then you can look on page 187 in the famous GR text “Gravitation,” by Misner, Thorne and Wheeler. The authors use work to describe gravitational red shift on a photon moving through a gravitational field. On page 187 the authors write

-------------------
The drop in energy because of work done against gravitation implies drop in frequency…
-------------------

Is that really true?  That is a rather clear definition of Gravity.  I wonder why Einstein didn't think of that.  Thanks.  Joe L. Ogan

I disagree with his definition. Hence why Einstein never said it. Gravity is not the same as matter. Matter is the “source” of gravity. Gravity also has potential energy in it and since energy has matter it contributes to itself. So gravity is also a source of gravity in this sense. But to say they are the same is wrong.

[Geezer]

I believe Joe's question may have resulted from a suggestion that gravity somehow does work to maintain bodies in orbit.

Joe, please correct me if that was not the thrust of your question. 

[Geezer]

Does Gravity do any work?  I think there is a school of thought that believes that Gravity does no work.  It appears to me that the concept may be in error.  I welcome comments.  Thanks.  Joe L. Ogan

I’m amazed at how much discussion there is on such a simple question. The answer to your question is yes. Gravity does work. And that is a result which both Newtonian Gravity and General Relativity (GR) give. If you’d like a textual statement of this then you can look on page 187 in the famous GR text “Gravitation,” by Misner, Thorne and Wheeler. The authors use work to describe gravitational red shift on a photon moving through a gravitational field. On page 187 the authors write

-------------------
The drop in energy because of work done against gravitation implies drop in frequency…
-------------------

Is that really true?  That is a rather clear definition of Gravity.  I wonder why Einstein didn't think of that.  Thanks.  Joe L. Ogan

I disagree with his definition. Hence why Einstein never said it. Gravity is not the same as matter. Matter is the “source” of gravity. Gravity also has potential energy in it and since energy has matter it contributes to itself. So gravity is also a source of gravity in this sense. But to say they are the same is wrong.

Pmb, please try to direct your comments to the author of the definition. "his definition" is ambiguous. I don't think it was Joe's definition.

Thanks

Mod.

[Joe L. Ogan (OP)]

No, I did not say that Einstein said that Gravity was the same as Matter.  Einstein said that he did not know what Gravity is.  Obviously Gravity can not be the same as Matter.  It is rather interesting that there is so much disagreement in the Scientific circles.   Thanks for your information.  Joe L. Ogan

[Pmb]

Pmb, please try to direct your comments to the author of the definition. "his definition" is ambiguous. I don't think it was Joe's definition.

Thanks

Mod.

Sorry. When I said that and said and followed it by "Hence why Einstein never said it." I was referring to the definition given by Mr. Scientist. That's why I wrote "Gravity is not the same as matter."

I'll be clearer next time. Thanks.

It is rather interesting that there is so much disagreement in the Scientific circles.  I believe that some of you need to go back and review the basics.

There is no disagreement in scientific circles. E.g. pick up any text on general relativity and they all agree on the physics I spoke of.

And thanks, but I don't need to review the basics.

[LeeE]

Wow! - six consecutive posts in this thread from the same person.  I think that must be a record.

Is anyone any clearer on whether gravity does any work yet?

[Madidus_Scientia]

I know, as if the frequent double/triple-posting wasn't enough

[Geezer]

Wow! - six consecutive posts in this thread from the same person.  I think that must be a record.

Is anyone any clearer on whether gravity does any work yet?

Six eh? Good catch! I missed that. BTW, Mr S has elected not to renew his membership at this time.

[Farsight]

Can we all agree on the following:

1) When a spacecraft orbits a planet in uniform circular motion, gravity is doing no work.

2) When a spacecraft performs a "slingshot" gravity-assist, and gains extra velocity that propels it towards Neptune, gravity is doing work.

3) When a spacecraft orbits a planet in an elliptical orbit, the question of whether gravity is doing work is debateable. One might say that gravity is "doing work" and thence "undoing work". Alternatively one might say that this is a stable configuration from which no energy is being removed, and hence gravity is doing no work.

IMHO the moot point concerning 3) above is that this situation is akin to a harmonic oscillator. The question is then "Does a harmonoc oscillator do work?". See http://en.wikipedia.org/wiki/Harmonic_oscillator. Since the photon is a form of harmonic oscillator, we then have to ask whether a photon does work when it simply travels through space. I think it's interesting how such a simple question can be so deep.

Pmb: I think it's better that the discussion should be carried on its merits rather than by deference to a 30-year-old textbook, particularly since at least one of the authors advocates time travel, which remains an unproven and unscientific conjecture.

[Joe L. Ogan (OP)]

Pmb, please try to direct your comments to the author of the definition. "his definition" is ambiguous. I don't think it was Joe's definition.



Mod.

Sorry. When I said that and said and followed it by "Hence why Einstein never said it." I was referring to the definition given by Mr. Scientist. That's why I wrote "Gravity is not the same as matter."

I'll be clearer next time. Thanks.

It is rather interesting that there is so much disagreement in the Scientific circles.  I believe that some of you need to go back and review the basics.

There is no disagreement in scientific circles. E.g. pick up any text on general relativity and they all agree on the physics I spoke of.

And thanks, but I don't need to review the basics.

[Geezer]

Can we all agree on the following:

1) When a spacecraft orbits a planet in uniform circular motion, gravity is doing no work.

2) When a spacecraft performs a "slingshot" gravity-assist, and gains extra velocity that propels it towards Neptune, gravity is doing work.

3) When a spacecraft orbits a planet in an elliptical orbit, the question of whether gravity is doing work is debateable. One might say that gravity is "doing work" and thence "undoing work". Alternatively one might say that this is a stable configuration from which no energy is being removed, and hence gravity is doing no work.

I think that's a very fair summary of the situation.

At the risk of restating the blindingly obvious (I have a knack for that), would it be true to say that whenever the orbiting body is displaced in the direction of the gravitational force (as is the case for an elliptical orbit or a "slingshot") work is done. The energy to do the work is obtained from changes in the potential and kinetic energies of the components of the system?

[Farsight]

At the risk of restating the blindingly obvious (I have a knack for that), would it be true to say that whenever the orbiting body is displaced in the direction of the gravitational force (as is the case for an elliptical orbit or a "slingshot") work is done. The energy to do the work is obtained from changes in the potential and kinetic energies of the components of the system?

Personally I'd say yes for the slingshot, but not for the elliptical orbit, because the gravity isn't adding any energy to the system, and you're not getting any work out of it. I think of gravity as a "pseudoforce" for this reaon: the total energy of a system of two planets does not increase when they accelerate towards one another. But I guess the question "does gravity do work?" is somewhat ambiguous, so I wouldn't argue too strongly against somebody who said work is being done when  potential energy is changed to kinetic energy and vice versa. Interesting topic.   

[yor_on]

Per

Does Gravity do any work?  I think there is a school of thought that believes that Gravity does no work.  It appears to me that the concept may be in error.  I welcome comments.  Thanks.  Joe L. Ogan

Perhaps the real question is.

What is 'forces'?

we like to see 'events' as created by 'forces' that we can count on. But probing their smallest constituents they are nowhere to be found.

[Pmb]

Pmb: I think it's better that the discussion should be carried on its merits rather than by deference to a 30-year-old textbook, particularly since at least one of the authors advocates time travel, which remains an unproven and unscientific conjecture.

I see nothing wrong giving a textual example of what I had said. It was not meant as proof of what I said but merely extra reading material for those who'd care for it.

[LeeE]

Perhaps the real question is.

What is 'forces'?

I think this is a very good question indeed.  While we can explain what causes forces, the forces themselves don't actually seem to consist of anything i.e. they aren't an entity in themselves but just seem to be an effect.

[Geezer]

While we can explain what causes forces, the forces themselves don't actually seem to consist of anything i.e. they aren't an entity in themselves but just seem to be an effect.

I wish we really could explain what causes the force. I think it's fair to say we really don't know at this point. However, we are very good at describing the force in terms of the effects it produces.

[LeeE]

While we can explain what causes forces, the forces themselves don't actually seem to consist of anything i.e. they aren't an entity in themselves but just seem to be an effect.

I wish we really could explain what causes the force. I think it's fair to say we really don't know at this point. However, we are very good at describing the force in terms of the effects it produces.

I think it sort of depends at what level of causality we're talking about.

We can say, for example, that gravity is caused by the presence of non-zero rest-mass matter, but we can't say why non-zero rest-mass matter appears to result in gravity.

An intrinsic problem with top-down analysis is that it can only result in an abstract, which can only be defined in terms of itself i.e. it is what it is and it does what it does because it is what it is.  As you burrow further down through the analysis, each time you reach a new lower, or more fundamental level, you are able to say why the higher level does what it does, defined in terms of the new lower level, but the lowest level can only be defined in terms of what happens.

For example, if we say that the presence of non-zero rest-mass matter results in gravity, and that gravity is the consequence of the mediation of gravitons, we've explained how non-zero rest-mass matter results in gravity by defining it in terms of gravitons, but we're now stuck with explaining why the presence and behaviour of gravitons manifests itself as gravity.

(Edited to remove a redundant 'at' in the penultimate paragraph)

[om]

Those interested in the role of gravity in the cosmos should go to

http://www.thenakedscientists.com/forum/index.php?topic=9197.0

To recap a small part:

1. Gravity is a nuclear force since essentially all mass is in the nucleus.

2. Gravity is a weak force over long distances.

3. Gravity is a very strong force over very short distances, but

4. Gravity cannot overcome repulsive forces between neutrons to convert neutron stars into black holes.

http://arxiv.org/pdf/0905.1667v1

In the Sun and in the cosmos, dynamic competition between long-range attractive gravitational forces and short-range repulsive forces between neutrons powers the objects that release energy and fill interstellar space with Hydrogen, a neutron-decay product.

If the universe is finite, then neutrons themselves may be the particle-sized black holes that were made in a Big Bang and compressed into massive, highly energetic neutron stars.

If the universe is infinite, then it may oscillate between expansion as interstellar space is filled with Hydrogen from neutron decay, and contraction after the neutron stars have evaporated and gravitational forces become dominant over repulsive forces between neutrons.

With kind regards,
Oliver K. Manuel
http://myprofile.cos.com/manuelo09

[yor_on]

Yep LeeE. It's very frustrating

Or if you accept the Higgs field then gravity 'accumulates' around mass. But where would that field come from, and why would it work at all. What's guaranteeing mass to exist even if that field existed? When we see two phenomena directly related to each other it is easy to wonder what came 'first', or did they came 'together'?

That's one of the reasons why I like 'emergences', as it allows them to come 'together' begetting new 'property's' (water to ice)

But if we look at it as having a 'beginning' involving 'forces' then we treat it as a chain of occurrences from a beginning to an end, and then that first 'force' must contain it all, in some manner of speaking, as from it all other will come.

Maybe there are other ways to look at it too?

[questioner]

Is gravity doing work if the energy that creates the action is within the atom to start with.