Science Question
 

Why does Mercury not get pulled into the Sun?

Start with Newton's First Law of Motion

This basically means that Mercury will continue moving through space until something exerts a force on it. The gravity of the sun pulls it in, but it already has a significant "forward" (or sideways, if you will) motion and therefore goes into an orbit. In other words, it is constantly "falling" into the sun, but it also is constantly moving forward. The end result of these two orthogonal forces is a circular orbit.

This is a very basic interpretation, but you get the idea.

Check out this website for more interesting info: http://curious.astro.cornell.edu/index.php



Mr Mephisto

and, since its forward motion was created by something initial (big bang?), and the suns gravitational force is constant, Mercury is very slowly getting pulled towards the sun, the same way the moon will eventually fall to the earth...but not for a looooong time

I'm not so sure it will eventually fall into the sun. As it's moving through a vacuum, only gravity is acting upon it. Therefore, what else will cause this "closed system" to change?

Unless we consider the fact that space is not really a vacuum (a true vacuum) and that there are other, very very small, forces that come into play. Remember, even light has an effect upon matter. As such, there is also a constant force (albeit an almost neglible force) "pushing" it away from the sun.

Mr Mephisto

I'm under the impression that the moon is leaving orbit, to the tune of some number of inches per year.

Ahhh, here we are.
Space.com

The fact that orbital motion is slowly spiraling inward is a highly debated topic. You asked a doozy of a question. It may or may not eventually hit the sun. There are several arguements, but the one I'm most familiar with is that due to the exact shape of the orbits (ovular, not circular), neither the planets nor the moon will ever stop orbiting with the way the galaxy is now unless something changes. However, I could always be wrong.

You could always be wrong?

Or, as always, you could be wrong?

:D


Mr Mephisto

Well he can not always be wrong, lest he be wrong about always being wrong, and thus not be wrong about always being wrong, and not always being wrong.

So the second one.

Even if Mercury was slowly moving toward the sun, I would think that the sun will became a red giant and engulf Mercury ( the sun basically grows and becomes a lot brighter) long before Mercury was have moved into the sun.

you are basically right, the sun will swell into a red giant and swallow not just Mercury but Venus as well. But it will not become alot brighter, it will actually get dimmer, because the there will be a larger surface to light intensity ratio.
Oh and the Earth will be fried when this happens as well, but it will not be swallowed.

The one thing that I'm not sure about is if the Earth will fall into the sun when this happens, because surely the change in size and density of the sun would cause changes in it's gravity. Would the earth fall into the sun, fly out of orbit, or would the changes forces maintain a balance and the earth continue to orbit?

I think it would fly out of orbit. The volume occupied by a star is dependant on the inward force of gravity on its gasses and the ourward force of the fusion reactions occuring inside the star. As the star ages it is losing mass, therefore the pressure created by the fusion reactions begin to win over the gravitational forces pulling the gasses inward. At this point the gravitational forces acting on the Earth will be much lesser than they are right now, so we should fly out of orbit.

I'm not sure if I'm correct, but I think I'm on the right track.

Well, I think this gives us a pretty good incentive to start funding space programs better.

I just "HAD" to put this in the memorable quotes thread.....truly classic

the earth will not fall into the sun when it becomes a red giant. the force of the sun on the earth depends only on the mass of the two objects (assuming each possess a certain symmetry, which they do to a very good approximation). since the mass of the sun and the mass of the earth will be essentially unchanged when the sun goes giant, the earth's orbit will be stable. unless the sun does "swallow" the earth, in which case friction from the earth traveling throught the sun's "atmosphere" will cause it to slow down and spiral inward.

well from what I last heard scientists when scientists had esitimated how large the sun would get, it would not swallow the earth.

As far as I know the pull of gravity is not just decided by mass but also by volume, otherwise nebulas would have a massive amount of gravity. So as far as I can understand it, as the sun expanded it would become less dense and therefor it's pull would be lessened somewhat.

no, it's determined by mass. the nebulas aren't really all that massive - any more than a cloud is massive. There's a LOT of space between the matter in a nebula.

Meaning that it's less dense. Density = Mass/Volume.
I'm pretty sure that gravity is not just a factor of mass, but also of the volume of that mass. But I'm only basing this on my reasoning, which is certainly not flawless, so feel free to find a formula to prove me wrong, or just say I'm wrong, either way works.

Oh, and by the way, nebula are pretty massive, stars form in them, from the gasses collapsing in until enough pressure is formed to start fussion.

well I didn't explain myself overly well. Here's a second attempt, which probably won't be any better ;)

if you take a sphere that's 1 inch in diameter and has a gravity factor of (making shit up here) 1, and you then increase the diameter of that sphere without increasing how much stuff makes up the sphere (i.e. you put more space between the atoms of the sphere) so that it's 2 inches, the gravity factor stays at 1.

Density does = mass/volume, but gravity does not = density.

Gravity is simply the measurement of attraction between two objects.

So if I'm orbiting the sun at the distance of Venus, and the sun expands without gaining or losing mass, and assuming the sun does not engulf my orbit, then I'll stay in the same orbit. If the sun engulfs my orbit and spreads out to, say, Jupiter, and I'm in a suit of unobtanium which allows me not to be burned up by the expanded sun, then I'll break orbit. Why? Because some of the mass that was pulling me <-- that way is now over --> there. My velocity didn't change, but the force pulling me inward has decreased (and if you wanna get REAL picky, there's now more force pulling me outward, namely the part of the sun that expanded beyond my orbit). So now my velocity will pull me out of orbit.

Wow. That's tangled. Did it make ANY sense?

Yes, but as the sun burns it loses mass, mostly as solar wind, and if you were to take that same sphere with a g factor of 1 and shrunk it down to a infinitly small radius, then you would have a singularity, however a very small one with a small event horizon, but a singularity with an increased g force. And if you expand the sun out enough it becomes nothing but, tada, a cloud of gas with a negligible gravitational pull. Gravity may not be solely determined by density, but density is a factor.

Gravity, in no small part, is only affected by mass, ie, the mass of each individual atom
that comprises the star. Within each atom, are four forces, strong and weak have
no affect whatsoever outside of the outer nucleus. Electromagnetic force, on the one
hand, expands just outside the entire atom before dimishing almost completely. Add
up all of the atoms and that extra bit of electromagnetic force, and you get one
hell of a push outwards. Gravity counters this outward force but not completely, being
that electromagnetic forces are stronger than gravitational ones, by a magnitude of
about 1000. Therefore, unless something completely knocks the atoms out of whack
and they forget they have an electromagnetic force, no planet will ever hurtle towards
the sun, and it would basically take another planet to even think about moving one
outwards.

ffffffffffhhhhhhhgggggggggn nbbbbb

About the gravity/density relationship, there is none. When it comes to spherical objects, as long as you are outside of the sphere you can treat the object as a point mass. The gravitational force of the sphere on an object is only a function of the distance betwen the two objects.

I agree with kutulu about that. If you wanted to increase the gravitational pull of that point mass, you'd add more mass to it. Whether the volume (and therefore, the density) changes, is totally irrelevant. Realistically, if you increase the mass of the point mass and bring a varying volume into the equation then its density is going to change. However, the mass and the distance from the object are the only two factors you need to worry about when calculating gravitational effects. Volumes and densities are a completely different story.

Ok, with this many people telling me I'm wrong I guess I shall have to concede.

I have a question though and it is based off of a half remembered statement, so if anyone can elaborate on it I would appreciate it.
Does a dense mass rotating at high speeds(and I mean HIGH speeds) create a stronger gravitational pull than the same mass not spinning?

No. Why should it? It's all about the Massiums.... erm, Mass. Look at Saturn, spins
slower than a mofo, but it definately exerts a hefty Gravitational force. Although
I really have nothing to compare it to, so that's probably a null statement.

I'm going to err on the side of caution here because i haven't read anything to confirm or deny the theory that things spinning faster makes their gravitational pull grow. However, my theory based on how much knowledge of the subject resides in my brain is that it would have no effect on the gravitational pull at all since the mass of the object isn't changing. If you were on the surface of the big spinning mass then it could possibly be that the centrifugal force is acting on you and if the spin becomes fast enough, it could propel you off the big spinning mass. (I'm not sure if centripetal force comes into this theory) But thats just one force being stronger than another, the gravitational force is still the same.

Seek a second opinion ;)

Saturn has a hefty pull because it has a large mass.
My question is theoritical, I was thinking of something about the size of a grapefruit that has a good bit of mass spinning really fast, like so fast that a point on the surface would be travelling at close to the speed of light. I could swear that I had heard this effect mentioned somewhere. :confused:

As something travels closer to the speed of light (and I mean real freakin close) then
it starts to gain an infinite amount of mass, but I don't believe it affects gravitational
pull in anyway. It might, I've never really read anywhere about correlations between
speed-of-light-induced mass and gravity. I could totally be wrong at this last point.

As stated above, any spherical mass can be treated as a point mass - so you have all of the gravity "coming from" the point in the center of the circle.

I'm not sure what would happen if you spun up the sphere to near-light-speed values, that would be an interesting question to ask an astrophysics professor, but as long as the mass of the object doesn't change, the force of gravity it puts out does not change.

The only time when rotational speed will affect gravity, it would be indirectly, with the loss or gain of mass.

I am very sure of this.

but what about the supposed mass gained by large increases in speed? As Paradise Lost said, as an object approaches the speed of light it gains mass, so wouldn't that cause it to have a stronger gravitational pull?

why does it gain mass and where does this mass come from?

It gains mass because e=mc^2 - and the mass comes from the energy put into the object in order to make it travel faster.

As far as I can understand the reasoning behind the whole gaining mass the closer to speed of light thing is: As the object approaches the speed of light it gains an incredible amount of kinetic energy. Now all of that energy can't just sit there and do nothing, it's energy it's got to do something, so lacking anything else to do it becomes mass.

Which is why it is theorized that you can't go faster than the speed of light, because as you approach it(theoretically) your mass approaches infinity and the energy it takes to accelarate that increasing mass approaches infinity also.

MageB
I hate to show that your wrong again but a star will become brighter when it becomes a gaint, at least brighter then it was at the end of if life. Link to a star's luminosity is posted below. The reason is somewhere on NASA's site I'm sure, it has to do with two different types of fusion, Initially it is H+H=He and during Giant phase He+He=something. I can't recall what it froms off the top of my head and I don't really feel like looking it up now as it is getting late.

http://imagine.gsfc.nasa.gov/docs/te...es/Image31.gif

Ok, sorry I guess I was wrong in my reasoning.
And don't feel bad about proving me wrong, it keeps me from having a huge ego that would make everyone who talked to me absolutely hate me. Besides being corrected and knowing the right answer and having a blow to the ego is much better than not being corrected and left to continue on in ignorance. :thumbsup: