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skpsuper
2003-Sep-17, 05:07 AM
Yes ! this is the section I was looking for ! I have some questions here :
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1 : What is the exact reason why earth is rotating and is there no
opposing force to this rotation and if so will it infinetly continue to
spin.

2 : What is the force which prevents the moon from falling to the earth
and is there some specific reason why moon's period of rotation and
revolution are the same.
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chuckb
2003-Sep-17, 09:15 PM
Wish I could answer these questions as well as these guys, so I will refer you to
http://www.physlink.com/Education/AskExper.../ae_answers.cfm (http://www.physlink.com/Education/AskExperts/ae_answers.cfm) in the
Astronomy, Cosmology and Astrophysics Section - there are some excellent answers posted by the pros that seem to match your questions...

Faulkner
2003-Sep-21, 11:49 AM
Another question could be added: why is Venus the only planet in the solar system that rotates the other way? Very strange.

Reading SkpSuper's 2nd question got me thinking. "In empty space, absolute motion can't be defined without a reference point" (to quote from an article in the above link). The moon is locked into an orbit with the Earth so that one side remains facing us - ie its rotation equals its orbit. That means, if you're standing on the Near Side of the Moon, you would the see the Earth hanging in the same place in the sky. Taking the above quote into consideration, then wouldn't it seem likely that a massive body like the Earth, which is NOT in orbit around the moon, would have to fall down ONTO the moon due to gravitational attraction?

It seems obvious FROM THE EARTH'S STANDPOINT that the Moon's orbit counteracts any gravitational attraction, so that it cannot possibly "fall" down on us. Instead, it falls "around" us. (In fact, I hear the Moon is getting gradually further away!).

HOWEVER, when you change your standpoint to that of the Moon, everything changes! The Earth is hanging in the sky. You would think it would be in attraction with the Moon, and getting closer!!!

I'm confused! Any enlightenment out there would be very welcome!!!
:blink:

skpsuper
2003-Sep-21, 04:09 PM
Yes I got the answers very specifically and clearly at :

http://www.physlink.com/Education/AskExperts/ae201.cfm

Thank u Chuckb for the link...I liked those questions and their answers as they were clear and specific....it was great

Thank u!

Fraser
2003-Sep-21, 04:58 PM
You're right Faulkner, it sure would be a puzzler to be standing on the moon looking up at an unmoving Earth. You would know that you're going around it, though, because the stars would be moving against the background.

I wonder if the Earth and Moon will eventually become tidelocked and always face each other?

Faulkner
2003-Sep-22, 11:32 AM
I'm still confused. What if the Earth-Moon system was isolated in some great intergalactic void, so that background stars could not be seen. Then, if you were standing on the Moon looking up at an unmoving Earth, you would have no reference point to assume one was orbiting the other. If there is no ABSOLUTE reference point - ie if everything is indeed "relative" in space - then why should a visibly moving background of stars make any difference? From the Moon's point of view, the Earth is motionless in the sky. I have read in "A Brief History Of Time" (I think it was) that using the Sun as the centre of the solar system is a purely arbitrary choice, it's just the easiest way to describe planetary orbits. But we could just as correctly make the Moon the centre of things, tho' we would have to develop complex formulae to describe the movements of the planets. In other words, there is no absolute reference point in space. Thus my question: If we were living on the Moon, looking up at an unmoving Earth, surely we would expect it to be getting closer?? I may be way off track here, forgive me, but these little things bug the crap outa me! :wacko:

kashi
2003-Sep-22, 12:10 PM
Both orbit around the centre of mass which is obviously closer to Earth. The only difference between the two observation points apart from their distance to this centre of mass, is that the Earth is rotating too fast for the moon to constantly stay visible from a given point on the planet, whereas the moon's rotation speed ensures that the Earth will always be in the "same spot" relative to the observer.

Faulkner
2003-Sep-22, 02:19 PM
Sorry to be a pain the arse here, but I'm still not convinced. Imagine the Earth & the Moon in deep deep space, where there are no visible stars. From the Moon, the Earth appears to hang stationary in the sky, in fact gradually getting further away. This would seem to go against all notions of gravitational attraction. Why is Earth's vantage point privileged over the Moon's vantage point? Does the unseen background universe have some mysterious influence here? If so, that would assume some absolute frame of reference at work. But we are told there IS no such thing. Is the Earth & Moon moving through an "ether"?? We are told there is no such "ether" either!

Am I barking up the wrong tree here?

This has actually been puzzling me for some time now. It's similar to something else that's been bugging me: centrifugal force. Imagine a wheel-shaped space station in deep space, rotating so as to exert an outward-pushing "pseudo-gravity". But against what standard do we judge the space station to be rotating? The background stars? Then the background stars must be exerting an influence on the space station!? From whose vantage point do we say the space station is rotating? It could equally be argued that it is stationary, and the background stars are moving around it. After all, everything is relative, right? Where does centrifugal force come from?

Maybe I'm missing something simple & obvious here... Are these valid questions? Or am I going senile? :(

major_eh
2003-Sep-24, 02:20 AM
Faulkner, from your original post in this discussion, whether or not you are viewing the phenomenon from Earth or the Moon...the fact of the matter is that the Moon is orbiting the Earth and consequently not falling into it due to gravity.

I believe your confusion lies in the different perceptions you would have from the different viewpoints. On earth you would have the luxury of being able to witness that the Moon is indeed orbiting you.

On the Moon, without background stars etc. to set you straight, you would only see a stationary Earth spinning away.

The only difference is that you wouldn't have enough information about the situation to come to the conclusions that you would if you were standing on Earth.
Nothing has physically changed about the situation.

An analogy: If you were standing close to a drag strip watching the cars race by from left to right they would seem as though they are going much faster than if you were viewing them from far off and the cars were going towards or away drom you. The fact remains that the cars are still going damn fast. ;)

major_eh
2003-Sep-24, 02:41 AM
OK.. and your second scenario of the space station...
If you were in a satellite just outside the space station and the two vehicles were stationary (except for the rotation), you would clearly see the rotation.

If the satellite you were in was in geosynchronous orbit to the SS, you would always see the SS the same. (of course then the satellite would have to rotate at the same rate as its orbit, like the moon)

The space station is rotating regardless of whether you can tell or not.

Relativity is useful for things like speed. Two cars side by side going the same direction at 100MPH are not moving relative to each other.
On the other hand, when one of them has a head on collision with oncoming traffic that's only going 30 MPH, the result is the equivalent of going 130MPH into a stationary vehicle because relative to the third vehicle that's how fast he was going.

Faulkner
2003-Sep-24, 03:35 AM
Hi Major-Eh, I appreciate the response. The more I'm thinking of this, the more I'm tearing my hair out.

BUT...I'm still not convinced! AAARGGHH!

You're saying regardless of vantage points, background stars, contexts etc etc, the Moon is orbiting the Earth. But it MUST be orbiting only when viewed from some outside reference point, which I feel Relativity Theory says doesn't exist!? If all motion in the universe is relative, then the fact that the Earth is spinning away from the Moon without changing its position in the sky seems to go against all notions of gravitational attraction. If the Earth & the Moon were moving through an "ether", I could understand the situation a lot better, because then we would have something to use to measure the movement of these 2 bodies. The only other alternative I can see is that somehow the "outside universe" is having some direct bearing on the situation. I think Mach's Principle relates to this, but not being a physicist, I'm not confident to delve into it! Ha. (But I'll look into it, for sure!)

About your scenario of rotating space station and a stationary satellite... The same problem here, I'm afraid! From what vantage point do we assume these 2 objects are stationary? Couldn't we say the space station is stationary, and the satellite is orbiting it?

I don't think you can say, well, "the fact of the matter IS: the Moon orbits the Earth, and the space station is rotating (thus causing centrifugal outward force)". All motion in the universe is relative (from what I read everywhere). So from what viewpoint do we judge something to be rotating/orbiting/moving etc????

Please don't get me wrong, I'm not playing devil's advocate here, nor am I an advocate of the "lunatic fringe", however this problem here really disrupts my understanding of a relativistic universe!!! All I'm asking for is some information that will put me right. Like I've said before, I may be missing something (maybe something obvious?)...but then, obviousness & common sense tend to go out the window when examing the cosmos, doesn't it?

I guess the idea of a "geosynchronous orbit" confuses me too. I know it exists, and it works. But I don't understand it. What is the Earth rotating "through"? From the Moon's point of view, the Earth is locked in a geosynchronous orbit. Is that right? How do we measure that the Moon is indeed rotating? Against what? It all makes sense when I think of an "ether". But I know it doesn't exist, and I want to believe that too. I guess I just need some coaxing into enlightenment!?! :wacko:

Faulkner
2003-Oct-01, 01:14 PM
Isn't anybody else as baffled by this as I am?

Well, I've tracked down an article by legendary Paul Davies that kinda touches on what I'm rambling about.

http://web.archive.org/web/19990117003745/...940922Mach.html (http://web.archive.org/web/19990117003745/http://www.physics.adelaide.edu.au/itp/staff/pcwd/Guardian/1994/940922Mach.html)

In it, he asks "how can something be said to be rotating or moving or accelerating with respect to empty space?". To answer this, he postulates what I consider to be an ether: the quantum vacuum (ie not empty space).

Kedas
2003-Nov-14, 08:16 PM
The key is 'acceleration' because you don't need to have an other point of view to know your own acceleration (you can just measure the force F)

A rotating object is constantly accelerating (changing direction, not absolute speed)

Don't ask me details but that should be the right direction to understand it.

Menikmati
2003-Nov-14, 09:27 PM
This has actually been puzzling me for some time now. It's similar to something else that's been bugging me: centrifugal force. Imagine a wheel-shaped space station in deep space, rotating so as to exert an outward-pushing "pseudo-gravity". But against what standard do we judge the space station to be rotating? The background stars? Then the background stars must be exerting an influence on the space station!? From whose vantage point do we say the space station is rotating? It could equally be argued that it is stationary, and the background stars are moving around it. After all, everything is relative, right?

Ok, I am trying to picture you and I could be way off. But you are picturing a wheel-shaped space station in nothing but black space, no stars, not planets, no anything, and you want to know from whose stand point is this station rotating? Well if the station was going at a constant speed with no accelerations and deaccelerations then the it would be impossible to tell that the station was rotating. But if you were to step out side the station and observe it, you would notice the station rotating.

Now lets add some stars to the back ground. Now you see stars making an orbit around your station. I can people can be easily confused that the stars are rotating around you, similar with how people thought the sun revolved around the earth. But I think it would be a question of asking yourself would it be more likely for the station to revolve around this fast to come back to your start point or would it be easier for a star to make an orbit around the universe and come back to the the starting point. My common sense would say that it would be more likely that the station make a complete turn in so that every 15 hours I see the same star again.
I dont' think I really answerd your question but I stated my opinon as if I was in the space station. I'm gonna look up some stuff and see how I can help you.

Kedas
2003-Nov-15, 10:09 AM
I have (had) that thought problem also.

About rotation they always rotate around each other until you you define a reference point. if you have only two object in the universe then you can't say which one is rotating around the other until you add a third reference point (your point of view)
If you change your reference point you can let them swap place.
you can also choose a reference point so that you see that they are actually rotating each other equally. So depending of your own movement in reference to the two objects you can see anything you like.
The only thing nature knows is they 'rotate each other' enough to compensate for the gravity force.
instead of 'rotate each other' you can also say 'accelerate each other'
If we add the logical reference of the stars then we can say this object rotates this one more than the other way around.
If we take the universe/stars as a reference then our sun is also rotating a little bit around us.

This is how I think about it. (I can't give you a link to an article)

This solves rotation of two objects but what about spinning.

A bal of water hanging in emty space how does it know when to change shape because of spinning?
I think it's the same but with many object the problem is that you very easily assume a reference point while it's really about water parts rotating/accelerating each other.

just my thoughts :)

Matthew
2003-Nov-15, 10:29 AM
The point of reference for a spinning, ceircular, space station is its centre.

The Earth could have started rotating by inertia. Now, most matter in the solar system was going one way, around the sun. When matter started to clump together it would have put its own energy into the big clump, Earth, which would have created a spin. With enough spin the inertia would keep it going.

An intresting point though is that the Earth's spin is slowly slowing down for various reasons. One of those is that we clump togeter water into reseviours and dams.

Guest
2003-Nov-15, 11:34 AM
Originally posted by matthew@Nov 15 2003, 10:29 AM
The point of reference for a spinning, ceircular, space station is its centre.


Just want to say that the point of reference is always a choice never something nature decides or act on it.

Kedas
2003-Nov-15, 11:37 AM
Sorry, that above message is from me 'Kedas'

Planetwatcher
2003-Nov-15, 02:04 PM
Boy you guys got me baffaled. I'll sit back and watch how this comes out.