Does Gravity do any work?

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Offline PhysBang

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Does Gravity do any work?
« Reply #100 on: 10/01/2010 00:04:00 »
The recording facilities are at your single observer's location, PhysBang. It's not sufficient to claim there's some kind of distortion between you and the astronauts that invalidates the evidence you observe. You can give each astronaut their own recording facilities, and see from afar that astronaut 1's tape moves slower, just like his light moves slower. When you retrieve their tapes you see that tape 1 has recorded say 999 feet as opposed to 1000 feet for astronaut 2, and that this tallies with what you saw during the experiment. It adds up to direct observable evidence that in a location where the gravitational potential is lower, the light goes slower.
The problem with what you say here is that you have not given any rules for when we collect the tapes. Do we collect the tapes according to how long they have been operating in their respective reference frames? Do we collect the tapes according to what we determine to be simultaneous in the reference frame of a third-party observer? These rules have to be laid out.

It is not possible to simply "see" what is going on at a distant location. The light from these distant locations will undergo a number of effects that must be accounted for to remove distortion. Relativity theory is not a theory about this distortion, it is about the rules for the descriptions of the time and place of events.
Quote
Einstein uses differential geometry, but he doesn't actually mention curved spacetime in The Foundation of the General Theory of Relativity. You can interpret the curvilinear motion as curved spacetime, but you mustn't let this distract you from the evidence that's in accord with what Einstein actually said. Here's a corrected translation from section 22 of Relativity: The Special and General Theory:

"In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the speed of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity. A curvature of rays of light can only take place when the speed of light varies with location."

Einstein's non-constant guv is observable as a non-constant speed of light in GPS and the Shapiro delay, as well as in the Gedankenexperiment I've described here.   
guv is not something that can simply be observed. It is a factor of the metric used to determine the distance between points of spacetime. It is used in this manner whether or not we are considering things in motion. It is the essence of curved spacetime and indeed can mean nothing else.

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Offline yor_on

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Does Gravity do any work?
« Reply #101 on: 10/01/2010 02:41:55 »
Okay, lets start with defining what work is.

"Work can be defined as transfer of energy. In physics we say that work is done on an object when you transfer energy to that object. If one object transfers (gives) energy to a second object, then the first object does work on the second object. "

work_energy_power


So, what do we mean by 'energy' in physics?

"There is no absolute measure of energy, because energy is defined as the work that one system does (or can do) on another. Thus, only the transition of a system from one state into another can be defined and thus measured."

Energy

And what do we mean by 'system' in physics?

Well, as far as I can see a 'system' seem to be about anything you can give a coherent description. So let us assume that gravity can be defined as a 'system'

So, do gravity transfer energy?

If gravity can be defined as a system, then it transfers energy, as energy is defined as "the work that one system does (or can do) on another" Notice how the two statements go into each other.

But does it go both ways?

Can you transfer energy to gravity by motion, that is by accelerating in a gravitational field? Well the gravitational field locally in the rocket will increase, right?

But do you then say that the gravity as a field or 'system' gets an added energy?


============

As for what a 'force' is in physics is rather simple.

"In physics, the concept of force is used to describe how a massive body is affected by acceleration or mechanical stress. Force can also be described by intuitive concepts such as a push or pull that can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate, or which can cause a flexible object to deform"

And gravity when working from that definition is a 'force' as it affects 'massive bodies'.

"What we now call gravity was not identified as a universal force until the work of Isaac Newton. Before Newton, the tendency for objects to fall towards the Earth was not understood to be related to the motions of celestial objects. Galileo was instrumental in describing the characteristics of falling objects by determining that the acceleration of every object in free-fall was constant and independent of the mass of the object. Today, this acceleration due to gravity towards the surface of the Earth is usually designated as \vec{g} and has a magnitude of about 9.81 meters per second squared (this measurement is taken from sea level and may vary depending on location), and points toward the center of the Earth. This observation means that the force of gravity on an object at the Earth's surface is directly proportional to the object's mass."

Force and Gravity

So defined like that gravity must be a 'force'

Now, the problem with those definitions is that we normally associate 'force' with matter of some kind transforming into energy, but when we speak about gravity there in no 'transforming'. Seen as a field gravity just is, although it will vary due to mass (black hole), acceleration (rocket), and uniform motion (Earth, more or less)

But if we accept those definitions then gravity has to be a 'force'.
But the definitions all go into each other, and none of them seem to 'isolate' what's happening.
« Last Edit: 10/01/2010 02:59:30 by yor_on »
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Offline Geezer

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Does Gravity do any work?
« Reply #102 on: 10/01/2010 06:18:40 »
Yes!

But does gravity do any work?
There ain'ta no sanity clause, and there ain'ta no centrifugal force ćther.

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Offline LeeE

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Does Gravity do any work?
« Reply #103 on: 10/01/2010 14:24:49 »
Yes!

But does gravity do any work?

Aha, so we're back to that old chestnut [:D]
...And its claws are as big as cups, and for some reason it's got a tremendous fear of stamps! And Mrs Doyle was telling me it's got magnets on its tail, so if you're made out of metal it can attach itself to you! And instead of a mouth it's got four arses!

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Offline Farsight

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Does Gravity do any work?
« Reply #104 on: 10/01/2010 15:33:02 »
Are we drifting from the original question here? Should we split some of this great discussion into a new or different topic (assuming I can figure out how to do that without fouling everything up!) What does everyone think?
I think yes, we were drifting from the original question, though the thread seemed to have quietened. How about if I start a new thread containing my "elucidation" along with some sort of question that invites discussion, and stick to "Does gravity do any work" here. Feel free to modify as you feel fit.


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Offline Geezer

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Does Gravity do any work?
« Reply #105 on: 10/01/2010 17:07:46 »
Are we drifting from the original question here? Should we split some of this great discussion into a new or different topic (assuming I can figure out how to do that without fouling everything up!) What does everyone think?
I think yes, we were drifting from the original question, though the thread seemed to have quietened. How about if I start a new thread containing my "elucidation" along with some sort of question that invites discussion, and stick to "Does gravity do any work" here. Feel free to modify as you feel fit.



OK - That would WORK too.
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Offline yor_on

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Does Gravity do any work?
« Reply #106 on: 10/01/2010 17:17:59 »
Yes!

But does gravity do any work?

As defined there it does, but if gravity is a direct result of mass, with acceleration and uniform motion being a 'secondary result' of us having three dimensions in an arrow of time, creating SpaceTime?

If mass is what creates 'space' (with our arrow of time) then 'gravity' is no ordinary 'force' to me, it's more of an invisible field where matter and 'energy' will introduce strains in that field. But it doesn't explain why gravity can increase locally inside that rocket, does it?

-----
But with the Higgs field included it might.
« Last Edit: 10/01/2010 17:19:32 by yor_on »
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Offline Farsight

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Does Gravity do any work?
« Reply #107 on: 11/01/2010 12:31:04 »
OK guys, I think this nice simple explanation nails it. See what you think:

Does gravity do any work? No. Imagine you carry a 10kg cannonball up a 100m tower. You puff and pant all the way up the stairs until finally you make it to the top. You did work, and you can feel the sweat running down the inside of your shirt. The work you've done on the cannonball has now given this cannonball potential energy.  Don't worry about where it is or how it's stored, we can come back to that later. The point is that when you lean over the balcony and let go of the cannonball, that potential energy is converted into kinetic energy as the cannonball falls to earth. Gravity effects this conversion from one form of energy to another, but it doesn't add any energy. It didn't do the work, you did.

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Offline Geezer

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Does Gravity do any work?
« Reply #108 on: 11/01/2010 18:30:27 »
OK guys, I think this nice simple explanation nails it. See what you think:

Does gravity do any work? No. Imagine you carry a 10kg cannonball up a 100m tower. You puff and pant all the way up the stairs until finally you make it to the top. You did work, and you can feel the sweat running down the inside of your shirt. The work you've done on the cannonball has now given this cannonball potential energy.  Don't worry about where it is or how it's stored, we can come back to that later. The point is that when you lean over the balcony and let go of the cannonball, that potential energy is converted into kinetic energy as the cannonball falls to earth. Gravity effects this conversion from one form of energy to another, but it doesn't add any energy. It didn't do the work, you did.

You're joking, right?

By definition, work is the change in kinetic energy of a body.
There ain'ta no sanity clause, and there ain'ta no centrifugal force ćther.

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Offline Madidus_Scientia

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Does Gravity do any work?
« Reply #109 on: 11/01/2010 18:53:00 »
lol

So the combustion of fuel in an engine doesn't do any work, it was the organisms that decomposed into oil millions of years ago that actually cause a car to move.
« Last Edit: 11/01/2010 18:59:49 by Madidus_Scientia »

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Offline Farsight

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« Reply #110 on: 12/01/2010 16:23:56 »
Madidus: yes of course the combustion of fuel does work. The "system" that is a gallon of petrol loses energy to the car and gets spat out as exhaust fumes. It isn't a gallon of petrol any more.

You're joking, right? By definition, work is the change in kinetic energy of a body.
I'm not joking, geezer. Look at what yor-on said above: Work can be defined as transfer of energy. In physics we say that work is done on an object when you transfer energy to that object. We transfer energy from that gallon of fuel to the car, so work is done. But drop that cannonball, and we transfer energy from the cannonball... to the cannonball. Look at http://en.wikipedia.org/wiki/Work_(physics)#Mechanical_energy and the bit where it says:

The mechanical energy of a body is that part of its total energy which is subject to change by mechanical work. It includes kinetic energy and potential energy... The principle of conservation of mechanical energy states that, if a system is subject only to conservative forces (e.g. only to a gravitational force), or if the sum of the work of all the other forces is zero, its total mechanical energy remains constant.

So yes, gravity gives that object some kinetic energy. But it came from the object's potential energy, so gravity didn't transfer any energy to the object. But I suppose it all depends on what the definition of work is. If you say it's a change in kinetic energy, then gravity does work. If you say it's change in an object's total energy, then gravity does no work. If you say it's force times distance, you're then left wondering whether gravity is a pseudoforce. For a pseudoforce, the force is always proportional to the mass of the object, which is the case for gravity. Think principle of equivalence, where Einstein was saying there's no force acting on a free-falling body, and instead the force is acting upward on a body that doesn't fall down. The object on the ground is accelerating, not the object in free-fall. That's why I take the total-energy view.


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Offline Madidus_Scientia

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Does Gravity do any work?
« Reply #111 on: 12/01/2010 16:52:23 »
Quote
Madidus: yes of course the combustion of fuel does work. The "system" that is a gallon of petrol loses energy to the car and gets spat out as exhaust fumes. It isn't a gallon of petrol any more.

The fuel is the stored energy. In the cannonball analogy the stored energy is the gravitational potential energy, which uses gravity to do work to convert the stored energy into kinetic energy, just as the fuel combusts to do the same. "It isn't a gallon of petrol any more" It isn't gravitational potential energy anymore.

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Offline Geezer

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Does Gravity do any work?
« Reply #112 on: 12/01/2010 18:29:35 »
Farsight, the same Wiki that you reference is quite clear on the subject.

Work is equal to the change in kinetic energy of a rigid body.

Gravity clearly altered the kinetic energy of the body. Therefore, gravity did work. The only way to argue that it didn't is to alter the definition of "work".

There ain'ta no sanity clause, and there ain'ta no centrifugal force ćther.

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Offline Farsight

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Does Gravity do any work?
« Reply #113 on: 13/01/2010 13:03:22 »
Madidus: I know what you mean. Burn the fuel, and the kinetic energy that's released drives the car. Drop the cannonball from the top of the tower and make it drive a wheel at the bottom, and it's the same situation.

Geezer: the definition of work is the problem here. If you look lower down on http://en.wikipedia.org/wiki/Work_(physics) you can see This definition is based on Sadi Carnot's 1824 definition of work as "weight lifted through a height". I had a look elsewhere and found multiple statements. Picking something at random, http://id.mind.net/~zona/mstm/physics/mechanics/energy/work/work.html says In physics we say that work is done on an object when you transfer energy to that object. For introductory thinking, this is the best definition of work. Another one at http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Work/DefinitionWork.html says When a force acts to move an object, we say that Work was done on the object by the Force. For our cannonball, these two statements are contradictory. If you just say W = F * d * cos θ and then ask whether gravity is a pseudoforce, you don't get a clear answer. See http://www.av8n.com/physics/fictitious-force.htm for an interesting article.

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Offline Geezer

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Does Gravity do any work?
« Reply #114 on: 13/01/2010 18:25:35 »
Geezer: the definition of work is the problem here. If you look lower down on http://en.wikipedia.org/wiki/Work_(physics) you can see This definition is based on Sadi Carnot's 1824 definition of work as "weight lifted through a height". I had a look elsewhere and found multiple statements. Picking something at random, http://id.mind.net/~zona/mstm/physics/mechanics/energy/work/work.html says In physics we say that work is done on an object when you transfer energy to that object. For introductory thinking, this is the best definition of work. Another one at http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Work/DefinitionWork.html says When a force acts to move an object, we say that Work was done on the object by the Force. For our cannonball, these two statements are contradictory. If you just say W = F * d * cos θ and then ask whether gravity is a pseudoforce, you don't get a clear answer. See http://www.av8n.com/physics/fictitious-force.htm for an interesting article.

I suspect the notion of transferring energy to a body is not necessarily correct. In the case where a ball is accelerated in outerspace, the ball has gained energy. However, in the case of lifting the ball to a greater height against the force of gravity, the ball only gained energy while it was being accelerated during the lifting process, and by the end of that process, it had lost that energy again. There was a redistribution of energy within the system that includes the ball and earth, but I don't think it's correct to say that the ball has gained energy (although I've probably said it often [:D]). Kinetic energy and potential energy are very different. Kinetic energy exists in the absence of gravitational force whereas potential energy only exists because of gravitational force.

I think the second definition is more accurate. Work was done when the ball was raised by the net force in the direction of motion and when the ball is released, work is done by the force of gravity to accelerate the mass of the ball and the mass of the Earth.

Whether or not gravity is a pseudoforce I'm not sure. But it seems to fit the definition of a force in terms of work.
« Last Edit: 14/01/2010 04:39:41 by Geezer »
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Offline yor_on

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Does Gravity do any work?
« Reply #115 on: 14/01/2010 15:48:38 »
Gravity is weird. I don't know what it is really but I believe it to be a 'field'. And as a field it exists in a 'continuum'. When we do something inside that 'field' it reacts 'instantly' as far as I know. Like 'inertia' shows us in outer space, That means if you change your rockets course, inertia will produce a instant 'gravitational effect' inside that rocket (frame of reference).

I really like this one.

It proves, at least to me :) that there still are some reasonable arguments versus the idea of Gravity propagating at 'c' as a 'force' (gravitons.)

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Offline Geezer

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Does Gravity do any work?
« Reply #116 on: 14/01/2010 17:26:11 »
Gravity is weird. I don't know what it is really but I believe it to be a 'field'. And as a field it exists in a 'continuum'. When we do something inside that 'field' it reacts 'instantly' as far as I know. Like 'inertia' shows us in outer space, That means if you change your rockets course, inertia will produce a instant 'gravitational effect' inside that rocket (frame of reference).

I really like this one.

It proves, at least to me :) that there still are some reasonable arguments versus the idea of Gravity propagating at 'c' as a 'force' (gravitons.)



Yoron - I agree. I don't happen to think gravity is a direct "force" between two bodies, and I'm not confident gravitons will ever be observed.

However, whether I'm right or wrong about that is of little consequence. When we talk about "gravitational force" in the context of work, it's just a convenient and sufficiently accurate way of describing the net force the bodies will experience due to gravity, or, if you prefer, due to mass interaction in spacetime.

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Offline yor_on

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« Reply #117 on: 14/01/2010 18:29:04 »
Yeah, when you look at the definitions I found, I can't help but think of permanent magnetism and wonder, anew, why that's not considered a 'force' too?
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Offline Geezer

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« Reply #118 on: 14/01/2010 18:54:28 »
Yeah, when you look at the definitions I found, I can't help but think of permanent magnetism and wonder, anew, why that's not considered a 'force' too?

"If it looks like a duck, and quacks like a duck, it's probably a duck!"

If we can anticipate a force and measure the force, it's probably OK to call it a force, even though we don't fully comprehend what causes the force.

May the Schwartz be with you.
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Offline Farsight

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« Reply #119 on: 15/01/2010 11:02:15 »
Geezer: hang the cannonball on a pendulum, and give it a push. You give it kinetic energy. As it swings upwards this gets converted into potential energy. As an alternative you could get some stepladders and carry the cannonball to the top of it swing. As to where this potential energy is, it's in the cannonball, not in the earth's gravitational field. This is obvious when you think of a cannonball way out in free space. It's beyond the earth's gravitational field, so its potential energy can't be in the earth's gravitational field. When however you move it and place it in the earth's gravitational field, it falls down, and then its potential energy gets converted into kinetic energy.

In terms of energy-conservation, a cold motionless cannonball at the surface of the earth is in a time-dilated environment as compared to similar cold motionless cannonball in free space, so all the atoms and electrons of the cannonball are moving at a reduced rate. Hence it has less total energy. The kinetic energy of the falling cannonball accounts for the difference. PS: I'm confident that gravitons won't be observed, because they essentially contradict relativity. 
« Last Edit: 15/01/2010 11:04:32 by Farsight »

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Offline Geezer

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« Reply #120 on: 15/01/2010 16:44:46 »
Farsight: The potential energy exists in the entire system that includes the ball, the Earth and the net force exerted between them.

Elevate the ball, as you say, to some stationary point above the Earth. Now remove the Earth. Does the ball go anywhere? I don't think so. What happened to the energy in the ball?

When you elevate the ball, you increase the potential energy of the ball (if there is a gravitational field) but do not forget that you have also increased the potential energy of the Earth by an equal amount. The Earth wants to fall towards the ball just as much as the ball wants to fall towards the Earth. It's only the inertia of the Earth that limits its acceleration towards the ball.

Potential energy only exists within a system, not within the individual components of the system. When you take the ball into outerspace, you have reduced the gravitational force so much that the ball, to all intents and purposes, is no longer part of the system.
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Offline Geezer

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« Reply #121 on: 15/01/2010 21:22:00 »
Farsight: We could also conduct this experiment.

Imagine two trucks sitting on a horizontal railway track. The trucks have very good axle bearings that do not produce any friction. We connect the two trucks by a long extensible spring.

Now we force the trucks apart with, say, a telescopic pole so that the spring is extended. Then we magically collapse the pole and observe what happens.

If the trucks have equal mass, they will both accelerate towards each other at the same rate.

We can load up one of the trucks so that it has a much greater mass than the other and repeat the experiment. The more massive truck will be accelerated less than the less massive truck. We can continue adding mass to the heavy truck to a point where its acceleration is imperceptible and only the lighter truck appears to accelerate.

In this model we did work by putting energy into the spring. The spring then did work by accelerating the trucks towards each other. While the trucks are stationary, the energy is clearly in the spring, not in either of the trucks. If you removed the extended spring from the trucks and kept it extended, it would still retain the energy.

In the case of gravity, there is a difference because the "spring" only exists by virtue of the masses. The energy is stored in the combined system, not in any single element of it.
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Offline questioner

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Does Gravity do any work?
« Reply #122 on: 16/01/2010 09:13:41 »
Does gravity do any work?
The energy that gravity uses appears to be inside every atom. Think of the energy in an atom as something like a spinning flywheel rotating at close to the speed of light with an insulating layer that stops it from reacting with other atoms around it.
Gravity could be a form of radiation (as yet undetected) that can penetrate all matter and upset this energy's equilibrium creating the action we know as the force of gravity.
This upsetting action is the actual work that gravity does. A weak force!

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Offline Geezer

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« Reply #123 on: 16/01/2010 17:20:20 »
Does gravity do any work?
The energy that gravity uses appears to be inside every atom. Think of the energy in an atom as something like a spinning flywheel rotating at close to the speed of light with an insulating layer that stops it from reacting with other atoms around it.
Gravity could be a form of radiation (as yet undetected) that can penetrate all matter and upset this energy's equilibrium creating the action we know as the force of gravity.
This upsetting action is the actual work that gravity does. A weak force!

Questioner: That does not seem to address the question about work. Perhaps you should start a new topic in "New Theories". Thanks!
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« Reply #124 on: 17/01/2010 01:20:54 »
Does gravity do any work?
The energy that gravity uses appears to be inside every atom. Think of the energy in an atom as something like a spinning flywheel rotating at close to the speed of light with an insulating layer that stops it from reacting with other atoms around it.
Gravity could be a form of radiation (as yet undetected) that can penetrate all matter and upset this energy's equilibrium creating the action we know as the force of gravity.
This upsetting action is the actual work that gravity does. A weak force!

Questioner: That does not seem to address the question about work. Perhaps you should start a new topic in "New Theories". Thanks!
Geezer : I think Gravity does it's work at the atomic level inside the atom. Is there any research in this area.

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Offline Geezer

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« Reply #125 on: 17/01/2010 02:22:14 »
Does gravity do any work?
The energy that gravity uses appears to be inside every atom. Think of the energy in an atom as something like a spinning flywheel rotating at close to the speed of light with an insulating layer that stops it from reacting with other atoms around it.
Gravity could be a form of radiation (as yet undetected) that can penetrate all matter and upset this energy's equilibrium creating the action we know as the force of gravity.
This upsetting action is the actual work that gravity does. A weak force!

Questioner: That does not seem to address the question about work. Perhaps you should start a new topic in "New Theories". Thanks!
Geezer : I think Gravity does it's work at the atomic level inside the atom. Is there any research in this area.

It might! But "work" in this context is an old mechanical concept which is defined as force times distance, or the change in kinetic energy of a rigid body. The discussion is not really about how gravity works. That is a very interesting question and there is much debate about it on this forum on other threads.
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Offline Pmb

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Does Gravity do any work?
« Reply #126 on: 17/01/2010 06:14:08 »
Does gravity do any work?
The energy that gravity uses appears to be inside every atom. Think of the energy in an atom as something like a spinning flywheel rotating at close to the speed of light with an insulating layer that stops it from reacting with other atoms around it.
Gravity could be a form of radiation (as yet undetected) that can penetrate all matter and upset this energy's equilibrium creating the action we know as the force of gravity.
This upsetting action is the actual work that gravity does. A weak force!

Questioner: That does not seem to address the question about work. Perhaps you should start a new topic in "New Theories". Thanks!
Geezer : I think Gravity does it's work at the atomic level inside the atom. Is there any research in this area.

It might! But "work" in this context is an old mechanical concept which is defined as force times distance, or the change in kinetic energy of a rigid body. The discussion is not really about how gravity works. That is a very interesting question and there is much debate about it on this forum on other threads.
I don't quite understand your reception to the idea that gravity can do work. Why do you believe this is

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« Reply #127 on: 17/01/2010 07:22:45 »
PMB: If you look up the definition for Mechanical Work (here is one example http://en.wikipedia.org/wiki/Work_(physics)  ) you will see that a force that produces a change in the kinetic energy of a rigid body has done (mechanical) work.

When you drop an object, gravity accelerates the object, therefore gravity has changed the kinetic energy of the object, therefore gravity has done work.
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Offline Farsight

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« Reply #128 on: 17/01/2010 08:55:19 »
Geezer, sorry to be slow getting back to you, my Internet has been playing up.

Farsight: The potential energy exists in the entire system that includes the ball, the Earth and the net force exerted between them. Elevate the ball, as you say, to some stationary point above the Earth. Now remove the Earth. Does the ball go anywhere? I don't think so. What happened to the energy in the ball?
No problem re the system, and no problem re the ball staying put. But the energy in the ball is still there. The ball is made of molecules / atoms / electrons etc, and they're all moving and spinning faster than they were when the ball was on the surface of the earth, where they were subject to gravitational time dilation.

When you elevate the ball, you increase the potential energy of the ball (if there is a gravitational field) but do not forget that you have also increased the potential energy of the Earth by an equal amount. The Earth wants to fall towards the ball just as much as the ball wants to fall towards the Earth. It's only the inertia of the Earth that limits its acceleration towards the ball.
Agreed. We could talk about separating two cannonballs or two planets etc, or even a shell of objects. We just tend to simplify things because the gravity caused by the cannonball is very slight and has no detectable effect on the Earth.

Potential energy only exists within a system, not within the individual components of the system. When you take the ball into outerspace, you have reduced the gravitational force so much that the ball, to all intents and purposes, is no longer part of the system.
True, we don’t talk about potential energy unless we’re talking about a system, and yes, once in outer space the ball is no longer part of the system. We wouldn’t talk about the potential energy of a ball on its own in space, but nevertheless, we can’t neglect conservation of energy.   

We could also conduct this experiment. Imagine two trucks sitting on a horizontal railway track. The trucks have very good axle bearings that do not produce any friction. We connect the two trucks by a long extensible spring. Now we force the trucks apart with, say, a telescopic pole so that the spring is extended. Then we magically collapse the pole and observe what happens. If the trucks have equal mass, they will both accelerate towards each other at the same rate.
No problem.

We can load up one of the trucks so that it has a much greater mass than the other and repeat the experiment. The more massive truck will be accelerated less than the less massive truck. We can continue adding mass to the heavy truck to a point where its acceleration is imperceptible and only the lighter truck appears to accelerate.
Yep, I'm with you.

In this model we did work by putting energy into the spring. The spring then did work by accelerating the trucks towards each other. While the trucks are stationary, the energy is clearly in the spring, not in either of the trucks. If you removed the extended spring from the trucks and kept it extended, it would still retain the energy.
Totally agree.

In the case of gravity, there is a difference because the "spring" only exists by virtue of the masses. The energy is stored in the combined system, not in any single element of it.
I'm sorry geezer, but for gravity, there is no spring. That demands action at-a-distance, which even Newton rejected. Separate the two trucks by a very large amount, and the spring doesn't pull the lighter truck back any more. I suppose one way to take a step from this analogy to what I've been saying, is to say that once the lighter truck gets to a certain distance, the spring comes unhooked from the heavier truck, and stays extended whilst the lighter truck carries it off. That means the spring is part of the lighter truck. If you imagine giving a cannonball 11.2km/s worth of kinetic energy, you’re giving it to the cannonball, not the earth. As the cannonball slows down there’s no detectable energy transfer out of the cannonball, so saying the potential energy is in the Earth’s gravitational field is relying on magic like gravitons. The cannonball doesn’t slow down to zero, it escapes the Earth. As a result the Earth’s gravitational field is slightly reduced, so it’s lost energy rather than gained energy, hence the cannonball must have taken the KE/PE energy with it. As an aside, people sometimes say a gravitational field is negative energy, but it isn’t. It’s a place where you could say the local ground state is lower, but it has a higher energy density than the surrounding space, and hence causes a little bit more gravity of its own.

By the by, I think the reason why we have a difference here is that you're taking a “classical” viewpoint whilst I’m taking the “relativity” viewpoint. Yor-on posted a useful link in another thread, see http://www.aei.mpg.de/einsteinOnline/en/elementary/generalRT/GeomGravity/index.html . Whilst I think this description is a bit too simplified, it does indicate the difference between the two viewpoints.     
« Last Edit: 17/01/2010 09:14:46 by Farsight »

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« Reply #129 on: 17/01/2010 10:03:18 »
PMB: If you look up the definition for Mechanical Work (here is one example newbielink:http://en.wikipedia.org/wiki/Work_(physics [nonactive])  ) you will see that a force that produces a change in the kinetic energy of a rigid body has done (mechanical) work.

When you drop an object, gravity accelerates the object, therefore gravity has changed the kinetic energy of the object, therefore gravity has done work.
Well put, thanks Geezer I'll work on something to put in new theories

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« Reply #130 on: 17/01/2010 18:09:20 »
In the case of gravity, there is a difference because the "spring" only exists by virtue of the masses. The energy is stored in the combined system, not in any single element of it.

I'm sorry geezer, but for gravity, there is no spring. That demands action at-a-distance, which even Newton rejected. Separate the two trucks by a very large amount, and the spring doesn't pull the lighter truck back any more. I suppose one way to take a step from this analogy to what I've been saying, is to say that once the lighter truck gets to a certain distance, the spring comes unhooked from the heavier truck, and stays extended whilst the lighter truck carries it off. That means the spring is part of the lighter truck. If you imagine giving a cannonball 11.2km/s worth of kinetic energy, you’re giving it to the cannonball, not the earth. As the cannonball slows down there’s no detectable energy transfer out of the cannonball, so saying the potential energy is in the Earth’s gravitational field is relying on magic like gravitons. The cannonball doesn’t slow down to zero, it escapes the Earth. As a result the Earth’s gravitational field is slightly reduced, so it’s lost energy rather than gained energy, hence the cannonball must have taken the KE/PE energy with it. As an aside, people sometimes say a gravitational field is negative energy, but it isn’t. It’s a place where you could say the local ground state is lower, but it has a higher energy density than the surrounding space, and hence causes a little bit more gravity of its own.

Indeed there is no spring. That's why I put it in double quotes. But there is, without any doubt, a resultant force that is produced by gravity, and it is that force that is responsible for doing the work that answers this topic - "Does gravity do work?"

BTW, in my analogy, if the spring became detached, the energy in it would be briefly converted into kinetic energy before it was dissipated as thermal energy.
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« Reply #131 on: 17/01/2010 19:32:33 »
Geezer: take a look at that link. It says "In the world of classical physics, if particles diverge from this behavior, it must be because there is a force acting on them". Then later on it says "However, there is another possibility...  In that situation, there is no force making the particles deviate from the straightest possible lines;" That's the difference I was on about between the classical viewpoint and the relativity viewpoint. You're taking the former, I'm taking the latter.   

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« Reply #132 on: 17/01/2010 20:51:49 »
Farsight:

However, my bathroom scales are unaware of that, and they continue to measure a force. And, as mechanical work is a classical idea that combines force and mass, gravity continues to do work.

BTW, the model in the link attempts to explain why moving particles might come together, but as far as I can see, it does not seem to explain why they continue to be attracted towards each other when they meet. Not that I'm arguing with Einstein of course, but models have their limitations.

 
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« Reply #133 on: 18/01/2010 12:59:12 »

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« Reply #134 on: 18/01/2010 19:21:50 »
Farsight: I think it would be more helpful if you could explain, in your own words, why work is not done by a force that we can clearly observe and measure. How that force comes about is interesting, but unless you are saying that it does not actually exist, I fail to see the relevance of your objection.
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« Reply #135 on: 18/01/2010 22:54:07 »
Geezer

Work is done when mass is moved from its resting state. The amount of work has to do with how far the mass is moved over a given time. The bathroom scale simply quantifies the force available to do work. The standard unit of work actually done is the joule which is equal to one newton meter.

Believe me. If I step off my bathroom scale and into an elevator shaft, my mass will not only move, it will move faster and faster over time. Accordingly, I think gravity works much like a rubber band. At the center of mass there is no gravity - like a relaxed rubber band. The surface of the earth is like a stretched rubber band. It holds potential to impart kinetic energy in the same way as stretched rubber band.

One analogy is something like this. I stretch a rubber band, an insect lands on it and then I release it. Assuming the insect hangs on, work will be done by accelerating the insect in much the same way falling down a well does the same for a dog.

You might reasonably object gravity is holding you down which is not the case with a static rubber band. Similarly, no work is done when an iron bar is held close by a big magnet. However, if you pull the bar down and an insect sticks to it, work will be done when you release the bar and it jumps to the magnet.

In all these cases potential energy can be converted to kinetic energy in a simple way. However, that energy depends on the independent variable: Specifically, potential energy is created by placing the object away from the center of attraction in the first place. Gun powder works the same way. Chemical energy is independently created and then released.

Both gravity and magnetism exert forces over long distances from many places. The mass that is subsequently accelerated in close proximity to either is simply a subtraction from the other sources.

This is just my simple minded way of thinking about this stuff. I could, and may very well, be wrong.

« Last Edit: 18/01/2010 23:19:07 by litespeed »

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« Reply #136 on: 19/01/2010 01:59:11 »
Thanks Litespeed. I have attempted to annotate your post with by comments:


LS. Work is done when mass is moved from its resting state. The amount of work has to do with how far the mass is moved over a given time. The bathroom scale simply quantifies the force available to do work. The standard unit of work actually done is the joule which is equal to one newton meter.

GE. Totally agree.

LS. Believe me. If I step off my bathroom scale and into an elevator shaft, my mass will not only move, it will move faster and faster over time. Accordingly, I think gravity works much like a rubber band. At the center of mass there is no gravity - like a relaxed rubber band. The surface of the earth is like a stretched rubber band. It holds potential to impart kinetic energy in the same way as stretched rubber band.

GE. OK, except I’m not quite sure what the surface of the Earth has to do with it.

LS. One analogy is something like this. I stretch a rubber band, an insect lands on it and then I release it. Assuming the insect hangs on, work will be done by accelerating the insect in much the same way falling down a well does the same for a dog.

GE. If you are saying work is done when a dog falls down a well, I completely agree and, I no longer understand why we are even having this debate!

LS. You might reasonably object gravity is holding you down which is not the case with a static rubber band. Similarly, no work is done when an iron bar is held close by a big magnet. However, if you pull the bar down and an insect sticks to it, work will be done when you release the bar and it jumps to the magnet.

GE. Well, yes. Work is only being done while there is motion. When the system is static, no work is being done. If you are saying that gravity is doing no work to keep me in place on my chair, I completely agree.

LS. In all these cases potential energy can be converted to kinetic energy in a simple way. However, that energy depends on the independent variable: Specifically, potential energy is created by placing the object away from the center of attraction in the first place. Gun powder works the same way. Chemical energy is independently created and then released.

GE. Right. Potential Energy only exists by virtue of the position of a body within a system. Some other form of energy had to be expended to achieve that position to create the potential.

LS. Both gravity and magnetism exert forces over long distances from many places. The mass that is subsequently accelerated in close proximity to either is simply a subtraction from the other sources.

GE. OK with the first sentence. You lost me on the second - not sure about "other sources".
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« Reply #137 on: 19/01/2010 15:01:15 »
Geezer - After a couple microseconds of simulated thought I subtract my comment about subtraction. However, I do not see work being done when gravity holds us in our chairs. For instance, is any work is done when a magnet holds a steel bar from falling down?

At an atomic scale, I don't see work done by the strong nuclear force until it is broken by, for instance, fission. Then all hell can break loose. I would simply speculate potential energy in all four cases has been created over time [(1) gravity; 2) magnet; 3) strong nuclear force; 4) gunpowder].

Let me speculate one step further. The only reason mass has energy E=MC2 is that at one time there was no mass. The mass 'precipitated' from the cooling big bang, thus transferring actual energy into a nicely compacted and stable form. Just about like gunpowder
« Last Edit: 19/01/2010 15:05:50 by litespeed »

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« Reply #138 on: 19/01/2010 16:22:00 »
Hey all,

If I could chime in I think there is a misunderstanding between Mr. Scientist and Geezer. All Geezer seems to be saying is there is no work if there is no displacement. This is true. Mr. Scientist seem to be saying that all masses at a certain height h have a gravitational potential energy GPE. This is also true. In fact I'm pretty sure you both agree with both of these statements, your arguments just don't seem to be 'in phase'.

Strictly speaking, gravity is an acceleration, not a force. Gravity generated by mass A acts on a mass B to give it a force pointing towards mass A and vice versa, as given by ForceGravity = G(massA * massB)/radius^2.

If mass B is displaced parallel to or anti-parallel to mass A, then there is an associated work, as given by Work = Force * Displacement.

Work is an energy, so here Mr. Scientist would point out that this Work is also equal to mass * gravitationalAcceleration * height, or mgh. This can be rewritten as Wh, since W or weight is a force. This is the same form as Work = Force(weight) * Displacement(height)...the point being that the GPE will not be 'used' unless the mass is actually displaced.

Sorry for the long-winded response, this is actually a good way to see whether I'm prepared for the physics test coming up [:D]

P.S. some how I missed that there are like 6 pages to this thread. I ended up replying to stuff on the first page; hope it's still relevant!
« Last Edit: 19/01/2010 16:23:56 by namaan »
Take it with a grain or two of salt...

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« Reply #139 on: 19/01/2010 17:09:22 »
Geezer - After a couple microseconds of simulated thought I subtract my comment about subtraction. However, I do not see work being done when gravity holds us in our chairs. For instance, is any work is done when a magnet holds a steel bar from falling down?

At an atomic scale, I don't see work done by the strong nuclear force until it is broken by, for instance, fission. Then all hell can break loose. I would simply speculate potential energy in all four cases has been created over time [(1) gravity; 2) magnet; 3) strong nuclear force; 4) gunpowder].

Let me speculate one step further. The only reason mass has energy E=MC2 is that at one time there was no mass. The mass 'precipitated' from the cooling big bang, thus transferring actual energy into a nicely compacted and stable form. Just about like gunpowder

Ahem! If you re-read my last reply, you will see that I completely agree with you that no work is being done while the system is static. I don't recall that I ever implied that it was. Sitting in chairs etc. included.

The only remaining question is: Is work done when a brick falls down a well? (As a dog owner, I prefer to use the brick model.)

If we can agree that it is, I think the question is answered. 
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« Reply #140 on: 22/01/2010 03:15:14 »
Farsight: I think it would be more helpful if you could explain, in your own words, why work is not done by a force that we can clearly observe and measure. How that force comes about is interesting, but unless you are saying that it does not actually exist, I fail to see the relevance of your objection.
I say work is not being done because I use the "transfer of energy" definition of work, as hinted at in http://en.wikipedia.org/wiki/Work_(physics) where it says "Likewise when a book sits on a table, the table does no work on the book despite exerting a force equivalent to mg upwards, because no energy is transferred into or out of the book". Gravity might appear to add energy to a falling object, but it's just converting some of the object's potential energy into kinetic energy. You can think of the object's potential energy as internal rotational spin energy, or as jiggle or something else, but it has to be there in the object rather than somewhere else. Otherwise as the object falls down there has to be a "magical" inflow of energy into the object, and there's no scientific evidence to support this. But I don't think I'm saying anything new here, and think it's probably best if we agree that there are ambiguities along with definition issues plus a mismatch between the classical and relativity viewpoints, and agree to differ.     

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« Reply #141 on: 22/01/2010 22:39:40 »
Farsight: Ah! So you are saying that, because no energy was added to the falling brick, no work was done on the brick?
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« Reply #142 on: 23/01/2010 11:46:15 »
Yes.

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« Reply #143 on: 23/01/2010 20:16:40 »
OK - but that seems to contradict the definition of work that states that work is done when the kinetic energy of a rigid body changes.

Could you perhaps be describing something other than work?
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« Reply #144 on: 25/01/2010 00:08:36 »
I don't think I'm describing something other than work, geezer. But the definition of work does seem to be at the core of the issue here. Take a look at at http://www.grc.nasa.gov/WWW/K-12/airplane/work2.html which talks about work done on a gas. The gas isn't a rigid body, and whilst pressing down on a piston involves force x distance and hence work and the addition of energy, we're dealing with pressure rather than kinetic energy. Then see http://hyperphysics.phy-astr.gsu.edu/Hbase/thermo/heat.html which says "This example of the interchangeability of heat and work as agents for adding energy to a system". It talks about adding energy but goes on to say: "neither the words work or heat have any usefulness in describing the final state of the system - we can speak only of the internal energy of the system." So what do you say if you have a method for converting microscopic internal spin motion, aka "jiggle" motion, into macroscopic linear motion, or vice versa? Called gravity? You're changing the internal potential energy into external kinetic energy, or vice versa, but you aren't actually adding any energy. All very confusing.   
« Last Edit: 25/01/2010 00:10:32 by Farsight »

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« Reply #145 on: 25/01/2010 01:10:17 »
I don't think I'm describing something other than work, geezer. But the definition of work does seem to be at the core of the issue here. Take a look at at http://www.grc.nasa.gov/WWW/K-12/airplane/work2.html which talks about work done on a gas. The gas isn't a rigid body, and whilst pressing down on a piston involves force x distance and hence work and the addition of energy, we're dealing with pressure rather than kinetic energy. Then see http://hyperphysics.phy-astr.gsu.edu/Hbase/thermo/heat.html which says "This example of the interchangeability of heat and work as agents for adding energy to a system". It talks about adding energy but goes on to say: "neither the words work or heat have any usefulness in describing the final state of the system - we can speak only of the internal energy of the system." So what do you say if you have a method for converting microscopic internal spin motion, aka "jiggle" motion, into macroscopic linear motion, or vice versa? Called gravity? You're changing the internal potential energy into external kinetic energy, or vice versa, but you aren't actually adding any energy. All very confusing.   

Well, that's all very nice, however, the work required to accelerate a brick (which is a fairly rigid body) is defined quite adequately by the change in its kinetic energy (and has been for rather a long time).

Unless you are willing to create a new definition for "work" and get it accepted by a rather large body of engineers and scientists, I think you should accept the current definition and try to accommodate it in your thinking. Of course, you can always define an entirely new concept based on your theory and try to get that accepted too.
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« Reply #146 on: 25/01/2010 03:26:57 »
Your definition of work is not adequate, and it isn't my theory - it's Einstein's. See http://www.perimeterinstitute.ca/Outreach/Explore_Our_Universe/Essence_of_General_Relativity/2/. This is an outreach article by the Perimeter Institute for Theoretical Physics: 

"Newton said that a falling apple is accelerating. Since acceleration requires a force, Newton had to invent the idea of a gravitational force that tugs on the apple while it is falling, making it fall faster and faster. Einstein said that it is precisely when the apple is falling that it is not accelerating (straight line trajectory in our space station example), and there is no need to introduce a mysterious gravitational force.

Newton said that an apple in your hand is not accelerating. No acceleration means no force. To arrive at no force, Newton imagined two exactly counterbalancing forces at work: gravity pulling the apple down and our hand pushing it up. Einstein said there is only one force at work: our hand pushing it up. This “unbalancing” force causes the apple to veer off its natural trajectory (and move instead on the circular trajectory in our space station example).

In short, what Newton got backwards was when the apple is accelerating and when it is not. This false starting point, although it is the common sense one, led him astray and required him to invent the idea of a gravitational force. Einstein took the diametrically opposite perspective, which showed “gravitational force” to be a red herring."



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« Reply #147 on: 25/01/2010 05:08:29 »
It's not my definition either. It's the one that been used for a very long time, and at least it's written down as I'm sure you know.

How does the PI define work, or is work too mundane a concept for the PI to theorize about? The article you cite does not provide a single mathematical relationship for anything. If you wish to refute the theory of work, you will have put forward a testable alternative theory, and provide a definition.

Perhaps you would like to say that gravity didn't do work, but the curvature of space-time did? I can accept something like that.
However, the kinetic energy of the brick increased, so, according to the definition of work, something did work to accelerate the brick. Or are you saying the brick didn't accelerate at all, or that the brick really has no mass.
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« Reply #148 on: 25/01/2010 15:22:41 »
The rigid-body-kinetic-energy description is the "mechanics" definition, geezer. I don't know how the Perimeter Institute define it, but I'll ask them. I'm not refuting the theory of work, I'm trying to inform of work and  gravity according to general relativity. No, your falling brick is not accelerating: Einstein said that it is precisely when the apple is falling that it is not accelerating. 

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« Reply #149 on: 25/01/2010 18:45:02 »
I think I see the problem. In absolute terms, the brick may not be accelerating. However, in relative terms, it is. Einstein may have said the brick is not accelerating because it is travelling in a straight line in spacetime due to its inertia, but he didn't say the brick and the Earth were not getting closer to each other at an increasing rate.

Within the Earth/brick system, the distance between the brick and the Earth did change. If you prefer to think of this as the Earth accelerating toward the brick, that's fine. We know this to be true because we can measure the effect as often as we want, and we will always get the same result. So, while the brick may have experienced zero force, relative to the Earth it really did accelerate (or the other way around if you prefer).

The velocity of the brick relative to the Earth changed. That's all we need to prove that work was done. The effect we refer to as gravity was responsible for doing the work, even without a direct force acting on the brick.
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