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Author Topic:   Sun-Earth-Moon Gravity
Ragged
Member (Idle past 3552 days)
Posts: 47
From: Purgatory
Joined: 10-26-2005


Message 1 of 119 (347450)
09-08-2006 12:09 AM


My astronomy teacher has asked me to calculate gravitation pull between Earth and Sun, Earth and Moon, and Sun and Moon. Well, it turned out that gravitational attraction between Sun and Moon ( 4.369 * 10^20 N) is more than twice that of between Earth and Moon (1.986 * 10^20 N). His (and my) question is why doesn't Sun pull Moon toward itself and away from its current orbit around the Earth.
I was thinking that is has something to do with angular momentum or something like that. Should I read more about planet and star creation, to find the answer?
I haven't seen many threads regarding our solor system in the Cosmolgy thread, but if you don't mind can you help me out? Thanks.

Replies to this message:
 Message 3 by Taz, posted 09-08-2006 2:43 AM Ragged has replied
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 Message 25 by AnswersInGenitals, posted 11-10-2006 7:29 PM Ragged has not replied
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AdminJar
Inactive Member


Message 2 of 119 (347452)
09-08-2006 12:11 AM


Thread moved here from the Proposed New Topics forum.

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


Message 3 of 119 (347484)
09-08-2006 2:43 AM
Reply to: Message 1 by Ragged
09-08-2006 12:09 AM


Before "we" answer your question, is this this out of your curiosity or is this part of a homework?
P.S. The answer is more obvious than you think.

This message is a reply to:
 Message 1 by Ragged, posted 09-08-2006 12:09 AM Ragged has replied

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Ragged
Member (Idle past 3552 days)
Posts: 47
From: Purgatory
Joined: 10-26-2005


Message 4 of 119 (347518)
09-08-2006 9:00 AM
Reply to: Message 3 by Taz
09-08-2006 2:43 AM


Well the class that Im taking turned out to be ridiculously easy, so the professor is trying to make it more interesting for me by giving me extra assignments such as that one, so its really not my homework or anything I'd be gettting a credit for. Its just something to give me more knowledge.

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Dr Jack
Member
Posts: 3514
From: Immigrant in the land of Deutsch
Joined: 07-14-2003
Member Rating: 8.7


Message 5 of 119 (347519)
09-08-2006 9:08 AM
Reply to: Message 1 by Ragged
09-08-2006 12:09 AM


Calculate the gravity between the earth and the moon, and consider what your frame of reference should be.

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nwr
Member
Posts: 6408
From: Geneva, Illinois
Joined: 08-08-2005
Member Rating: 5.1


Message 6 of 119 (347520)
09-08-2006 9:20 AM
Reply to: Message 1 by Ragged
09-08-2006 12:09 AM


His (and my) question is why doesn't Sun pull Moon toward itself and away from its current orbit around the Earth.
The moon is in orbit around the sun - essentially the same orbit as earth. The moon/sun gravitation is what keeps it in that orbit.
The moon/earth gravitation explains the perturbations of the moon orbit, the cyclic motions within the orbit that we think of as the moon going around the earth.

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Son Goku
Inactive Member


Message 7 of 119 (347552)
09-08-2006 12:56 PM


This won't add much beyond what nwr said, but consider the moon's motion around the earth as simple "noise" in its motion around the sun.
Zoom out far enough and you wouldn't notice it.
Edited by Son Goku, : My amazing ability to constantly forget the word "you".
Edited by Son Goku, : God lord, moon not monn.

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Chiroptera
Inactive Member


Message 8 of 119 (347583)
09-08-2006 3:05 PM
Reply to: Message 7 by Son Goku
09-08-2006 12:56 PM


Good answers. I will add one more thing: think of the sun's gravity keeping the earth in its orbit and the sun's gravity keeping the moon in its orbit. These happen to be essentially the same orbit. The earth and moon's gravity then keeps these two bodies close together while they both are orbiting the sun.

"The fact that a believer is happier than a skeptic is no more to the point than the fact that a drunken man is happier than a sober one." -- George Bernard Shaw

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


Message 9 of 119 (347639)
09-08-2006 9:20 PM
Reply to: Message 1 by Ragged
09-08-2006 12:09 AM


While other people have certainly answered your question, permit me to present it in what I think as a more understandable approach.
In space, as long as any two objects have mass and are close enough to each other so that their graviational influence could be felt, you can bet that they orbit each other. More accurately, though, you would have to say that they orbit a point in space between them known as the center of mass between the two objects.
Believe it or not, since the Earth has mass, there is a center of mass (or center of gravity) between the Earth and the Sun that is not at the center of the Sun. But because the Sun's mass is considerably greater than the Earth's, the center of mass of the Earth-Sun system is at a point in space that is not at the center of the Sun but beneath its surface.
In physics, or at least classical mechanics, you have to view everything in terms of a classical system of objects. If you haven't gone over center of mass (or center of gravity), you will later in your course.
And as such, the Earth and the Moon represents a classical system where both objects orbit around the center of gravity between the two. Because the Earth's mass is considerably greater than the Moon, the center of gravity of this system is closer toward the Earth than the Moon, which gives the impression of the moon orbiting the Earth.
Because there exists two primary objects in the vicinity of Earth (Earth and Moon), we have to temperarily consider the Earth and the Moon as one object whose center of mass representing the center of this object. Let's call this imaginary object EM. The Sun is another object. Objects EM and the Sun make up another classical system where the center of gravity of this system lies underneath the surface of the Sun (and in fact very very close to the center of the Sun).
Ok, perhaps this explanation isn't as easy to digest as I originally thought, but I think it gives a better outline of what is happening here in regard to Sun-Earth-Moon relationship.
Hope that helps.
Added by edit.
Further thought on what I have gone through.
Because we can view everything in space in terms of classical systems, we have been able to use this very simple concept to find extra-solar planets. What these planet hunters do is observe a particular star night after night to see if it wobbles. The wobbling of a star indicates that it is orbiting around a center of gravity that lies underneath its surface but is not at its center. In other words, there must be another object of significant mass in close proximity for the star to behave that way.
Unfortunately, our current technology can only permit us to confirm a wobble of a star to a certain degree that the companion object (planet) of the star has to be at least as massive as Jupiter. Anything smaller and the wobbling would not be detected at all by our instruments.
Anyway, through this very simple concept, we have been able to detect dozens of jupiter size and more massive planets orbiting other stars.
Edited by gasby, : No reason given.

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Ragged
Member (Idle past 3552 days)
Posts: 47
From: Purgatory
Joined: 10-26-2005


Message 10 of 119 (347789)
09-09-2006 2:46 PM


I thank all of you for your feedback. Your replies have certainly clarified alot of things for me. There is still an inconsistency in my understanding, though.
As far as I know gravitational pull depends on combined mass and distence between objects. Since Earth and Moon have essentially the same orbit around the Sun, it means that they are the same distance away from the Sun at any given point in time (disregarding Earth-Moon motion relative to each other). It would make sense for Earth-Sun atraction to be greater then Moon-Sun atraction, since mass of the Earth is considerably greater. I could even buy if the two forces where nearly the same, but why would Moon-Earth gravitation field be so much greater?
Thanks again.
Edited by Ragged, : No reason given.

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


Message 11 of 119 (347795)
09-09-2006 3:34 PM
Reply to: Message 10 by Ragged
09-09-2006 2:46 PM


You are still not thinking in terms of classical systems. While the Moon and Earth are locked in orbit around their center of gravity, you have to treat the two objects as one when you add the Sun into the equations.
Just try to think of it like this. The Earth and the Moon together makes up one system (EM system). Now, think of EM as a single object. EM and the Sun makes up another system.

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Replies to this message:
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jar
Member (Idle past 393 days)
Posts: 34026
From: Texas!!
Joined: 04-20-2004


Message 12 of 119 (347798)
09-09-2006 3:40 PM
Reply to: Message 10 by Ragged
09-09-2006 2:46 PM


...but why would Moon-Earth gravitation field be so much greater?
Distance.

Aslan is not a Tame Lion

This message is a reply to:
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Replies to this message:
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 Message 14 by Ragged, posted 09-09-2006 4:34 PM jar has replied

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


Message 13 of 119 (347799)
09-09-2006 3:47 PM
Reply to: Message 12 by jar
09-09-2006 3:40 PM


G*M*m
--------- = F
d*d
G = universal gravitational constant
M = mass of one object in Kg
m = mass of the other object
d = distance between them in Meters
F = force in Newton

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Ragged
Member (Idle past 3552 days)
Posts: 47
From: Purgatory
Joined: 10-26-2005


Message 14 of 119 (347809)
09-09-2006 4:34 PM
Reply to: Message 12 by jar
09-09-2006 3:40 PM


Distence is the same. The only difference I see is in mass, of which Earth has alot more.
Gasby, I made two separate calculations. One separate for Earth-Sun the other is for Moon-Sun. Moon-Sun gravitational force turned out to be much greater. Were my calculation wrong? Or is it even wrong to separate the two?
From those calculations it turns out that Sun "requires" much greater force to keep Moon in orbit, then it does the Earth. Thats the part that I dont get.

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Replies to this message:
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Coragyps
Member (Idle past 734 days)
Posts: 5553
From: Snyder, Texas, USA
Joined: 11-12-2002


Message 15 of 119 (347812)
09-09-2006 4:41 PM
Reply to: Message 14 by Ragged
09-09-2006 4:34 PM


Were my calculation wrong?
Must have been! Distance, as you say, is very nearly the same, and the Moon's mass is 0.0123 that of the Earth. So Sun-Moon attraction will be 0.0123 that of Sun-Earth.
As far as "cocktail-napkin" calculations, try this:
Earth-Moon attraction = 1 X 0.0123/12 = 0.0123
Earth-Sun attraction = 1 X 333,000/3902 = 2.18
Moon-Sun = .0123 X 2.18 = 0.0269
Edited by Coragyps, : add calculations

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