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Author Topic: Does Gravity do any work?  (Read 69803 times)

Offline Geezer

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Does Gravity do any work?
« 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.
 

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
 

Offline Geezer

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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.
 

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 »
 

Offline questioner

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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 http://en.wikipedia.org/wiki/Work_(physics [nofollow])  ) 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
 

Offline Geezer

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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.
 

Offline Farsight

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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.   
 

Offline Geezer

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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.

 
 

Offline Farsight

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

Offline Geezer

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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.
 

Offline litespeed

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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 »
 

Offline Geezer

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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".
 

Offline litespeed

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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 »
 

Offline namaan

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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 »
 

Offline Geezer

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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. 
 

Offline Farsight

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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.     
 

Offline Geezer

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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?
 

Offline Farsight

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« Reply #142 on: 23/01/2010 11:46:15 »
Yes.
 

Offline Geezer

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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?
 

Offline Farsight

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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 »
 

Offline Geezer

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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.
 

Offline Farsight

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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."


 

Offline Geezer

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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.
 

Offline Farsight

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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. 
 

Offline Geezer

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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.
 

The Naked Scientists Forum

Does Gravity do any work?
« Reply #149 on: 25/01/2010 18:45:02 »

 

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