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Offline yor_on

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How much does the Earth weigh?
« Reply #100 on: 03/12/2010 18:17:52 »
I think I can present a argument speaking for my interpretation of 'proper mass'. Imagine yourself inside that 'box' described above, being 'at rest' with one of the 'particles'. When being so you will subtract the 'added' 'mass' as all motion can be seen, or transformed, into heat, and also as 'energy'. Being 'at rest', unmoving relative the particle will allow you to see it in its 'original state' and that state is also what I would call its 'rest mass' or if you like 'invariant mass' and those definitions are the equivalence to a piece of matter being 'proper mass', invariant in all 'frames of reference'.
 

Offline QuantumClue

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« Reply #101 on: 03/12/2010 18:29:03 »
Fool's definition of weight may be correct, but he'll also have to be consistent. That means for example, that every time he accelerates his mass and jumps a few millimeters in the air, he is weightless (strictly speaking he'd need to be in a vacuum of course, which, come to think of it, might not be such a bad idea.)

I will refrain from expressing an opinion on what kind of person he is.
But hopefully not your opinion...

Even if his weight is zero, it implies there is no atraction between bodies.
 

Offline QuantumClue

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« Reply #102 on: 03/12/2010 18:30:01 »
All that weight is reconfigured into a force. And vice versa.
 

Offline Geezer

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« Reply #103 on: 03/12/2010 20:22:20 »
All that weight is reconfigured into a force. And vice versa.

Well, it kinda depends. While you are in freefall and rapidly approaching the surface of the Earth, notwithstanding any screaming that might be going on at the time, it's impossible to determine your weight. You can only determine your "Earth weight" when you are static in a direction relative to your center of mass and the Earth's center of mass.

So, the tricky bit is deciding whether you weighed anything during your fall or not. If you can't measure it, arguably, it's anybody's guess.

On the other hand, if you say that you are "weightless" during your fall, you have to wonder why you are falling at all.

The situation is slightly different when it comes to large bodies like the Earth and the Sun, because, when they are falling towards each other, it's actually possible to measure their relative weights based on the center of rotation of the orbiting system (Solar wobble if you like). That situation suggests that you can actually weigh the Earth while it's zipping round the Sun, and, therfore, it aint't weightless.

I think JP had it right when he said it's best to completely avoid the use of the term "weight".
 

Offline Foolosophy

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« Reply #104 on: 04/12/2010 07:27:15 »
All that weight is reconfigured into a force. And vice versa.
I think JP had it right when he said it's best to completely avoid the use of the term "weight".

All that weight is reconfigured into a force. And vice versa.
So, the tricky bit is deciding whether you weighed anything during your fall or not. If you can't measure it, arguably, it's anybody's guess.

On the other hand, if you say that you are "weightless" during your fall, you have to wonder why you are falling at all.



why?

photons don't seem to worry too much about their zero mass when moving about - are they weightless?

« Last Edit: 04/12/2010 07:32:22 by Foolosophy »
 

Offline yor_on

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« Reply #105 on: 04/12/2010 09:51:55 »
Heh, that was a new one
I like that.

"Waiter two ordinary photons, one relative, three Rindler virtual specials and a weightless. And make it snappy, Chop chop"

Well, they are weightless, are they not?
In fact about the only thing there is :)
 
 

Offline Foolosophy

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« Reply #106 on: 04/12/2010 10:46:42 »
Heh, that was a new one
I like that.

"Waiter two ordinary photons, one relative, three Rindler virtual specials and a weightless. And make it snappy, Chop chop"

Well, they are weightless, are they not?
In fact about the only thing there is :)
 

perhaps
 

Offline rosy

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« Reply #107 on: 04/12/2010 11:34:30 »
Quote
On the other hand, if you say that you are "weightless" during your fall, you have to wonder why you are falling at all.

Quote
why?

photons don't seem to worry too much about their zero mass when moving about - are they weightless?

Urrr.... "weightlessness" is no bar to movement. Once an object is moving, it will continue to move with unchanged velocity until a force acts on it. If no force acts it will stay still/continue to move at constant speed in a straight line for ever (depending on initial conditions). That's Newton, that is. 

Anything falling freely under gravity is accelerating all the time. Even if speed is unchanged, as in a circular orbit, the direction changes, which requires an acceleration (one at right angles to the instantaneous direction of travel).
 

Offline Foolosophy

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« Reply #108 on: 04/12/2010 12:11:38 »
Quote
On the other hand, if you say that you are "weightless" during your fall, you have to wonder why you are falling at all.

Quote
why?

photons don't seem to worry too much about their zero mass when moving about - are they weightless?

Urrr.... "weightlessness" is no bar to movement. Once an object is moving, it will continue to move with unchanged velocity until a force acts on it. If no force acts it will stay still/continue to move at constant speed in a straight line for ever (depending on initial conditions). That's Newton, that is. 

Anything falling freely under gravity is accelerating all the time. Even if speed is unchanged, as in a circular orbit, the direction changes, which requires an acceleration (one at right angles to the instantaneous direction of travel).

You must remember that according to Geezer's Law, "one must wonder why anything is falling at all if it is weightless"

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Offline rosy

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« Reply #109 on: 04/12/2010 13:00:32 »
yor_on.. to go back a few posts (sorry):
Quote
I know one thing. when discussing 'mass' in physics it's important to agree on what the he* we are discussing :) People use the same synonyms for totally different properties at times it seems.
Actually, in the case of considering the orbit of the earth and sun, or the freefall of a skydiver, it really doesn't matter at all which definition of "mass" we use since none of the bodies involved are traveling at anything approaching lightspeed and thuse relativistic effects don't have any significant effect.

Fool:
Quote
Notice how the object with mass "m" on the surface of the earth is assumed to be stationary?
If that same body was free falling towards the surface of the earth, what would be its weight then? Zero right?
Well the earth is in free fall motion around the sun therefore its weight = zero

Do you, then, believe that there is no force between objects in orbit about one another? That there is no force attracting a satellite toward the earth? I can't see any other interpretation of this post, but am trying one last time to work out what it is you think.
 

Offline Foolosophy

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« Reply #110 on: 04/12/2010 13:20:56 »



...there are people who argue that the centrifugal force is NOT real
 

Offline rosy

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« Reply #111 on: 04/12/2010 13:31:38 »
What does the centrifugal force have to do with anything? The centrifugal force is a mathematical convenience in certain reference frames, but let's stick to an earth/satellite system in the reference frame of the earth (or, strictly, the centre of mass of the orbiting system).

Which forces do you then believe are needed to keep the satellite in its orbit? Please explain in your own words without copy-pasting someone else's diagram.
 

Offline Foolosophy

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« Reply #112 on: 04/12/2010 13:42:45 »
What does the centrifugal force have to do with anything?

You are asking as to the relevance of the centrifugal force in a classic "earth/satellite" orbit problem??

Interesting......

No wonder you still cannot grasp the simple fact that the earth is weightless as it orbits the sun.

(some equations are very useful in order to make a point clearer - notice how for a given mass and radius a body must attain a certain velocity in order to balance the centrifugal and gravitational forces? Hence free fall orbit conditions = weightlessness
« Last Edit: 04/12/2010 13:54:56 by Foolosophy »
 

Offline rosy

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« Reply #113 on: 04/12/2010 14:26:11 »
Quote
You are asking as to the relevance of the centrifugal force in a classic "earth/satellite" orbit problem??

Yup. Well, actually, not really. There's no question. In the reference frame of the centre of mass, there is no centrifugal force. It simply does not exist. The centripetal force is what we're interested in. If you don't know what the centripetal force is, look it up. It's key to understanding orbits of all sorts.

 

Offline Foolosophy

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« Reply #114 on: 04/12/2010 14:36:50 »
Quote
You are asking as to the relevance of the centrifugal force in a classic "earth/satellite" orbit problem??

Yup. Well, actually, not really. There's no question. In the reference frame of the centre of mass, there is no centrifugal force. It simply does not exist. The centripetal force is what we're interested in. If you don't know what the centripetal force is, look it up. It's key to understanding orbits of all sorts.



But you claim that the earth IS NOT weightless as it orbits the sun even though it is free fall motion

You must pay attention Rosy


« Last Edit: 04/12/2010 14:44:22 by Foolosophy »
 

Offline yor_on

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« Reply #115 on: 04/12/2010 15:59:43 »
Rosy, as a practical matter forces is a very useful concept, for me though I believe (I'm a believer:) in Einsteins SpaceTime 'geodesics'. According to that concept 'SpaceTime' is bent, wrinkled and bumpy. And while forces is the alternative way to look at it, it's no 'forces' involved for me, except possibly as some 'paths of least resistance' in SpaceTime, to why the planets orbit each other. Well, that's how I see it. To me gravity is no force even though I use the word sometimes, inside apostrophes mostly.

As for the comment on 'mass' that was just a common observation I've made at times, coming up when we discussed 'proper mass' and 'rest mass'. As for the rest of your comment I agree, for what we call a 'free fall' the mass doesn't matter :) all masses live in a 'free fall' depending on how you define your system, sort of?
 

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« Reply #116 on: 04/12/2010 16:34:12 »
Centrifugal force is a fictive force. It only exists in a local frame of reference, such as the guy in the picture swinging the weight. From our frame of reference, a mass in motion will continue in a straight direction unless it is accelerated. In this instance the weight is accelerated toward the guy by a centripetal force applied by the rope. In other words, the guy is applying the only force in the example. There is no centrifugal force unless you confine your frame of reference to the guy. By the way, gravity is also a fictive force that we see from our frame of reference, but from Einstein's frame of reference it is not. In fact, I am pretty sure that Einstein said that the centrifugal frame of reference problem helped him with understanding gravity. I think that this may be what Yor_on is going on about.
 

Offline yor_on

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« Reply #117 on: 04/12/2010 17:28:18 »
Most things I can think of, except gravity, can be transformed into 'energy', well, with the possible exception of 'anti matter' but if anti matter strikes anti matter there still should be a 'kinetic energy' created inside SpaceTime. If I would define anything as a 'force' then it is light, as it is the purest definition of 'energy' I know of? When you see a orbit 'break down' to the 'force' of gravity, then, from my point of view the geodesic 'pointed' to where the object moves. The only thing opposing geodesics are transforming and expending 'energy', like light is.
« Last Edit: 04/12/2010 17:35:40 by yor_on »
 

Offline CPT ArkAngel

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« Reply #118 on: 04/12/2010 17:33:50 »
The problem is Foolosophy is in a spacecraft going around the earth and he doesn't feel gravity. Any objects with him appear to have no weight. We are all down to earth and we see him in a free fall around the earth and we say this free fall is due to his weight. Like Geezer said, there is a lack of precise definition of what is the weight. I search the internet but some definitions are more general, speaking only of force and others are more specific, talking about weight relative to the experimenter.
« Last Edit: 04/12/2010 17:39:43 by CPT ArkAngel »
 

Offline yor_on

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« Reply #119 on: 04/12/2010 17:45:56 »
No CPT, from my view he's following a geodesic, created by gravity as balanced by his 'relative' motion. all orbits in 'free fall' are uniform motions, as soon as you need to 'correct' them by expending energy you are in fact breaking the geodesic path. And 'uniform motion' is a very slippery subject as you in a 'black box scenario', following a geodesic, can't differ between 'speeds', making all 'speeds' equivalent from that perspective. The only truth I think I know in this case to break a geodesic is to 'expend energy'. To define a possible speed you always need coordinates, making those your 'system' and a relative one as such. The only time you can define a speed in this black box is when having a non-uniform acceleration, as I see it. A uniform acceleration is equivalent to gravity, if in a black box scenario.
 

Offline yor_on

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« Reply #120 on: 04/12/2010 18:04:05 »
Thinking of it, when we say that something accelerates you need to ask if you feel a gravitation. If you don't you're not accelerating, you can't accelerate if following a geodesic. So, as I see it, as the apple falls of the branch, 'accelerating', it's in fact having a 'uniform acceleration' :)

Sorry, not motion.
Or?

I'm not sure about that one, uniform acceleration is equivalent to gravity, that's correct, But from the view point of the apple there is no 'acceleration' separable from a 'uniform motion' as it still follows a geodesic, being 'weight-less'. A tricky one..
==

You might possibly expand on the fact that all uniform 'speeds' are equivalent of course, and that way create a proof (?) For this type of acceleration being a 'uniform motion'?

Anyone have a view on this?
« Last Edit: 04/12/2010 18:18:51 by yor_on »
 

Offline CPT ArkAngel

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« Reply #121 on: 04/12/2010 18:14:26 »
yes, you are right... But he is still in a free fall, but is relative motion prevent him to change of geodesic trajectory. Truly, he feels a small angular acceleration for his change in speed direction around the earth... No?
« Last Edit: 04/12/2010 18:26:28 by CPT ArkAngel »
 

Offline Geezer

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« Reply #122 on: 04/12/2010 18:21:55 »
Quote
On the other hand, if you say that you are "weightless" during your fall, you have to wonder why you are falling at all.

Quote
why?

photons don't seem to worry too much about their zero mass when moving about - are they weightless?

Urrr.... "weightlessness" is no bar to movement. Once an object is moving, it will continue to move with unchanged velocity until a force acts on it. If no force acts it will stay still/continue to move at constant speed in a straight line for ever (depending on initial conditions). That's Newton, that is. 

Anything falling freely under gravity is accelerating all the time. Even if speed is unchanged, as in a circular orbit, the direction changes, which requires an acceleration (one at right angles to the instantaneous direction of travel).

You must remember that according to Geezer's Law, "one must wonder why anything is falling at all if it is weightless"

One of the 7 wonders of the modern intellectual realm of challenges

Ah yes! Photons, another "bum steer" if ever there was one.  However, I'm glad you finally acknowledge my vastly superior intellectual prowess.
 

Offline rosy

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« Reply #123 on: 04/12/2010 18:26:47 »
I would still like an answer to these questions, please, Fool:

A couple of questions, Fool...

1. If you were in a rocket, accelerating upwards from the earth's surface, would your weight have increased relative to when you were stationary?

2.a)If you think your weight would not have changed, how is this different to the situation of being in a rocket in freefall orbit about the earth?

2.b)If you think your weight would have changed, if the rocket were accelerating not upwards, but sideways at a tangent to the earth's surface, what would your weight be then? Would it still pull you toward the earth's centre or would it suddenly have a "backwards" component?
 

Offline yor_on

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« Reply #124 on: 04/12/2010 18:37:13 »
It's a good idea CPT, but I don't think so myself. The definition of a true geodesic have to be perfect weightlessness, in a black box scenario. If you introduce a angular gravitational influence, influencing his motion, inside that black box, it's no longer a perfect 'free fall', following a geodesic.

==

If you mean that he have two gravitational 'forces' acting on him, but we find that he still floats free in the exact middle of that black box the whole time, then it will be what I call a geodesic though. And now I'm mixing metaphors :)
« Last Edit: 04/12/2010 18:42:28 by yor_on »
 

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