# The Naked Scientists Forum

### Author Topic: How much does the Earth weigh?  (Read 56663 times)

#### yor_on

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##### How much does the Earth weigh?
« Reply #150 on: 06/12/2010 21:43:51 »
Inside my black box?

hmmm :)

#### SteveFish

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##### How much does the Earth weigh?
« Reply #151 on: 06/12/2010 21:46:40 »
Yor_on:

Q- How to tell free fall straight down from free fall in orbit while in a black box? A- Coriolis force (also a fictive force).

Steve

EDIT-- I withdraw this pending some hard thought, or help.
« Last Edit: 06/12/2010 21:54:15 by SteveFish »

#### Geezer

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##### How much does the Earth weigh?
« Reply #152 on: 06/12/2010 21:49:49 »
It seems we are mixing the concept of weightlessness with net forces and gravity then?

I don't think so. Anyway, the question was, "How much does the Earth weigh?"

I believe I have proposed a satisfactory method of actually weighing it even while it's in orbit around the Sun. If I can weigh it, I think it's reasonable to assume that it's not weightless.

#### yor_on

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##### How much does the Earth weigh?
« Reply #153 on: 06/12/2010 21:59:47 »
Let me see if I got it right. You mean that by measuring the orbit of the moon knowing the mass of Earth, you can make a educated guess about the mass of the moon, That seems correct to me. But assuming that I didn't know the mass of the Earth too, then measured the distance of the moons orbit around Earth, I would have a he** of a problem 'splitting' the number I got into two, also you could get the same distance/orbit, if so, with different masses it seems to me? But you're thinking of having one known value, and then the distance I presume?

#### yor_on

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##### How much does the Earth weigh?
« Reply #154 on: 06/12/2010 22:04:07 »
No Steve, you're correct, I forgot to specify there.
But I think it's included in the 'frame dragging' as such
Heh ::))
==

Rereading you, missed the way you split the Q.
Yep, I think you're spot on.

If we imagine someone in a geodesic in deep space he won't have a Coriolis force acting on him.
« Last Edit: 06/12/2010 22:19:24 by yor_on »

#### yor_on

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##### How much does the Earth weigh?
« Reply #155 on: 06/12/2010 22:47:21 »
Rosie I think you are right, I looked and I didn't find any proofs for that he considered it 'weightless'. What he did consider it as was that the object in orbit was 'pulled over the horizon' due to its speed with the net forces taking themselves out, as I understands it?

In a way that is semantics though as when they 'equal out' they are equivalent to being weightless as I see it? There is a difference though in that Einstein went one step further, exchanging those net forces for his geodesics, and stated that when you were weightless it was actually possible to consider it equivalent to there being no gravity at all.

Gotta love physics :)

#### Geezer

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##### How much does the Earth weigh?
« Reply #156 on: 06/12/2010 23:31:57 »
Let me see if I got it right. You mean that by measuring the orbit of the moon knowing the mass of Earth, you can make a educated guess about the mass of the moon, That seems correct to me. But assuming that I didn't know the mass of the Earth too, then measured the distance of the moons orbit around Earth, I would have a he** of a problem 'splitting' the number I got into two, also you could get the same distance/orbit, if so, with different masses it seems to me? But you're thinking of having one known value, and then the distance I presume?

It's a simple beam balance. If you can determine the "fulcrum" (the point about which bodies both rotate) you can determine their relative weights. The unit of weight would be relative e.g. the Earth weight = x Moons, but weight is always a relative measurement, so that makes no difference. The point is, if you can determine the weight of a body, how can it be considered weightless?

You seem to be having a slight problem with this orbiting thing. "Free fall" applies whenever a body is not constrained and falls towards another body. It happens every time both your feet leave the ground. You don't have to be orbiting the Earth to experience it. Whether you are actually orbiting the Earth, or running for a bus, it makes no difference. You are either weightless in both cases, or not weightless in both cases. You can't be weightless in one case but not in the other case.

#### Foolosophy

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##### How much does the Earth weigh?
« Reply #157 on: 07/12/2010 02:35:25 »
Gotta love physics :)

Indeed!

To semanticise (my word) this weight/mass polemic trivialises and misrepresents what objects in free fall motion "feel". The fact that astronauts are weightless in space stations is not a matter of semantics - actually its a very accurate and apt description of their physical state (and supported by the laws of physics)

If you wish to stop the earth and place it on a set of scales under some known gravitaional field then I am sure that you will get a value for the earth's weight.

But that is another question

#### Geezer

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##### How much does the Earth weigh?
« Reply #158 on: 07/12/2010 03:45:50 »
Gotta love physics :)

But that is another question

No it isn't. It was the question.

Did you actually read any of this? The weighless condition an astronaut "feels" is exactly the same condition that you feel when you are falling to Earth from any height, and that, BTW, is entirely supported by the laws of physics.

#### JP

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##### How much does the Earth weigh?
« Reply #159 on: 07/12/2010 03:55:10 »
To semanticise (my word) this weight/mass polemic trivialises and misrepresents what objects in free fall motion "feel".

To semanticise this topic is over-complicating what is actually an extremely simple question.  You all agree on the laws of physics, but are arguing over definitions of the word "weight."  Since there is no universally accepted definition, as pointed out earlier, what's the point in arguing that your particular definition is the right one?

By the way, an interesting thing about the astronaut in orbit around the earth is that he's not in an inertial reference frame.  If he drops an object it will appear to hover next to him, but if he instead throws it along the direction of his orbit, he'll see it curve up away from him rather than in a straight line.  An astronaut in deep space would see the ball travel in a straight line if he threw it in any direction.  I was needlessly confused for a while while learning physics, since both were described as "weightless."  It was only after learning Newtonian mechanics that I actually realized that the concept of weight isn't really all that descriptive...

You can make things even more complicated if you consider one of those artificial-gravity space station designs that rotates so that the centripetal force is equivalent to the force of gravity on earth.  The same problem applies that although you would "feel" right, the motion of objects isn't identical to being on the earth, since you're no longer in an inertial reference frame.

#### Foolosophy

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##### How much does the Earth weigh?
« Reply #160 on: 07/12/2010 05:50:00 »
To semanticise (my word) this weight/mass polemic trivialises and misrepresents what objects in free fall motion "feel".

To semanticise this topic is over-complicating what is actually an extremely simple question.  You all agree on the laws of physics, but are arguing over definitions of the word "weight."  Since there is no universally accepted definition,

Untrue - there isnt any controversy over what weightlessness means.

The earth must by definition be wieghtless as it orbits the sun in free fall motion.

This fact is irrelevant to the earth's mass which is an intrinsic property of matter

(although one can argue over relativistic mass which is a thread topic in itself)

#### JP

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##### How much does the Earth weigh?
« Reply #161 on: 07/12/2010 06:48:09 »

To semanticise this topic is over-complicating what is actually an extremely simple question.  You all agree on the laws of physics, but are arguing over definitions of the word "weight."  Since there is no universally accepted definition,

Untrue - there isnt any controversy over what weightlessness means.
[/quote]

I said the definition of weight was controversial, not weightlessness.  You can argue with me on that point, but you'd be wrong.

#### Foolosophy

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##### How much does the Earth weigh?
« Reply #162 on: 07/12/2010 11:16:21 »

To semanticise this topic is over-complicating what is actually an extremely simple question.  You all agree on the laws of physics, but are arguing over definitions of the word "weight."  Since there is no universally accepted definition,

Untrue - there isnt any controversy over what weightlessness means.

I said the definition of weight was controversial, not weightlessness.  You can argue with me on that point, but you'd be wrong.
[/quote]

????

By your very own logic if the definition of weight is controversial then weightlessness must also be controversial.

But in the case of a free falling orbiting body, its weight must equal to zero.

I am sure that its mass is not offended

#### yor_on

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##### How much does the Earth weigh?
« Reply #163 on: 07/12/2010 11:47:09 »
Well Geezer, I thought I knew what it was until stumbling on this thread :)
Never mind, I'll stick to my geodesics then :)

Well, in a way I see your point there. Let's agree on that planets have a 'weight' even if relative, as I see it. Proper mass isn't though. Cause if it is we will have to redefine the whole darn thing with matter I think

===
the proof for that is simple.
Place Earth on Jupiter, then ask it if it feel a weight :) If it complains do the same with it on the moon, and ask it if it feels better now :)

doctors order..
« Last Edit: 07/12/2010 11:51:02 by yor_on »

#### Foolosophy

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##### How much does the Earth weigh?
« Reply #164 on: 07/12/2010 12:03:25 »
Foolosophy - perhaps if you spent a moment with a basics physics text or even on wikipedia; the recommended reading topic is vector quantities with reference to velocity and acceleration  (magnitude and direction).  You will soon learn that scalars such as speed are not same as vectors such as velocity.  Both forms of reference will also have a section on circular motion - that will fill the most obvious gaps.

the issue of scalar and vector quantities isnt disputed by anybody in here.

its about weightlessness of bodies in free fall.

are you still disputing the basic scientific fact that the earth is in free fall motion around the sun and therefore weightless by definition?

#### Foolosophy

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##### How much does the Earth weigh?
« Reply #165 on: 07/12/2010 12:12:32 »

Well, in a way I see your point there. Let's agree on that planets have a 'weight' even if relative, as I see it. Proper mass isn't though. Cause if it is we will have to redefine the whole darn thing with matter I think

#### imatfaal

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##### How much does the Earth weigh?
« Reply #166 on: 07/12/2010 13:09:26 »
the issue of scalar and vector quantities isnt disputed by anybody in here.

Quote
as far as your claim that the earth is accelerating you must remember that acceleration is the change in velocity (ie dv/dt)
how is the earths velocity changing?)
This quote and others from the first page of this thread make it fairly clear you do not understand the vector nature of velocity and acceleration

Quote
its about weightlessness of bodies in free fall.
I think you should give your definition of free fall - because it it quite clearly not the same as the more accepted version of the motion of a body which only experiences force from gravitational attraction

Quote
are you still disputing the basic scientific fact that the earth is in free fall motion around the sun and therefore weightless by definition?
I dispute your assertion that one follows from the other.  you have provided no proof or argument other than saying it is a basic fact.  Please provide the definitions that confirm your assertion.
« Last Edit: 07/12/2010 14:15:19 by imatfaal »

#### JP

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##### How much does the Earth weigh?
« Reply #167 on: 07/12/2010 13:40:35 »
Quote from: JP
To semanticise this topic is over-complicating what is actually an extremely simple question.  You all agree on the laws of physics, but are arguing over definitions of the word "weight."  Since there is no universally accepted definition,

Untrue - there isnt any controversy over what weightlessness means.

I said the definition of weight was controversial, not weightlessness.  You can argue with me on that point, but you'd be wrong.

By your very own logic if the definition of weight is controversial then weightlessness must also be controversial.

No.  That's by your logic, which is still wrong no matter how emphatically you state otherwise.

#### yor_on

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##### How much does the Earth weigh?
« Reply #168 on: 07/12/2010 16:53:41 »
:) Foolosophy ::))

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

This thread will explode soon ::))
==

I liked the way you explained your equation by the way.
It's a good habit.
« Last Edit: 07/12/2010 16:55:46 by yor_on »

#### yor_on

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##### How much does the Earth weigh?
« Reply #169 on: 07/12/2010 16:58:45 »
and yes, I'm as sure as one can be, only depending on what the boffins might cook up next :) Proper mass is a very defined property, hopefully correct too.
==

Eh, thinking of it, anyone disagreeing?

(counting down 3 .. 2.99 ... 2.98 ... \B00M/)

And some day I will grow up.
But not tonight :)
« Last Edit: 07/12/2010 17:22:49 by yor_on »

#### Geezer

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##### How much does the Earth weigh?
« Reply #170 on: 07/12/2010 17:02:54 »
I'm afraid I have to weigh in on this one more time.

I'm less concerned about the definition thing than I am about consistency. The conditions effecting a body in orbit are no different from the conditions effecting a ball that's moving in a parabolic (or elliptical) arc because somebody threw it in the air.

If a body in orbit is "weightless", a ball thrown in the air must also be "weightless". Everyone keeps wanting to discuss the complicated case of orbiting without understanding the uncomplicated case of chucking a !@#\$%^& ball in the air. And, if it follows that a thrown ball is weightless, it also follows that a human is weightless every time they lose contact with Terra Firma, or not! Either way, I'm pretty sure the various cases have to be consistent (unless somebody invented a new branch of Dynamics without telling me about it.)

« Last Edit: 07/12/2010 17:07:38 by Geezer »

#### yor_on

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##### How much does the Earth weigh?
« Reply #171 on: 07/12/2010 17:19:29 »
"Weight is the force that results from the acceleration by gravity on the mass of an object.[1]

Sometimes it is defined in operational terms of the weighing process as the force exerted by an object on its support,[2] which also illustrates the condition of weightlessness.

Standing at rest on a weighing scale on Earth, a person's weight equals its mass multiplied by the gravitational acceleration of Earth. However, in a free falling elevator, a scale indicates a zero weight, as no net force is exerted by the body on the support; the person experiences weightlessness.

Similarly, in a space craft in orbit around the Earth the weight is also zero, as the orbit represents a free fall. On the surface of the Moon, an object's weight is approximately one sixth of the weight at rest on Earth, as the gravitational force exerted by the Moon is much smaller than that of Earth."

Take it up with the wiki instead.
They have a 'talk page', that's what I would do.

From Weight.

#### yor_on

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##### How much does the Earth weigh?
« Reply #172 on: 07/12/2010 17:40:53 »
Answers.com I like, a lot. Lots of info on most subjects. This is some of what they have to say about 'Zero Gravity'.

"Newton proposed the law of universal gravitation, which states that two bodies of matter in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.

According to this law, even a small increase in the distance between bodies will produce a large decrease in the gravitational force, since the force decreases with the square of the distance. As a body moves from the Earth's surface to a location an infinite distance from the Earth, the gravitational force approaches zero and the body approaches weightlessness. In the true sense, a body can be weightless only when it is an infinite distance from all other objects.

Weightlessness is also defined as a condition in which no acceleration, whether of gravity or any other force, can be detected by an object or organism within the system in question.

According to Albert Einstein's principle of equivalence, there is no way to distinguish between the forces of gravitational fields and the forces due to inertial motion. When a gravitational force on a body is opposed by an equal and opposite inertial force, a weightless state is produced.

This is based on the fact that the mass that determines the gravitational force of a body is the same as the mass related to the acceleration produced by an inertial force of any kind. These inertial forces have no external physical origin, but are the consequences of an accelerated state of motion.

Because of inertia, a moving object always tends to follow a straight line. 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) tends to keep the bucket moving in a straight line, while the bucket holder exerts a counterforce constraining the bucket to move along the circle.

A similar situation exists in a spaceship orbiting the Earth 200 mi (320 km) above the Earth's surface, where the gravitational field is only slightly weaker than at sea level. The ship, in free fall with negligible atmospheric drag, 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."

From weightlessness.
« Last Edit: 07/12/2010 17:46:18 by yor_on »

#### SteveFish

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##### How much does the Earth weigh?
« Reply #173 on: 07/12/2010 18:15:43 »
I have been following and enjoying this argument, and I now believe that JP is correct. There is a relatively simple confusion regarding definitions of what the word “weight” really means. Definitions can be important. The Wiki article is pretty good and it discusses some of the ambiguities. It says “Weight is the force that results from the acceleration by gravity on the mass of an object.” Force is mass X acceleration (of gravity in this case). So here is my take on this:

Weight in everyday life is relatively simple. You bring your trout to a hanging scale (having cleverly forced a few stones down its mouth) and watch how much the calibrated spring stretches to proclaim your trophy. So where is the spring scale in the earth sun system? As the earth orbits the sun it is being constantly accelerated at a right angle to its direction of movement by the centripetal force resulting from the mutual attraction between the earth and sun, and this force on the sun is evidenced by a wobble in the earth’s orbital plane. We on the earth feel weightless, relative to the sun, because we are being accelerated by the same amount as our environment, but we and the earth are very certainly under the effect of a great force. It isn’t a great stretch (intended) to see the orbit and wobble as the stretch indicator of the spring in this scale.

Again for effect- If we are falling to earth (or in orbit around the earth) we may feel weightless because we and all our internal organs and all our baggage are falling at the same rate, but this doesn’t mean that there isn’t a considerable force acting on us, and there is considerable evidence of the force in this situation. F= m x a.

If one wishes to define weight as meaning the force exerted by a mass in a specific gravity field, but only if the mass is stationary relative to the gravity field, then OK.  Yor_on, you have to be very careful when using the concept of centrifugal force because it can lead you astray, as it has the author of the piece you linked. Steve
« Last Edit: 07/12/2010 18:23:10 by SteveFish »

#### yor_on

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##### How much does the Earth weigh?
« Reply #174 on: 07/12/2010 18:53:03 »
"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) tends to keep the bucket moving in a straight line, while the bucket holder exerts a counterforce constraining the bucket to move along the circle. " This Steve?

And where?

#### The Naked Scientists Forum

##### How much does the Earth weigh?
« Reply #174 on: 07/12/2010 18:53:03 »