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Q: Grafity, how does it work. ( Answered,   3 Comments )
Subject: Grafity, how does it work.
Category: Science > Physics
Asked by: utrechtsedomtoren-ga
List Price: $10.00
Posted: 30 Nov 2006 04:45 PST
Expires: 30 Dec 2006 04:45 PST
Question ID: 786955
How can mass see eachother, f.e. how does the moon
'know' that it has less mass then the earth, so it turns around the
earth instead of the earth turns around the moon.
Subject: Re: Grafity, how does it work.
Answered By: justaskscott-ga on 18 Dec 2006 01:52 PST
Hi utrechtsedomtoren,

Ian G.'s comment is a good starting point for this answer.  As
Courtney Seligman, an astronomy professor, observes:

"The Earth and Moon are each moving around the center of mass of the
Earth-Moon system.  Since the Earth is much more massive, it is much
closer to the center of mass, which is actually inside the Earth."

Prof. Seligman notes that the Earth and the Moon ""each exert[] a
force on the other which, according to Newton's Third Law of Motion
(the Law of Action and Reaction), is equal and opposite to the force
that the other is exerting on it."  However, "although the forces are
equal, their effects are not, because the more massive Earth is
accelerated less by the same force, than the less massive Moon."

"Gravitational Interactions of the Earth and Moon"
Personal website of Courtney Seligman, Author / Professor of Astronomy

For further explanations of the Earth-Moon system, see:

"Question #1526" (answered by Louis A. Bloomfield)
How Things Work

"The Moon's Motion," by Dr. Jamie Love
Merlin Science

In short, the more massive Earth does not move as much around the
barycenter as the less massive Moon.

As your question indicates, an orbit is a result of gravity.  Gravity
is a complex phenomenon, still being studied.  However, for purposes
of this answer, I think that it suffices to say that a massive object
warps space-time, causing other objects to move towards it (rather
than, say, continue in a straight line).

"Gravity" (27 Dec 1997)
The Why Files

As explained by Dr. Jamie Love in the document cited earlier, the
Moon, in orbiting the Earth, is essentially moving toward the Earth
but never hitting it.  (And likewise, the Earth is moving toward the
Moon and never hitting it -- though its movement is not as great,
since, as noted before, it is much more massive.)

For more information about gravity (and related subjects), see:

"Special & General Relativity Questions and Answers," by Dr. Sten Odenwald
Gravity Probe B

Please let me know if you need any clarification of this answer.

- justaskscott

Search strategy --

Searched on Google for:

"earth orbit the moon"
"earth orbits the moon"
earth moon barycentre
earth moon barycenter
Subject: Re: Grafity, how does it work.
From: markvmd-ga on 30 Nov 2006 06:51 PST
Brad Gasrret quipped in his routine from pre-Star Search, "The woman
I'm dating is Swedish, six feet two inches. Her name is Inertia. Large
Subject: Re: Grafity, how does it work.
From: siliconsamurai-ga on 30 Nov 2006 08:33 PST
You want to post this in religion -
the concept of assigning thought and feelings to
inanimate objects might be best addressed by
a Shinto or someone who thinks in terms of 
dryads or other spirits of plants or mountains or something,
 perhaps a druid would be a 
good source of an answer.

Although he was a christian philosopher, C.S. Lewis assigned god-like 
attributes to the planets, perhaps a reading of his "adult" books (not Narnia)
would be a help.
Subject: Re: Grafity, how does it work.
From: iang-ga on 30 Nov 2006 08:39 PST
Rather than thinking of the Earth and the Moon as two separate bodies,
think of them as a system - they're working together. The Earth and
the Moon attract each other, but the Earth's more massive so it pulls
harder.  It's not quite true to say the Moon orbits the Earth - the
Moon and Earth both orbit their centre of mass, called the barycentre.
 It happens that the barycentre is within the Earth, but the Earth
wobbles as the Earth / Moon system revolves.

Ian G.

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