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What is gravity ?

Posted by: Rolf Guthmann - Fri Dec 02, 2005 7:57 pm
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What is gravity ? 
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Post    Posted on: Fri Dec 09, 2005 5:52 am
Granted, that's why I got into AE in the first place......

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Post I'm faaaaaaaaaaaaaaaaaaaaaaaaaaaaalling! Ahhhhhhhhhhhhhhhhh!   Posted on: Tue Jan 03, 2006 8:46 pm
campbelp2002 wrote:
After countless pages of posts showing that to be impossible, all the people who initially believed that it was possible still do.


For the simple reason that things WILL spiral into the sun. If something in orbit gets pushed closer to the sun, in relation to a line drawn from the sun to the object, the gravity will increase. The additional pull of the sun on the object will be equalled out by decrease in velocity need to stay in a lower orbit- but the object will pull on the sun with greater force. Therefore, ipso facto, sum total, in the end and at the end of the matter, the object will spiral into the sun. Heavier objects fall faster than lighter ones.
8)

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Post    Posted on: Thu Jan 05, 2006 4:56 am
SuperShuki wrote:
campbelp2002 wrote:
For the simple reason that things WILL spiral into the sun. If something in orbit gets pushed closer to the sun, in relation to a line drawn from the sun to the object, the gravity will increase. The additional pull of the sun on the object will be equalled out by decrease in velocity need to stay in a lower orbit- but the object will pull on the sun with greater force. Therefore, ipso facto, sum total, in the end and at the end of the matter, the object will spiral into the sun. Heavier objects fall faster than lighter ones.
8)


Except for that they don't. Objects fall at the same rate regardless of their mass (I assume you're referring to mass when you talk about "heavier" objects), all other variables being equal. Think of it this way: two baseballs will fall with almost exactly the same acceleration, whether they're seperated by a millimeter or a thousand kilometers. They wouldn't any faster simply because they're glued together, right? You can view any material object as simply being an amalgam of these balls, although in the real world mass is carried in quarks and leptons. True, denser objects have a higher terminal velocity on Earth than looser objects, but this is entirely caused by atmospheric drag. In the vacuum of space, where drag is negligable, all objects fall with the same acceleration, hence a hammer and a feather dropped on the Moon will hit the ground at the same time.

Objects' masses don't change as they close distance with their orbital partners. Certainly their weight does, but by stipulation mass remains constant regardless of of the object's position in the universe. Remember, gravity is determined by three things, the mass of each of the two bodies involved, and their distance from each other. Since neither object's mass changes as the orbiting body nears the stationary one, the gravitational force does not increase as a result of weight gain. As you said, the gravitational force increase created by the change in distance is countered by an increase in speed of the orbiting object, and therefore no change in energy occurs.

Think of it this way. A satelitte has a certain amount of energy at all times, some of it potential and some of it kinetic. The law of conservation of energy (as well as simple common sense) shows that the total of kinetic and potential energy of the object will always remain the same. If this is true, then an object cannot spiral into the Sun because doing so would require it to decrease its potential energy without fully converting it into kinetic energy. TANSTAAFL is a two-way street.

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Post    Posted on: Thu Jan 05, 2006 5:08 pm
Senior Von Braun wrote:
Objects fall at the same rate regardless of their mass (I assume you're referring to mass when you talk about "heavier" objects), all other variables being equal.


More massive objects fall faster. F=MA ; If F stays constant, and M changes, A must also change.
In this case, F is increasing (i.e. the object weighs more), and M stays the same, so A must change.
F is increasing because the object is getting closer to the Sun.
If the change in acceleration was exactly equalled out by the change in velocity, the difference in the accelerations of two different masses would not be taken into account.
A bowling ball, sent towards the sun, would have the same change in velocity as Alpha Centauri (another star) sent towards the sun, from the same orbit, but they would have vastly different changes in acceleration towards the sun. Alpha Centauri would pull the sun alot more than the bowling ball.
The reason that things don't appear to spiral into the sun is that the spiraling is so slow when dealing with small objects. But slow and small are relative. Kinda like cousins, a thousand times removed.
:D
P.S
About preservation of energy: An object in orbit of the sun is like a ball on a shelf. Push the object off the shelf, and it falls!

P.P.S. Energy isn't always preserved. Sometimes its converted to mass. Or light. Or some such. But then it doesn't go so well with pancakes.
:lol:

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Post    Posted on: Thu Jan 05, 2006 6:48 pm
SuperShuki, without any added force, except gravitation and an objects current velocity. I think an object can "not" spiral into the sun.

If an object is placed or moving with a velocity to the sun "I think", it has 2 options:
1) hit, or fall into the sun without going arround the sun one time.
2) if it spirraled / orbitted the sun atleast 1 time, I think you can be 100% sure, it will "never" fall into the sun, without any added force. If something is orbitting the sun (or earth) and doomed to fall into it, it means the object was placed with an added force (rocket engine) in that position at first, but if you push an object at larger distance, to the earth or the sun, it will hit, or aftern 1 orbit, never hit...

Can someone else tell me if this is right or wrong ? I haven't checked my facts :P

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Post    Posted on: Thu Jan 05, 2006 7:13 pm
Eyelids getting heavier, unable to stiffle yawn...zzzzzzzzzzzzzzzz :)

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Post    Posted on: Thu Jan 05, 2006 8:35 pm
Look, just don't go there, people... this argument is doomed by either a failure of some proponents to grasp the fundamental concepts or a really anal semantic interpretation on the part of others; or both.

Sigurd, the flaw in all of Shuki's examples is that they fail to account for the conservation of angular momentum, but folks whom can't wrap thier heads around that never will...

And yes, to all of you law school types out there, in some arbitrarily large segment of time, all orbits would either decay to thier focus or to escape trajectories, meaning that some objects do "spiral in;" but those are all cases involving some (alibiet miniscule) external force. And in the case of heliocentric orbits the "arbitrarily large segment of time" is typically longer than the sun's dwarf-phase lifespan, meaning that any such orbit will more probably be absorbed by the expanding sun than actually meet it via decay.

We could ask Cowboy to have one of his profs answer but then he'd have to make himself look foolish for not being able to fend for himself... and the people whom thought they could hand-toss a ball into the sun from LEO would still proably believe it anyway...

Best to let sleeping dogs (and Andy too, it seems) lie in any case.


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Post    Posted on: Fri Jan 06, 2006 9:35 pm
Just to be joyous,

Why don't electron orbits decay?

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Post    Posted on: Fri Jan 06, 2006 9:55 pm
Ahh, but they do; it is simply a matter of (drum roll, please)...

EXTERNAL FORCES! (bet nobody saw that coming, huh?)

Electron orbits acted upon by external forces can decay to thier focus (the production of neutron stars is an example) or to escape trajectories (we call this a plasma) These things happen every day in the great wide universe, and the conservation of angular momenta in all of these systems is strictly observed (unless a waiver is obtained by filing in triplicate at your local galactic center)


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Post    Posted on: Sat Jan 07, 2006 6:36 am
And I thought it was just mould...

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Post external forces   Posted on: Sun Jan 08, 2006 4:45 am
In this case then, the gravitys caused by the "orbiting" object is external.

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Post    Posted on: Sun Jan 08, 2006 5:10 pm
SawSS1Jun21 wrote:
These things happen every day in the great wide universe, and the conservation of angular momenta in all of these systems is strictly observed (unless a waiver is obtained by filing in triplicate at your local galactic center)


That would be at the local planning center located in the Alpha Centauri system, or would we have to go directly to Ursa Minor Beta and conduct negotiations directly with the Guide?

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Post    Posted on: Mon Jan 09, 2006 3:55 pm
Shuki: Not according to Issac Newton. But you should believe whatever makes you happy. Happy is more important than reliable space transportation.

Cowboy: I believe that the office at Alpha Centauri is closed for remodeling until the next millenia, so if you plan to violate any conservation laws, you're going to need superluminal travel... of course, to achieve superluminal travel, you'll probably have to violate some conservation laws; hence we see that the primary obstacle to interstellar travel is bureacracy (no great surprise there, I guess).


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Post    Posted on: Mon Jan 09, 2006 11:06 pm
Damned Vogons.

As far as the actual discussion goes, I think my professors would be much happier if I stuck with the opinions of Newton and Company. So would I, since I'd like to get at least passing grades.....

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Post    Posted on: Tue Jan 10, 2006 7:36 am
Oh no, not Vogons :shock: . The last time we met up with them they demolished Earth and that's why we all feel like somethings missing. Anyone checked on the dolphins lately? And spare us the bad poetry please please :cry:

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