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Author Topic: Does Gravity do any work?  (Read 69592 times)

Offline Mr. Scientist

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Does Gravity do any work?
« Reply #50 on: 14/12/2009 17:54:59 »
I can't understand your analogy - so will require some math - you know.. the vector calculus you are aware of?
 

Offline Mr. Scientist

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« Reply #51 on: 14/12/2009 18:00:09 »
I'll save you the trouble of the math - lets stick to terminology.

Something which form a right angle is perpendicular meaning that they are orthagonal - Force IS a vector quantity, so having the information of two vectors form a vertical point of saturation. Are you telling me that we cannot associate two vectors which are perpendicular together and not define each other using calculus?
 

Offline Bored chemist

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« Reply #52 on: 14/12/2009 19:43:08 »
"You can quite easily, for instance, measure weight in Newtons. You can also measure the weight of the earth, because of it being made of units of kilograms. Corresponding the two, 1 kilogram is about 9.8 Newtons. So your presumptious nature was wrong."

Just plain wrong.
You measure mass in kilograms and weight in Newtons (at least in physics you do). If you like you can switch units and use pounds and slugs, but the units of weight and mass are different because (whether you like it or not) they are different things.
That's why on the moon I would remain about 70Kg in mass, but I would only weigh about 12 Kgf.
Coloquially you weigh things in kilograms but strictly you are using the Kgf as a unit, not the Kg.

It would also help if you stopped posting word-salad like "Force IS a vector quantity, so having the information of two vectors form a vertical point of saturation. Are you telling me that we cannot associate two vectors which are perpendicular together and not define each other using calculus?"
« Last Edit: 14/12/2009 19:46:32 by Bored chemist »
 

Offline Mr. Scientist

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« Reply #53 on: 14/12/2009 19:45:54 »
The definition of 1kg (which is about the same mass as an apple) is 1 newton.

YOU ARE WRONG. Ok?

Just accept it. You're making a fool of yourself.
 

Offline Geezer

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« Reply #54 on: 14/12/2009 19:56:19 »
Yes. Gravitrons may explain how the gravitational force is communicated. But how would that force be able to impart a force that is orthagonal to it?

Vector calculus. You could work orthogonal vectors to suit what you wanted to measure.

Mr S, I do understand vectors, and it is quite impossible to derive any force that is orthogonal to another force by vector analysis, vector calculus or anything else. This is not a math problem.

I'll give you a model and you can try to knock it down:

Attach a string to a pebble. Now swing the pebble around your head. It orbits around your hand. Easy! Right?

Now try to repeat the experiment without moving your hand in a circle. It's impossible because you cannot impart any rotational movement to the pebble.

Think of gravity as the string and your hand as the center of mass of the earth. Unless the center of mass of the earth executes a circular path relative to you (and it doesn't), it can do nothing to propel you.

Are we certain on this? Why not show me some of this understanding then...? Teach me something new.

We are certain of this. The only way to produce a component of force in the direction of motion is create a vector that has a component of force in that direction. By moving your hand in a circular arc, that is precisely what you are doing. So the force in the string (the vector) has two components at right angles. One that propels the stone, and another that acts towards the center of rotation (which in this case is not where your hand is) that maintains the orbit of the stone.

Now, when the vector (the string) goes straight to the center of rotation, the component of the force in the direction of rotation is zero.

Does that help?
 

Offline Bored chemist

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« Reply #55 on: 14/12/2009 19:59:00 »
The definition of 1kg (which is about the same mass as an apple) is 1 newton.

YOU ARE WRONG. Ok?

Just accept it. You're making a fool of yourself.

F***ing big apples where you come from.

The definition of the kilogram is the mass of a cylinder of a platinum iridium alloy in France.
It was originally defined from the mass of a cubic decimetre of water and the metre was, in turn defined as a fraction of the eath's meridian through Paris.

I'm not the one making a fool of myself here, other than because I'm wasting my time arguing with an idiot.
 

Offline Mr. Scientist

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« Reply #56 on: 14/12/2009 20:02:01 »
It would also help if you stopped posting word-salad like "Force IS a vector quantity, so having the information of two vectors form a vertical point of saturation. Are you telling me that we cannot associate two vectors which are perpendicular together and not define each other using calculus?

Give me strength - now your supporting him? Just you two keep together. You make a good couple.

 

Offline Mr. Scientist

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« Reply #57 on: 14/12/2009 20:03:08 »
The definition of 1kg (which is about the same mass as an apple) is 1 newton.

YOU ARE WRONG. Ok?

Just accept it. You're making a fool of yourself.

F***ing big apples where you come from.

The definition of the kilogram is the mass of a cylinder of a platinum iridium alloy in France.
It was originally defined from the mass of a cubic decimetre of water and the metre was, in turn defined as a fraction of the eath's meridian through Paris.

I'm not the one making a fool of myself here, other than because I'm wasting my time arguing with an idiot.

1 Newton i meant to say corresponds to the general mass of an apple. Ok? It's you that's messing with my head.
 

Offline Bored chemist

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« Reply #58 on: 14/12/2009 20:08:06 »
It would also help if you stopped posting word-salad like "Force IS a vector quantity, so having the information of two vectors form a vertical point of saturation. Are you telling me that we cannot associate two vectors which are perpendicular together and not define each other using calculus?

Give me strength - now your supporting him? Just you two keep together. You make a good couple.


Since we are both right it should be no problem to keep together.
Anyway, while I accept that a typical apple has a mass of about 0.1Kg and a weight (on earth near sea level) of about 1 Newton, that has nothing to do with the fact that weight and mass are different. Ask an astronaut.
 

Offline Farsight

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« Reply #59 on: 16/12/2009 00:13:35 »
Joe: imagine a cubic light year of space. I nearly said "empty space", but it isn't quite empty, because that space has vacuum energy. But anyway, put a planet in the middle of this apace. In doing this you've added a mass, and since E=mc² you've added a concentration of energy. As a result, you've also altered the surrounding space to add a gravitational field. Now add a second planet, thus adding further energy to your cubic light year. Now let the two planets fall together and coalesce. There will be fireworks, and radiation, and that radiation will escape your cubic light year. This is the transference of energy, and at first glance gravity caused it, so we might say "gravity does work". But instead of making two planets, you could have made a star, and then the radiation would have escaped your cubic light year, and gravity didn't make it happen. Since a concentration of energy causes gravity, and a ooncentration of energy in a star emits radiation, it's better to say that non-uniform energy usually does work. Energy tends to spread out evenly, that's entropy, and when it does this, you have a transfer of energy, which is work. 

PS: a black hole is something of an exception, in that then the concentration of energy doesn't spread out.     
« Last Edit: 16/12/2009 00:15:29 by Farsight »
 

Offline Pmb

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Does Gravity do any work?
« Reply #60 on: 16/12/2009 00:53:18 »
Quote from: Joe L. Ogan
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…
-------------------

Quote from: Joe L. Ogan
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.
 

Offline Geezer

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« Reply #61 on: 16/12/2009 01:09:35 »
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. 
 

Offline Geezer

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« Reply #62 on: 16/12/2009 01:15:32 »
Quote from: Joe L. Ogan
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…
-------------------

Quote from: Joe L. Ogan
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.
 

Offline Joe L. Ogan

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« Reply #63 on: 16/12/2009 02:48:01 »
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
« Last Edit: 19/12/2009 20:55:11 by Joe L. Ogan »
 

Offline Pmb

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« Reply #64 on: 16/12/2009 05:59:19 »
Quote from: Geezer
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.
Quote from: Joe L. Ogan
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.
 

Offline LeeE

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« Reply #65 on: 16/12/2009 07:12:21 »
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?
 

Offline Madidus_Scientia

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« Reply #66 on: 16/12/2009 07:37:34 »
I know, as if the frequent double/triple-posting wasn't enough
 

Offline Geezer

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« Reply #67 on: 16/12/2009 17:39:23 »
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.
 

Offline Farsight

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« Reply #68 on: 16/12/2009 23:04:31 »
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.
 

Offline Joe L. Ogan

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« Reply #69 on: 16/12/2009 23:20:25 »
Quote from: Geezer
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.
Quote from: Joe L. Ogan
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.
« Last Edit: 19/12/2009 20:52:02 by Joe L. Ogan »
 

Offline Geezer

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« Reply #70 on: 17/12/2009 01:05:12 »
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?
 

Offline Farsight

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« Reply #71 on: 19/12/2009 19:34:38 »
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.   
 

Online yor_on

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« Reply #72 on: 26/12/2009 16:54:18 »
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.
 

Offline Pmb

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« Reply #73 on: 27/12/2009 03:12:05 »
Quote from: Farsight
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.
 

Offline LeeE

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« Reply #74 on: 28/12/2009 11:33:31 »
Quote
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.
 

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