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Author Topic:   Sun-Earth-Moon Gravity
2gud
Junior Member (Idle past 6149 days)
Posts: 6
Joined: 05-27-2007


Message 46 of 119 (402550)
05-27-2007 10:49 PM
Reply to: Message 45 by Taz
05-27-2007 10:41 PM


Thanks for letting me know it is a joke.
I was addressing the first post of this topic - in which the thread starter no where asked "why doesn't the moon orbit the sun?"
His question of why doesn't moon get pulled away from Earth-Moon system into a separate orbit of its own around the Sun is still valid.
And I answered it with the exact scientific explanation.
If you are more interested in understanding under what conditions a planet can have a satellite, read about "Roche limit" and "Roche Lobe" in particulare. The conept of Roch Lobe is what which I explained in so many words.
Edited by 2gud, : Rephrased certain sentences

This message is a reply to:
 Message 45 by Taz, posted 05-27-2007 10:41 PM Taz has replied

Replies to this message:
 Message 47 by Taz, posted 05-28-2007 2:24 AM 2gud has replied

  
Taz
Member (Idle past 3291 days)
Posts: 5069
From: Zerus
Joined: 07-18-2006


Message 47 of 119 (402563)
05-28-2007 2:24 AM
Reply to: Message 46 by 2gud
05-27-2007 10:49 PM


2gud writes:
His question of why doesn't moon get pulled away from Earth-Moon system into a separate orbit of its own around the Sun is still valid.
Well, not really. This is a problem that I often had to face while trying to explain to my students about orbital mechanics. When the person asked why doesn't the moon just leave its current orbit and have an orbit of its own around the sun, the person isn't really thinking about tidal forces and such. The person who wrote the OP had a very common misconception about how gravity works.
To give you an extreme example of this misconception, a while back there was a kid that thought he could disprove the theory of gravity all together by pointing out that rocks don't orbit mountains and that astronauts don't orbit the space shuttles. The misconception about gravity is that the smaller object must orbit the bigger object. It's a very simple (and wrong) view of gravity. This is why I always tried to stress the fact that you have to look at these things as systems rather than single objects.
The earth-moon is a system. The earth orbits the center of gravity of earth-moon and the moon orbits around the center of gravity of earth-moon. The point of center of gravity of this system in turn orbits around the center of gravity between earthmoon-sun system. Because the sun is so much more massive than the earth and the moon, the center of gravity in this particular system is very very close to the center of the sun.
What I'm trying to say in too many words is that we need to recognize the misconception behind the question. Without understanding why, the person who wrote the OP thought that smaller objects must orbit bigger objects. This misconception must be addressed first before we can assume that the person actually was referring to tidal forces.
And I answered it with the exact scientific explanation.
Well... I tend to get nervous everytime the words "scientific explanation" are used, especially when we often have to talk to people who believe science to be just another religious doctrine.


We are BOG. Resistance is voltage over current.
Disclaimer:
Occasionally, owing to the deficiency of the English language, I have used he/him/his meaning he or she/him or her/his or her in order to avoid awkwardness of style.
He, him, and his are not intended as exclusively masculine pronouns. They may refer to either sex or to both sexes!

This message is a reply to:
 Message 46 by 2gud, posted 05-27-2007 10:49 PM 2gud has replied

Replies to this message:
 Message 48 by 2gud, posted 05-28-2007 3:06 AM Taz has not replied
 Message 49 by mpc755, posted 07-29-2007 11:35 PM Taz has not replied

  
2gud
Junior Member (Idle past 6149 days)
Posts: 6
Joined: 05-27-2007


Message 48 of 119 (402565)
05-28-2007 3:06 AM
Reply to: Message 47 by Taz
05-28-2007 2:24 AM


Tazmanian Devil writes:
To give you an extreme example of this misconception, a while back there was a kid that thought he could disprove the theory of gravity all together by pointing out that rocks don't orbit mountains and that astronauts don't orbit the space shuttles. The misconception about gravity is that the smaller object must orbit the bigger object. It's a very simple (and wrong) view of gravity. This is why I always tried to stress the fact that you have to look at these things as systems rather than single objects.
Though the conclusion that the kid drew can be quite easily proved false, but still I am impressed that a kid asked a question (what is his age?) like "why don't rocks on earth orbit mountains". It is a interesting question indicating a inquisitive mind.
Sometimes we tend to ask some questions whose answers are beyond our conception at that point of time. It happened with me several times. I distinctly remember asking my physics teacher in 7th standard, that why all the planets have to rotate in more or less in the same plane, why can't their orbits be tilted at angles like 20/30 degrees to each other, and why does only pluto have such orbit, the teacher couldn't give me an answer. But he was nice to have referred the question to a professor who could answer the question.

This message is a reply to:
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mpc755
Member (Idle past 6071 days)
Posts: 31
Joined: 07-29-2007


Message 49 of 119 (413234)
07-29-2007 11:35 PM
Reply to: Message 47 by Taz
05-28-2007 2:24 AM


quote:
The point of center of gravity of this system in turn orbits around the center of gravity between earthmoon-sun system.
If that is the case, then shouldn't the sun's gravitational force on the earth-moon system be balanced and have no effect on the earth's tides?

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Chiroptera
Inactive Member


Message 50 of 119 (413284)
07-30-2007 8:50 AM
Reply to: Message 49 by mpc755
07-29-2007 11:35 PM


No. How do you derive your conclusion from the quoted sentence?

I've done everything the Bible says, even the stuff that contradicts the other stuff! -- Ned Flanders

This message is a reply to:
 Message 49 by mpc755, posted 07-29-2007 11:35 PM mpc755 has replied

Replies to this message:
 Message 51 by mpc755, posted 07-30-2007 10:53 AM Chiroptera has replied

  
mpc755
Member (Idle past 6071 days)
Posts: 31
Joined: 07-29-2007


Message 51 of 119 (413320)
07-30-2007 10:53 AM
Reply to: Message 50 by Chiroptera
07-30-2007 8:50 AM


If the Sun is interacting with the Earth-Moon system as a single entity, then it should be interacting with it as a whole.
Is there a contradiction with saying the Sun interacts with the Earth-Moon system as a single entity, but will still have an effect on the Earth's tides? If the sun can still impact the Earth's tides, then isn't it interacting with the Earth as a single entity?

This message is a reply to:
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Replies to this message:
 Message 52 by Chiroptera, posted 07-30-2007 11:08 AM mpc755 has replied
 Message 56 by Taz, posted 07-30-2007 8:20 PM mpc755 has not replied

  
Chiroptera
Inactive Member


Message 52 of 119 (413327)
07-30-2007 11:08 AM
Reply to: Message 51 by mpc755
07-30-2007 10:53 AM


I'm not sure how you're coming to these conclusions. Something can interact with a "system" as a whole and still have individual effects on the individual pieces that make up the system. In fact, the effect something has on a "system" is usually the sum of the individual effects on the parts.
But I can see how this might be confusing. I wouldn't explain this as the sun interacting with the earth/moon as a system. I would just say that when one does the calculations for the motions of the earth and moon due to their mutual gravitational attraction, one finds that the earth and moon both orbit their common center of mass.
And when one does the calculations of the sun interacting with the eartha and with the moon, one sees that the earth and the moon orbit their common center of mass as this center of mass orbits the sun in a Keplerian orbit.

I've done everything the Bible says, even the stuff that contradicts the other stuff! -- Ned Flanders

This message is a reply to:
 Message 51 by mpc755, posted 07-30-2007 10:53 AM mpc755 has replied

Replies to this message:
 Message 53 by mpc755, posted 07-30-2007 12:07 PM Chiroptera has replied

  
mpc755
Member (Idle past 6071 days)
Posts: 31
Joined: 07-29-2007


Message 53 of 119 (413337)
07-30-2007 12:07 PM
Reply to: Message 52 by Chiroptera
07-30-2007 11:08 AM


Has anyone come up with a reason why gravity exists, or how it works?
My crack-pot theory is that all objects displace space, and it is this displaced space that causes objects to remain in orbit around one another.

This message is a reply to:
 Message 52 by Chiroptera, posted 07-30-2007 11:08 AM Chiroptera has replied

Replies to this message:
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 Message 55 by Chiroptera, posted 07-30-2007 1:14 PM mpc755 has replied
 Message 57 by Taz, posted 07-30-2007 8:23 PM mpc755 has replied
 Message 69 by AnswersInGenitals, posted 07-31-2007 8:09 PM mpc755 has replied

  
NosyNed
Member
Posts: 8996
From: Canada
Joined: 04-04-2003


Message 54 of 119 (413340)
07-30-2007 12:21 PM
Reply to: Message 53 by mpc755
07-30-2007 12:07 PM


General Relativity
Has anyone come up with a reason why gravity exists, or how it works?
My crack-pot theory is that all objects displace space, and it is this displaced space that causes objects to remain in orbit around one another.
I'm afraid that is, indeed, crackpot.
General relativity is the best current explanation of gravity. Object orbit one another moving in something that might loosly be called a "straight line" in spacetime but spacetime itself is bent by the mass of objects. Gravity is what we see from this bending.
The math of special relativity is pretty simple but GR is much, much harder (too hard for me). GR is also extremely well supported by many experiments. It is far and away the best answer we have right now.

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Chiroptera
Inactive Member


Message 55 of 119 (413353)
07-30-2007 1:14 PM
Reply to: Message 53 by mpc755
07-30-2007 12:07 PM


Has anyone come up with a reason why gravity exists, or how it works?
No. Newton only stated that bodies attract one another, and he gave a formula for this attraction. The formula very accurately describes the motions of objects in real life, so there is something to it, but he never made a guess as to why bodies attract one another.
Einstein only stated that space-time is curved, and he gave some equations to describe this curvature. The equations very accurately (more accurately than Newton, even) describes the motions of objects in real life, so there is something to it, but he never made a guess as to why space-time is curved.
In the language of Newton, it is simply the nature of material bodies to attract one another. Mass is simply the measure of how much a particular body attracts others, but no one has come up with a reason why this attraction exists.
In the language of Einstein, it is simply the nature of space-time to be curved. Mass is simply the measure of how curved a region of space-time is, but as far as I know no one has come up with a reason why space-time should be curved.

I've done everything the Bible says, even the stuff that contradicts the other stuff! -- Ned Flanders

This message is a reply to:
 Message 53 by mpc755, posted 07-30-2007 12:07 PM mpc755 has replied

Replies to this message:
 Message 58 by mpc755, posted 07-30-2007 9:48 PM Chiroptera has replied

  
Taz
Member (Idle past 3291 days)
Posts: 5069
From: Zerus
Joined: 07-18-2006


Message 56 of 119 (413409)
07-30-2007 8:20 PM
Reply to: Message 51 by mpc755
07-30-2007 10:53 AM


mpc755 writes:
Is there a contradiction with saying the Sun interacts with the Earth-Moon system as a single entity, but will still have an effect on the Earth's tides? If the sun can still impact the Earth's tides, then isn't it interacting with the Earth as a single entity?
And here you've stumbled into another very common misconception, even among science enthusiasts and many physics students.
First of all, it's our moon that causes earth's tides more than anything else. The misconception is that the moon's gravity itself somehow pulls on the water of the oceans and causing them to rise. Ask anyone here among the science enthusiasts and they will tell you the same thing. This is not entirely correct.
To fully understand why we have tides, you first need to have a little imagination to understand what I'm about to explain.
The earth's tides are not caused by the direct gravitational pull of the moon on earth's ocean waters. Think of it this way. You don't feel any lighter everytime the moon passes over your head, do you? The moon's gravity acts on earth more or less as a whole rather than the individual objects.
The true explanation for earth's tides is the following. Even though we tend to think of gravitational force acting on an object like earth more or less as a whole, we have to put into account the distance between one side of the earth to the moon and the other side of the earth to the moon. The fact is the side of the earth that is closer to the moon experiences a slightly greater pull from the moon than the other side. The effect of it is the earth's shape changes back and forth very slightly. The changes in shape are so minutely slight that you can't actually observe any difference here on earth. But this changes in shape actually cause the earth's oceans to appear to rise up and down.
Here is something you can do to help you better visualize this effect. Get a plastic container like a milk bottle or something. Pour water into it. Now, compress it and release it. Observe how the water wobbles back and forth.
This same effect is what is causing one of Jupiter's moons to have active volcanos. Jupiter's gravity is doing the same thing to its moon what earth's moon is doing to earth but at a much greater scale. The changes in the shape of jupiter's moon is causing a lot of friction within the moon which in turn is giving it a very active core.
Anyway, if my above explanation doesn't make any sense, I guess it's fine if you want to continue to believe that the moon is actually pulling onto the earth's ocean water and causing it to rise giving us tides.

Disclaimer:
Occasionally, owing to the deficiency of the English language, I have used he/him/his meaning he or she/him or her/his or her in order to avoid awkwardness of style.
He, him, and his are not intended as exclusively masculine pronouns. They may refer to either sex or to both sexes!

This message is a reply to:
 Message 51 by mpc755, posted 07-30-2007 10:53 AM mpc755 has not replied

  
Taz
Member (Idle past 3291 days)
Posts: 5069
From: Zerus
Joined: 07-18-2006


Message 57 of 119 (413410)
07-30-2007 8:23 PM
Reply to: Message 53 by mpc755
07-30-2007 12:07 PM


mpc755 writes:
Has anyone come up with a reason why gravity exists, or how it works?
Why does gravity exist? I don't know. But I doubt it would help us understand the universe any better if we answer because it is god's will. Catch my drift?

Disclaimer:
Occasionally, owing to the deficiency of the English language, I have used he/him/his meaning he or she/him or her/his or her in order to avoid awkwardness of style.
He, him, and his are not intended as exclusively masculine pronouns. They may refer to either sex or to both sexes!

This message is a reply to:
 Message 53 by mpc755, posted 07-30-2007 12:07 PM mpc755 has replied

Replies to this message:
 Message 60 by mpc755, posted 07-30-2007 10:16 PM Taz has not replied

  
mpc755
Member (Idle past 6071 days)
Posts: 31
Joined: 07-29-2007


Message 58 of 119 (413437)
07-30-2007 9:48 PM
Reply to: Message 55 by Chiroptera
07-30-2007 1:14 PM


In the language of Einstein, it is simply the nature of space-time to be curved. Mass is simply the measure of how curved a region of space-time is, but as far as I know no one has come up with a reason why space-time should be curved.
Space-time is curved because the space that would exist where the object is has been displaced. The object's mass displaces space. For example, the Earth displaces space. This displaced space is what keeps the Moon in orbit around the Earth.

This message is a reply to:
 Message 55 by Chiroptera, posted 07-30-2007 1:14 PM Chiroptera has replied

Replies to this message:
 Message 59 by Taz, posted 07-30-2007 10:11 PM mpc755 has replied
 Message 64 by Chiroptera, posted 07-30-2007 11:10 PM mpc755 has not replied

  
Taz
Member (Idle past 3291 days)
Posts: 5069
From: Zerus
Joined: 07-18-2006


Message 59 of 119 (413443)
07-30-2007 10:11 PM
Reply to: Message 58 by mpc755
07-30-2007 9:48 PM


Just so you know, everytime you want to put forward an idea that is different than what the conventional science tells us, you have to answer at least the following: What's wrong with the conventional view? Why is your idea better than the conventional view? What predictive power does your "theory" have?

Disclaimer:
Occasionally, owing to the deficiency of the English language, I have used he/him/his meaning he or she/him or her/his or her in order to avoid awkwardness of style.
He, him, and his are not intended as exclusively masculine pronouns. They may refer to either sex or to both sexes!

This message is a reply to:
 Message 58 by mpc755, posted 07-30-2007 9:48 PM mpc755 has replied

Replies to this message:
 Message 61 by mpc755, posted 07-30-2007 10:21 PM Taz has replied

  
mpc755
Member (Idle past 6071 days)
Posts: 31
Joined: 07-29-2007


Message 60 of 119 (413448)
07-30-2007 10:16 PM
Reply to: Message 57 by Taz
07-30-2007 8:23 PM


Tazmanian Devil writes:
< !--UE-->But I doubt it would help us understand the universe any better if we answer because it is god's will. Catch my drift?
Not sure why you went here. I am not religious.

This message is a reply to:
 Message 57 by Taz, posted 07-30-2007 8:23 PM Taz has not replied

  
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