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[Vampyr]

I was curious about something, and was wondering if someone can explain this to me.

Our sun has an extremely large gravitational field... based primarily on the fact that our planets keep revolving around it.

If the Earth were to suddenly stop spinning around the sun... would the sun's gravitational force suddenly start pulling the earth straight towards it?

Is it the centrifigual force the only thing keeping the planets from smashing into the sun?

Also... I would imagine that at least a few planetary objects have at one time gotten pulled into the sun. So is their matter within the core of the sun?

(What made me ask this is that I was reading an article about a probe that is orbiting Mercury and taking pictures and I thought.. "How is that probe able to withstand the force of the sun's gravitation pull? Surely the sun is strong enough to pull that probe away from Mercury's small gravitation)

Love to hear your thoughts on this

Also... I'm confused about another piece of information I read. I read that the sun's rotation takes 27 days... and they say "You can measure this by looking at the sun's sun spots and other visual indicators" etc...
But I ALSO read that an object must travel FASTER around the object with a much larger mass, to keep from having the smaller object be pulled into the larger mass' gravity.
So HOW can you truly know the Sun's rotation if you are looking at it from an object that is traveling much faster than the object you are looking at? What confuses me is... we are stating that the sun rotates in 27 days. But TAHT measurement is based on how fast WE rotate on our own axis. If we sat stationary in space, would the sun still appear to rotate for 27 days?

Which brings me to another question. Is it truly possible to even sit stationary in space? With all the gravitational objects in space... SOMETHING would grab hold of you and start to pull you towards it.

[A Poster He or I]

If the Earth were to suddenly stop spinning around the sun... would the sun's gravitational force suddenly start pulling the earth straight towards it?

Yes. But for the very same reason (namely, gravity) it is impossible for Earth to suddenly stop dead in space unless a major external force was introduced such as a foreign planetoid ripping through our solar system.

Is it the centrifigual force the only thing keeping the planets from smashing into the sun?

Yes, but it is almost perfectly balanced against the sun's pull.

Also... I would imagine that at least a few planetary objects have at one time gotten pulled into the sun. So is their matter within the core of the sun?

Most likely, the objects would completely ionize (turn into plasma) at the sun's surface or even in the solar atmosphere and be jettisoned into space by solar radiation. The sun's surface is about 10,000 degrees Fahrenheit, and the solar wind (radiation) is strong enough to rip away electrons from matter.

(What made me ask this is that I was reading an article about a probe that is orbiting Mercury and taking pictures and I thought.. "How is that probe able to withstand the force of the sun's gravitation pull? Surely the sun is strong enough to pull that probe away from Mercury's small gravitation)

Gravity's strength fades with the square of the distance between objects. That means the probe doesn't have to worry about the sun's gravity too much because, being in Mercury orbit, it is completely in the grip of Mercury's gravity which is far, far stronger at orbital distances than the sun's gravity. Now if the probe intends to return to earth, that's trickier. It will have to power away from Mercury in an exact way that leverages Mercury's gravity as a slingshot to hurl it away faster than the sun's gravity can overtake it.

I read that the sun's rotation takes 27 days... and they say "You can measure this by looking at the sun's sun spots and other visual indicators" etc...
But I ALSO read that an object must travel FASTER around the object with a much larger mass, to keep from having the smaller object be pulled into the larger mass' gravity.
So HOW can you truly know the Sun's rotation if you are looking at it from an object that is traveling much faster than the object you are looking at?

Scientists know that all movement and measurement of celestial objects is relative (that is, there is no such thing as a stationary point from which to measure) and they take this into account. A 27-day solar rotation signifies the time needed in terms of elapsed days on Earth for a spot on the sun to rotate all the way around the sun back to its original position from the Sun's point of view (i.e, observing from the sun's north or south pole.

Is it truly possible to even sit stationary in space? With all the gravitational objects in space... SOMETHING would grab hold of you and start to pull you towards it.

You're correct. There is no such thing as a stationary object in space. Even in deep intergalactic space free from any measurable gravitational field, stationary position is impossible because "fixing" your position means fixing it relative to something else--but everything else is moving. For this same reason, measuring the speed of how fast an object is moving is only possible from a pre-selected frame of reference (like I illustrated to explain a 27-day solar rotation). Change the frame of reference and your speed measurement changes too. Einstein proved there is no such thing as an absolute frame of reference, nor is there any "preferred" frame of reference except the one you choose by which to measure.

[Vampyr]

Thanks for the reply... helps me understand it better.

Some other things in questioning how gravity works.

Is there a 'tangible' thing that defines what gravity is?

Or, another way of putting it... if humans could 'build' an object the size of the moon... would it create it's own gravitational force like the moon? Or is there some unknown 'element' that determines what objects are given gravity?

Another thought is the idea of a black hole.... it's gravitational force being far greater than anything else. But in most graphical displays/artist renderings, it shows it 'spinning', as though with a cone shaped trajectory.
Is it possible for an object's mass in space to somehow be uneven in that the gravitational pull is MUCH greater from a certain angle than it is from the opposite position?

Another thought is... if you were able to travel to the very center of the moon... would the gravitational force be greater than on the surface of the moon? and how could you pull yourself out of something where the gravity is surrounding you? I guess, what I'm saying is... if you stood at the center of an objects gravitational mass, would you get stuck inside a 'gravity bubble'?

[A Poster He or I]

Is there a 'tangible' thing that defines what gravity is?

No one knows what gravity is, ontologically speaking. But science has at least 3 pretty good theories of gravity that almost fully allow us to predict its behavior, so it doesn't end up mattering too much what gravity is...unless you're a philosopher I suppose.

Anyway, in one theory, gravity is one of the 4 "forces" that hold the universe together--just don't ask what a force is. In the 2nd theory, gravity is the exchange of particles called "gravitons" between other types of particles. Trouble is, no one has ever actually found a graviton experimentally, so they may not even exist. In the 3rd theory, gravity is literally the shape of the universe at any given point. Where gravity is strongest is where the universe is most "curved." Where there is no gravity measured, the universe is "flat." Just don't think curved or flat that you can see--the curvature is in 4-dimensional space-time.

As you can see, not a lot of help. But the theories work great!

Or, another way of putting it... if humans could 'build' an object the size of the moon... would it create it's own gravitational force like the moon? Or is there some unknown 'element' that determines what objects are given gravity?

In each of the 3 theories I mentioned, gravity is concomitant with matter, not something extra added on. So yes, a man-made moon would have gravity.

Another thought is the idea of a black hole.... it's gravitational force being far greater than anything else. But in most graphical displays/artist renderings, it shows it 'spinning', as though with a cone shaped trajectory. Is it possible for an object's mass in space to somehow be uneven in that the gravitational pull is MUCH greater from a certain angle than it is from the opposite position?

This is just a guess, but it sounds like those cone-shaped trajectories were attempting to show graphically a trajectory in Einsteinian 4-dimensional space-time which is a very common way for black hole properties to be presented for a better understanding of their properties. In "normal" 3-dimensional space any object approaching a black hole at an oblique angle would spiral toward the black hole's "event horizon" (the distance at which nothing could escape).

Another thought is... if you were able to travel to the very center of the moon... would the gravitational force be greater than on the surface of the moon? and how could you pull yourself out of something where the gravity is surrounding you? I guess, what I'm saying is... if you stood at the center of an objects gravitational mass, would you get stuck inside a 'gravity bubble'?

Yes, gravity intensifies (at the square of the distance again) as you approach the "center of gravity" which is where the gravitational field is the largest summation of all the gravity of the component atoms. For the moon or a planet, that would be dead center. Once there you'd be stuck in terms of not falling anywhere else (there's nowhere left to fall to), but if you had a rocket with the power to achieve "escape velocity" (to overcome such intense gravity) you could get out.

[Belinda]

I don't want to interrupt this most useful conversation but I'm just thinking about something I heard about how you should not walk clse to speeding heavy lorries because their gravitational pull tends to pull you towards them. An urban myth maybe.

[Vampyr]

Ok... some other thoughts then.
I understand that some moons are shaped differently due to the pulls of gravity being made by the planet it orbits.
(I believe I read that Ion is actually stretched and pulled due to the gravitation pulls it endurs).
So... if gravity is always pulling things towards it... then what is the force that is keeping ir from tearying apart (it's own gravity would be doing that, as it just keeps itself together is all)? In fact, how is it that our universe is 'expanding' if everything is atracted to each other by gravity?

I would imagine that eventually the universe would reach a point that it begins to collapse on itself from the very nature of gravity pulling everything towards everything else?

Unless of course there is some other variable that comes into play that has the opposite effect on gravity?

Going back to the example of the moon that stretches from the gravitational pull of the planet. For something the pull so hard on an object that you are physically changing it's shape, there MUST be something of equal force pulling it the other way to keep it from falling into the planet?

And given that an object's mass is equal on all sides, and therefore it's gravity is equal on all sides... how is that its moon does not travel in an equal path rather than an its current 'elliptical' path?

Even our moon was recently 'closer' to the earth than ever before... why wouldn't our Earth just pull it completely in? Also... with all the gravitational pulls going on in just out solar system... how is that we are still spinning and rotating? Wouldn't the spinning and rotating eventually come to a halt due to finding an equal balance of all gravitational forces pulling it and slowing it down to whatever path it is on?

as far as what actually constitutes gravity... It would seem to me to represent the actual speed at which the object rotates in space. The reasoning behing this is that I had heard once (or read) that if the Earth suddenly stopped... we would suddenly start to float to space.
Even an object in our orbit must travel faster than the earth's own speed on its axis to stay in orbit. But that if it slows below the speed of the Earth's rotation, then it gets pulled back in. But what I don't understand is 'how' are our planets maintaining a speed faster than the Sun's rotation?

I'd be curious to know.. exactly how far form Earth, does an object need to be to no longer be influenced by Earth gravity?

I've always wondered... what if we could truly become stationary in space... without being effected by any movement what so ever. How far would we be from home, from our own solar system, or even galaxy? (I'm thinking the same principle as being on a fast moving train. We may walk very slowly on the train, but in reality we are actually moving faster than the train at that point. So the same principle could be applied to our Earth and even our Galaxy, and all object in space. We could be being thrown through space at billions of miles an hour if we were to somehow find a way to truly stop moving in space?... just thinking outside the original topic here)

[A Poster He or I]

(LOL) I think you've been hired to exhaust my book-knowledge of astrophysics. I should be charging you for this, but here we go...

...if gravity is always pulling things towards it... then what is the force that is keeping it from tearying apart...?
...
Going back to the example of the moon that stretches from the gravitational pull of the planet. For something to pull so hard on an object that you are physically changing it's shape, there MUST be something of equal force pulling it the other way to keep it from falling into the planet?

Centrifugal force is the answer. But it isn't a "force" of its own, actually. In this context, it's simply movement in a different direction from gravity's pull, "hampered" by gravity's pull. In other words, the moon is falling to Earth all the time, just as you think it should. But the moon also has its own path through space, lateral to the direction of Earth's gravity. This movement would be in a straight line through space except that Earth's gravity is bending it toward Earth...bending it all the time...bending it, bending it, bending it, until the moon's path bends all the way around the earth and the moon ends up right where it started (relative to Earth).

Imagine yourself standing on a small asteroid with very weak gravity. You throw a baseball very hard. With gravity so weak it goes a long way before it falls, and with the asteroid so small it ends up landing beyond the horizon, on the other side of the asteroid. Now throw the ball even harder. This time it goes so far that it comes all the way around the asteroid. You step out of the way so that it doesn't hit you in the back. With no atmosphere to slow it down, the ball has the same speed at which you threw it, which means it will continue to fall...all the way around the asteroid again...and again...and again. You just threw the ball into orbit. That is all an orbit is: a falling object with its own lateral movement balancing the effect of the fall.

And given that an object's mass is equal on all sides, and therefore it's gravity is equal on all sides... how is that its moon does not travel in an equal path rather than an its current 'elliptical' path?

Have you ever spun a small child in the air, round-and-round, holding his arms while his legs fly through the air? (Little boys like this more than girls, in my experience). If you have, you probably discovered that you couldn't stand in one place, but found yourself moving in a circle, especially your upper body where you and the child were connected by your 4 arms. That's because the child is pulling on you too. His pull is not as strong, so you don't move as much as he does.

Gravitational attraction between 2 objects goes both ways, always. You may not notice it if one of the objects is much smaller, but it is there. So any orbit is always eliptical. And an elipse, unlike a circle, has two centers (called foci). The planet being orbited will wobble between one focus and the other.

In fact, how is it that our universe is 'expanding' if everything is atracted to each other by gravity?

The premier theory on this for the last 80 years or so is the "Big Bang" theory which says that the universe's expansion is a residual effect of its initial origin. So far, gravity has not overcome the inertia of this initial "explosion." (The theory is in trouble, however. See below).

I would imagine that eventually the universe would reach a point that it begins to collapse on itself from the very nature of gravity pulling everything towards everything else?

The Big Bang theory says that whether gravity will stop expansion and begin collapse depends on the overall "shape" of the universe in 4-dimensional space-time. If the universe is "flat" or "negatively curved" gravity will never be powerful enough to slow expansion to a full stop, let alone reverse it. But if it "positively curved" it will.

Unless of course there is some other variable that comes into play that has the opposite effect on gravity?

In 1997, such a variable was discovered: Dark Energy. It has wreaked havoc on the Big Bang Theory as well as other cosmological theories. Nobody knows what dark energy is, but scientists all agree that (1) over 70% of the entire universe seems to be made of it, and (2) it is causing the expansion of the universe to accelerate. With accelerating expansion, gravity doesn't have a chance to reign it in.

Although I have not read of any scientist saying so(because most scientists are conservative in their profession and do not go out on limbs without solid evidence to back them up), others have suggested that dark energy's behavior suggests it may be a fifth force of nature--one that manifests at large (cosmic) scales, and acts to counter gravity.

Even our moon was recently 'closer' to the earth than ever before... why wouldn't our Earth just pull it completely in?

Because it speeds up when it gets closer. That may sound magical but it actually is a natural aspect of orbital dynamics.

Also... with all the gravitational pulls going on in just out solar system... how is that we are still spinning and rotating? Wouldn't the spinning and rotating eventually come to a halt due to finding an equal balance of all gravitational forces pulling it and slowing it down to whatever path it is on?

You're correct that gravity will eventually render any orbit unstable if there is the slightest disturbance in the gravitational fields involved. Planetary rotation is far more easily affected by external gravitational pulls. Our own moon once rotated but long ago ground to a halt under Earth's influence. The Sun has done the same thing to its nearest planet, Mercury.

... I had heard once (or read) that if the Earth suddenly stopped... we would suddenly start to float to space.

Not correct.

Even an object in our orbit must travel faster than the earth's own speed on its axis to stay in orbit. But that if it slows below the speed of the Earth's rotation, then it gets pulled back in. But what I don't understand is 'how' are our planets maintaining a speed faster than the Sun's rotation?

Not correct. There is no relation between orbital speed and planetary (or solar) rotation.

I'd be curious to know.. exactly how far form Earth, does an object need to be to no longer be influenced by Earth gravity?

There is no theoretical limit, but since gravity fades at the square of the distance between Earth and any object, an object beyond lunar orbit will soon fall under the sway of solar gravitation, overriding Earth's pull. Strictly speaking, however, the Earth's pull pervades the solar system and beyond and has a measurable influence on the orbits of all the planets, as well as the wobble of the sun.

I've always wondered... what if we could truly become stationary in space... without being effected by any movement what so ever. How far would we be from home, from our own solar system, or even galaxy? (I'm thinking the same principle as being on a fast moving train. We may walk very slowly on the train, but in reality we are actually moving faster than the train at that point. So the same principle could be applied to our Earth and even our Galaxy, and all object in space. We could be being thrown through space at billions of miles an hour if we were to somehow find a way to truly stop moving in space?... just thinking outside the original topic here).

An understanding of Einstein's Special Relativity is needed to see why stationary position (i.e., absolute position) is impossible, even in principle, let alone practice. It is beyond me to explain relativity theory accurately--I only understand the gist of it plus a few particulars.

[Vampyr]

(LOL) I think you've been hired to exhaust my book-knowledge of astrophysics. I should be charging you for this, but here we go...

Even an object in our orbit must travel faster than the earth's own speed on its axis to stay in orbit. But that if it slows below the speed of the Earth's rotation, then it gets pulled back in. But what I don't understand is 'how' are our planets maintaining a speed faster than the Sun's rotation?

Not correct. There is no relation between orbital speed and planetary (or solar) rotation.

.

I do appreciate you responding. I'd rather understand it correctly than have a self perceived idea that isn't correct.

In relation to my question above... I was thinking in reference to the fact that astronauts must orbit the Earth at a specific speed (I think I read something like 17,000 MPH).
I had thought I understood that man made objects must go a certain speed around in earth's orbit to maintain their altitude, otherwise, any slower and they begin to fall back to earth?
So with that thought in place... I understand the Earth rotates around the sun at something like 65,000 MPH. So If we rotated around the sun even faster.... would that push our orbit out farther from the sun? And if slowed down... would it bring us closer to the sun?
If that were true to some extent... then could that same principle be applied by just stating that the effects of gravity are based on the speed at which an object rotates around another object?

your example of spinning a child is spot on.. thank you. But given that example... say I was able to spin an adult around the same way. The larger mass would indeed throw me off my axis more, but at the same time... it would require me to hold on with much more strength, regardless of how far out the adult was swinging. Or in other words... if we based the mass of an object with the strength of its gravity... why then are the planets aligned in their current orbits? Wouldn't the largest planets be closest to the sun? After all, their mass and the sun's mass is so much greater than little mercury that I would think they would be more attracted to each other.

I guess what I'm trying to understand is how I can measure gravity to determine how to manipulate gravity and create gravity without movement being required? In all applications that I can think of (spinning a child through the air)... there is some measure of movement.

Something else I was wondering...
Given the speed at which the Earth rotates, you would think we would fly off the surface. But the Earth's gravity keeps us on the surface. If the Earth rotated faster, would we weigh less? If it rotated slower, would we weigh more? (more or less.. is Earth's gravity a constant measurement? and is it the Earth's rotation a different variable in relation to how gravity is measured?)

What IS the measurement of gravity? If we need to plot a course through space... and wanted to make sure our spacecraft flew in a straight line... wouldn't the ship need to compensate for various gravity fields that would try to pull it off its trajectory?

[A Poster He or I]

I had thought I understood that man made objects must go a certain speed around in earth's orbit to maintain their altitude, otherwise, any slower and they begin to fall back to earth? So with that thought in place... I understand the Earth rotates around the sun at something like 65,000 MPH. So If we rotated around the sun even faster.... would that push our orbit out farther from the sun? And if slowed down... would it bring us closer to the sun?

I think I see the problem. Your original question said "rotate" but you meant "revolve." Planets don't rotate around the sun: they revolve around the sun and rotate around their axis. Examples: The moon takes 28 earth days to revolve around the earth and has no rotation (relative to Earth). Earth takes 365 days to revolve around the sun and fully rotates once every 24 hours.

So to answer your question: yes, if Earth started revolving faster around the sun, its orbit would spiral outward from the sun, creating a higher (farther) orbit. (Due to existing dynamics of the solar system as a whole, the new orbit would be "eccentric", i.e. very eliptical). If it reached "escape velocity" (overcoming solar gravity) it would stop orbiting and head off into deep space in a straight line.

If Earth slowed down in its revolution, then its lateral motion could no longer compensate for solar gravity. Earth would spiral into the sun and be destroyed.

...could that same principle be applied by just stating that the effects of gravity are based on the speed at which an object rotates around another object?

In short, you've got it backwards. Orbital speed is based on the effects of gravity, or in the case of the swinging child, centrifugal force. (In this context, it is better to think of centrifugal force as being acceleration, not the lateral motion against gravity I described for orbits. You are creating an acceleration by continually applying energy into the swinging. Wherever there is acceleration, there is always a force. You are countering that force with your arms--just don't let go!)

your example of spinning a child is spot on.. thank you. But given that example... say I was able to spin an adult around the same way. The larger mass would indeed throw me off my axis more, but at the same time... it would require me to hold on with much more strength, regardless of how far out the adult was swinging. Or in other words... if we based the mass of an object with the strength of its gravity... why then are the planets aligned in their current orbits? Wouldn't the largest planets be closest to the sun? After all, their mass and the sun's mass is so much greater than little mercury that I would think they would be more attracted to each other.

If you tried to swing an adult instead of the child, you would indeed be swinging each other. The foci of your elipse would be so far apart that the circle of your own motion would overlap that of your partner: you two would be orbiting each other. This is what happens between binary stars of similar mass for instance. Now it is true that it would be harder to get the swinging going (there is more "inertia" for your strength to overcome in this more massive set-up) but once you two got swinging, it would NOT take more strength per se to keep the spinning going. You would get exhausted quicker because there is much more motion for you to manage in yourself, but brute strength wouldn't help there.

As to why the giant planets aren't closer to the sun, you have to appreciate that our solar system wasn't designed--it evolved. As the solar system evolved, different concentrations of matter stratified at different points. If this matter achieved stable orbit relative to the nascent solar system's center of mass (center of gravity) then it doesn't matter WHERE in that distribution the largest proto-planets were orbiting. (I'm sure there's a more specific theory on why the large gas giants in our solar system are all together where they are, but I don't know it).

It is very true that Jupiter and Saturn have a huge gravitational pull on the sun (and the rest of the planets) compared to the four inner planets. The motions of the planets and the sun include this effect: it is part of their own orbital dynamics. But so long as their orbital dynamics are stable, the big planets won't be pulled in no matter how far or close to the sun.

I guess what I'm trying to understand is how I can measure gravity to determine how to manipulate gravity and create gravity without movement being required? In all applications that I can think of (spinning a child through the air)... there is some measure of movement.

There is a fixed number called the gravitational constant. It lets you predict the strength of gravity at any point so long as you know the distance from the center(s) of gravity and their respective masses. You do have to know what units of measure to use for everything to make sense. The only way I know to create gravitational effects is to create centrifugal forces by setting something into circular motion. Amusement park rides where you go upside-down or the floor drops out while you spin all work that way.

If the Earth rotated faster, would we weigh less? If it rotated slower, would we weigh more?

No, as long as Earth still had the same mass.

What IS the measurement of gravity?

I don't remember the formula but in space you take the square of the distance to the center(s) of gravity associated with local objects, multiplied by their masses, and apply the gravitational constant. On the surface of a planet, you can generally ignore all objects except the planet itself since its field vastly overrides all other fields. An exception would be if you want to measure tidal forces which are caused by the moon pulling at Earth.

If we need to plot a course through space... and wanted to make sure our spacecraft flew in a straight line... wouldn't the ship need to compensate for various gravity fields that would try to pull it off its trajectory?

Yes, definitely.

[Kowalskil]

If the Earth were to suddenly stop spinning around the sun... would the sun's gravitational force suddenly start pulling the earth straight towards it?

Yes it would. And a progressive decrease of the orbiting speed would result in the spiral motion of our planet toward the sun.

Ludwik
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[Vampyr]

Something else I wondered... in regards to gravity and water.

Water flows down because of gravity... that's obvious.
I guess what I'm asking is... if gravity is strongest at the poles... then why doesn't water 'gravitate' towards those ends of the earth?

Also.. if gravity is weakest at the equator... does that mean that the water level in the ocean is slightly higher along the equator than elsewhere?

[Kowalskil]

Something else I wondered... in regards to gravity and water.

Water flows down because of gravity... that's obvious.
I guess what I'm asking is... if gravity is strongest at the poles... then why doesn't water 'gravitate' towards those ends of the earth?

Also.. if gravity is weakest at the equator... does that mean that the water level in the ocean is slightly higher along the equator than elsewhere?

Because the gravitational force (in any place on earth) is directed toward the center of our planet.
.

[Kowalskil]

Something else I wondered... in regards to gravity and water.

Water flows down because of gravity... that's obvious.
I guess what I'm asking is... if gravity is strongest at the poles... then why doesn't water 'gravitate' towards those ends of the earth?

Also.. if gravity is weakest at the equator... does that mean that the water level in the ocean is slightly higher along the equator than elsewhere?

Because the gravitational force (in any place on earth) is directed toward the center of our planet.
.