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Author Topic: How much does the Earth weigh?  (Read 56665 times)

Offline yor_on

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How much does the Earth weigh?
« Reply #175 on: 07/12/2010 18:59:12 »
'Pseudo force' or not, that's also a question where people seem to have different definitions. And that I know :) Been there done that as they say, long time ago :)

All centrifugal 'forces' are 'straight lines' constrained, as far as I understands it. With the constraint being the 'centripetal force' as i see it?
==

Like me on the inside of a hollow tube in space, the tube spinning. Then I have a 'gravity' but the 'gravity' can also be seen as a centrifugal force constricted by a centripetal force that in this case is the wall of the tube. Am I right?
« Last Edit: 07/12/2010 19:07:09 by yor_on »
 

Offline Geezer

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« Reply #176 on: 07/12/2010 19:31:39 »
"When a person swings a bucket by the handle in a large circle, he or she feels a pull on his or her hand, because inertial force (also called centrifugal force in this case)


No they don't  :D. What they feel is a reaction to the centripetal force that's necessary to maintain the bucket in a circular path. Just as "there ain'ta no sanity clause", "there ain'ta no centrifugal force" either.
 

Offline yor_on

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« Reply #177 on: 07/12/2010 19:44:13 »
well, a matter of opinion it is :)
But the quote comes from Answers.com.
« Last Edit: 07/12/2010 19:47:16 by yor_on »
 

Offline yor_on

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« Reply #178 on: 07/12/2010 19:58:45 »
For those of you that enjoy Newton. Why not take a look at Newton's Philosophiae Naturalis Principia Mathematica, from Stanford university encyclopedia of philosophy. I love that site, so many cool thoughts about 'everything'.
 

Offline yor_on

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« Reply #179 on: 07/12/2010 20:31:39 »
Newton surprises me :) We talk about it as singular 'forces' but he seemed to have another view.

"It is important to recognize that, in calling the referents of the defined terms “quantities,” Newton is assigning them to the ontological category of quantity in Aristotle's sense. Thus force and motion are quantities that have direction as well as magnitude, and it makes no sense to talk of forces as individuated entities or substances.

Newton's laws of motion and the propositions derived from them involve relations among quantities, not among objects. In place of “no entity without identity,” we have “no quantity without definite proportions;”[19] and the demand on measurement is to supply values that unequivocally yield an adequate approximation to these definite proportions."

Didn't know that.
 

Offline Geezer

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« Reply #180 on: 07/12/2010 22:06:31 »
well, a matter of opinion it is :)
But the quote comes from Answers.com.

Well, if somebody discovered the existence of centrifugal force, they must have done so in the last 45 years or so, 'cos it sure didn't exist when I was being taught this stuff. Either that, or it's a reflection on the deplorable state of education these days that causes educators to adopt a "politically correct" stance so they don't offend the "true believers".

If we run around inventing special names for all the reactions to actual forces, we're going to get into a real pickle.
 

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How much does the Earth weigh?
« Reply #181 on: 08/12/2010 00:00:23 »
Yor_on:

There is no controversy among physicists regarding centrifugal force. Centrifugal force is actually just a calculation shortcut when designing devices that are spinning at high speed. When you look at these situations as a physicist does, not from any specific viewpoint (e.g. that of the guy holding on to the spinning weight), it is obvious that when gravity acts on a mass moving perpendicular to the direction of the acceleration of gravity, there is only a centripetal force (gravity). As I already said, a moving object will continue unless acted upon, that is, accelerated by a force (centripetal in this case). 

The Answers.com article goes sideways when it says that the ship “is pulled toward the Earth by the Earth's gravitational attraction force, while the inertial or centrifugal force of the moving ship is directed radially outward from the Earth; consequently, the force of gravity on the orbiting ship is opposed and nullified by the centrifugal force, and apparent weightlessness results."

A ship in this situation if pulled by gravity that is momentary, or continuous, or either but much weaker or stronger, will result in some specific orbital change, such as flying away, or crashing, or a different higher or lower orbit, but in all these situations the ship and its inhabitants will continue to be weightless. They would not even be able to detect a change without navigational instruments. The Answers author seems to think that there is some specific centrifugal force value that will result in a delicate balance with centripetal force that will in turn result in weightlessness, but this is not true and one doesn’t make this mistake if they keep in mind that there is actually no centrifugal force.

Steve
« Last Edit: 08/12/2010 00:09:05 by SteveFish »
 

Offline Geezer

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« Reply #182 on: 08/12/2010 00:28:47 »
Yeaahh Steve! (I was beginning to wonder if my memory was even more dodgy than I already know it is.) What I do remember is that our Physics teacher (aka "Bilko", due to an uncanny resemblance to the Sargeant of the same name) would beat the tar out of anyone who proposed the existence of "centrifugal" force.

This is another good example of why it's a good idea not to believe everything you read on the Internet (unless, of course, the author happens to be yours truly).
 

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Offline Foolosophy

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« Reply #183 on: 08/12/2010 02:04:17 »
Shrunk
Yeaahh Steve! (I was beginning to wonder if my memory was even more dodgy than I already know it is.) What I do remember is that our Physics teacher (aka "Bilko", due to an uncanny resemblance to the Sargeant of the same name) would beat the tar out of anyone who proposed the existence of "centrifugal" force.

This is another good example of why it's a good idea not to believe everything you read on the Internet (unless, of course, the author happens to be yours truly).

So were you frequently beaten up in class?
 

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Offline Foolosophy

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« Reply #184 on: 08/12/2010 06:01:02 »
Shrunk
This is another good example of why it's a good idea not to believe everything you read on the Internet

including Wikileaks

(what hope is there for this Physics chat forum if the most posts for a thread topic is "How much does the earth weigh?)
« Last Edit: 08/12/2010 06:04:05 by Foolosophy »
 

Offline yor_on

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« Reply #185 on: 08/12/2010 08:19:25 »
Well that's why I avoid those expressions normally. To me if you spin something, you are accelerating it, breaking its geodesic path, and expending energy, but also confining its motion from 'any' of the geodesics it otherwise would have. And the 'engine' is the guy rotating. But I remember having this discussion before, and there seems to be different views on what consists of 'fictitious forces'.

So if you looked at my example with that tube spinning, would you say that I interpreted the 'forces' wrongly? I don't like 'forces' at all :)
==

Rereading :) To me it always have an importance who expends the energy. Ignoring that may give you an 'objective idea' of what 'forces' there exist, but to me it's always something 'inducing' something when it comes to spending energy.

Maybe there exist examples where you can't say who's doing that though? It's still to early in the morning for me :)
==

Yep, I'm gonna get me ass out of here for a while. Need to raid a supermarket..

C000FFFFEEEE
« Last Edit: 08/12/2010 08:44:19 by yor_on »
 

Offline yor_on

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How much does the Earth weigh?
« Reply #186 on: 08/12/2010 10:16:37 »
Back, three cups after, keyed up and ready. Throw it at me, I can take it :)

(But be quick, or I will need more coffee.. :))
 

Offline yor_on

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How much does the Earth weigh?
« Reply #187 on: 08/12/2010 10:18:05 »
To late, off to the kitchen..
(Quick metabolism)

I think?

 

Offline Foolosophy

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« Reply #188 on: 08/12/2010 11:53:14 »
:) Foolosophy ::))

You gonna mix relativistic mass into this *** thread?


why not - we are all friends here arent we?
 

Offline yor_on

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« Reply #189 on: 08/12/2010 12:03:51 »
Yep..

And those that ain't I'll ...

   :)
 

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« Reply #190 on: 08/12/2010 17:23:31 »
Yor_on:

Either you have had too much coffee or I haven't had enough because I don't understand what you are talking about. It almost sounds like you are saying that an object can't produce a force due to gravity because no work is done.

Steve
 

Offline Foolosophy

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« Reply #191 on: 09/12/2010 03:51:20 »
It almost sounds like you are saying that an object can't produce a force due to gravity because no work is done.

Steve


You could say that

 

Offline yor_on

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« Reply #192 on: 09/12/2010 16:22:04 »
Steve, I guess I'm saying that :)
Think of it this way

A apple falls to the ground. From the viewpoint of Earth the apple is accelerating but from the viewpoint of the apple it's just following a geodesic. Which one is correct? To me it's the geodesic, and it's easy to see what constitutes one, it's, ahem, weightless :) So the acceleration you observe is not a 'uniform acceleration' but just a geodesic that relative a proper mass will bend 'SpaceTime' into a steeper slope, making the apple behave as it is accelerating to you looking at it, having the same 'frame of reference' as Earth.

We had a discussion before here on TNS discussing if a accelerating charge would radiate, where I think we discussed this type of acceleration? But maybe you have a counter-example proving ah, otherwise :) It's the way I look at it now :) But I'm not cast in iron :) We're all here to learn I think.
 

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« Reply #193 on: 09/12/2010 23:58:59 »
Yor_on, when the apple hits the ground it certainly exerts a force, doesn't it? Further, for it to fall at all a force had to be applied to accelerate it.

As an aside, my understanding of the use of the word "geodesic" apparently doesn't match your usage. Please explain. Steve
 

Offline yor_on

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« Reply #194 on: 15/12/2010 13:13:46 »
In what way do you mean Steve?

A geodesic, to me, that is :) is the path any uniformly moving object, not expending energy, will take in SpaceTime, including when 'accelerating' due to gravity. The last one ('accelerating' due to gravity) is how I see it though, but that I said too? And in mathematics a geodesic is the idea of a shortest path between two points, depending on the 'surface' described. Like Earth, or on a paper it will express itself differently to us observing both, but they are both the shortest 'paths' and they both 'save energy'.

==
Although 'shortest' is a matter of definition I agree. I like to describe it in form of expending energy myself. And yes, you can see it as a 'force' if you like but to me gravity is no force, and that also include the apple falling. As for it's 'potential energy' relative Earth it exist.. In the impact, not before as I see it. To me it's as relevant as discussing that apples 'potential energy' relative the moon when it comes to describing it from its own frame of reference, while being 'weight-less' in a 'free fall', well, to me that is :)

As in a black box scenario you would have no way of differing whether you were falling towards the moon, Earth, or just coasting the space-lanes. Coriolis force and frame-dragging excluded that is. They might tell you that space was twisted :) Maybe gravity waves can do the same though? Which would make the equivalence absolute, all as I see it?
« Last Edit: 15/12/2010 13:25:55 by yor_on »
 

Offline yor_on

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« Reply #195 on: 15/12/2010 13:39:55 »
A way to see my definition of 'acceleration' is to look at it from the point of 'energy spent'. the apple doesn't expend any energy and so I think of it as following a geodesic. Does that mean that its 'room time geometry' doesn't change?

Assume that it is free falling towards a Black Hole, or a neutron star. I would expect its 'room time geometry' to change if so, but it still doesn't expend any energy, it's the room time itself that are 'twisted' where it is as I see it.

Now, does that make sense?
It depends on how you look at motion, and matter I admit.
To me it make a twisted sense though :)
==

And if you want to argue that by spending a finite amount of energy you can 'force' the whole of SpaceTime to 'age' into entropic equilibrium you will make me very curious, I don't see how that is possible? And that is what you will need to do if treating it as a 'force' I think? (Ah, this is when discussing 'normal acceleration, expending energy)

If we now use that idea on our apple, not expending any energy what so ever?

Then if I look at as a 'force' it will without expending any of its energy, as measured by you in a 'exact same' free fall beside it, 'twist' our SpaceTime? And that's one step further into a very interesting subject, filled with traps. I'm sure we can create the thread of threads debating this one :)
« Last Edit: 15/12/2010 13:56:48 by yor_on »
 

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« Reply #196 on: 15/12/2010 18:03:36 »
Yor_on:

It appears to me that you are using some terms that you have redefined to mean something different than common usage. This makes communication difficult. My version of Webster’s dictionary says that geodesic is “the shortest line between two points that lies in a given surface.” This is a pretty tight definition and you should use it accurately. It is possible that I am just ignorant, and if so I am quite willing to be educated, but I need something more than your opinion regarding how you like to use the term.

Gravity, from a Newtonian perspective, is a force between two objects related to their mass and distance. You can measure this force directly, yourself, with an object that has a known mass.  This force of gravity is often measured in newtons, a unit of force. You can measure the force of gravity by observing how an object is accelerated, whether free falling straight down or in orbit. If you, or I, were in a closed room, moving or not, under the effect of gravity or an inertial frame, or many thousands of light years from any massive object, we could detect what our situation was with some relatively simple but highly accurate instruments that measure force. Just because you, or I, in our relatively imperfect bodies would not feel any force doesn’t mean anything.

Now if all you are doing is switching to an Einsteinian frame of reference in which gravity is a fictive force, like centrifugal and coriolis forces, then OK, but I still don’t see what your point is in the context of this thread. This is because- If you wish to say that something in free fall among some massive bodies is not acted upon by any force, because gravity is a fictive force, then you also have to say that when you stand on a scale you are weightless because you aren’t really exerting a force on the scale. If somebody asks me how much something weighs (e.g. the earth) I would either try to pin down what their frame of reference was, or try to use the most obvious one to answer their question. In addition to word meanings, it is also important to define your frame of reference in order to communicate accurately and, then, to be consistent.

Steve
« Last Edit: 15/12/2010 18:10:23 by SteveFish »
 

Offline yor_on

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« Reply #197 on: 15/12/2010 23:18:33 »
'Weight' is what you feel when 'proper mass' is acting on (propping up) 'proper mass' :)

==

Also there is its equivalence to a uniform acceleration at one G. If you was in that spaceship and weighted yourself the scale would give you the same weight as on Earth. Now you might want to say that as the spaceship moves, energy is expended and a 'force' created. But it is in a 'black box scenario' only that this equivalence exist. Any other way you will know that you are 'traveling', and so destroy its equivalence as I understands it.
==

Thinking of it again :) I better add that all acceleration, in where you expend energy, will give you a 'weight'. but no 'acceleration' will if you are following a geodesic, as falling of the ladder, or that apple falling. And that's how I differ it.
==

Anyway, my 'proper mass' bending the 'room time geometry' for me as I stand on the scale. The scale answering as if my 'proper mass' was a 'force' applied on it. You can also see the scale as a sort of intermediary between me and the planet it rests on, reacting to me, as it is 'nulled' versus that planets gravity well. That is, being at Zero when nothing is placed on it. As I see it, gotta admit it sounds kind of zen that one :)

Yep it does. I think I need to stop eating vegetarians, vegetarian?.

Anyway, think of a laser shooting that beam at the moon. It's a ruler in a sense and the path it takes will be the straightest possible, although, its path 'bends'? Why? Because of 'SpaceTime Geodesics', they bend 'Space'. In fact, anything moving will follow those weird ahem, 'ley-lines' (Druid is my middle name)

And that's the way I see it, except I call them Geodesics, not ley-lines ( Too many fantasy novels there :)And I agree, they are very different from the concept of 'gravity's force', but they suit my way of looking at it.




« Last Edit: 16/12/2010 00:33:17 by yor_on »
 

Offline yor_on

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« Reply #198 on: 15/12/2010 23:40:53 »
Proper mass will create a 'gravity' when 'unmoving'. Not that we can prove anything to be so, except as compared relative something else. That's a unique 'property' belonging only to 'proper mass'. So I bend 'SpaceTime', just as you do, creating 'gravity wells'. And when two objects of 'proper mass' are placed together they 'attract' each other. What the scale measures is that 'attraction', graded in some common nominator to us, be it pounds or kilos. But it's not a 'force' even though it sounds like one. Our preconceptions is based on a Euclidean geometry in where that lasers beam 'shouldn't bend', but we know it will. We live in a place where matter 'distort SpaceTime', just as momentum, relative mass and 'pure energy' does. As it is I'm not entirely sure that it is the momentum 'bending space'? After all, don't we expect the 'momentum' to 'push'? Can it do both? Isn't 'energy' the better description for whats bends/distorts 'space' when two beams 'travel' parallel to each other so that they ahem, 'gravitate' towards each other?
 

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« Reply #199 on: 16/12/2010 00:13:51 »
Thinking of it, that's a nice concept illustrating the idea. Photons are 'mass less' according to mainstream physics, so why do they bend? And should I then expect them to weight something if they do so? If you think they don't weigh, and accept that they still gravitates towards each other then you will see the concept I'm thinking of. It's a concept relating to everything we know, even anti-matter as I understands it.
 

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