Okay, the earth is attracted by the sun - as everything else in this solar system is. Very nice. There it is, the earth, nicely going around the sun. And the sun attracts the earth ...... why in the name is the earth and the rest still where it is??? It should come closer and closer to the sun no??

Is there an opposite force pushing us lil back or what?

Why is there this equilibrium.

Hi c'est moi,

Roughly speaking, the centrifugal force and gravitational force nicely compensate in case of the planets. However, earth loses small bits of energy (because of the little drag there is in space, because of tidal motion, ...) so slowly but surely the earth is slowing down and will eventually wind up crashing into the sun.

Luckily, the energy disposal is very tiny and it will take several tens of billions of years before we crash into the sun. By that time, the sun will have already gone nova and burnt us all to a ...

Crisp.

Crisp

I was about to leap into an explanation on pressure density, gravitational fields and Lagrange points.

You’ve summed it quite nicely.

Btw – “burnt to a … Crisp”. lol :)

Crisp,

I don't think that you understood what c'est moi was asking. I think what he meant was why in the last 5 billion years didn't the Earth fly into the sun or out of the solar system??? Why is the balance so perfect??

You might say that it's a coincidence that the Earth found a perfect orbit in which the pull of the sun is equal to the centrifugal force of the earth, but it seems very unlikely. It would probably be easier to balance a needle on its tip than to create a perfect orbit. What makes it even weirder is that there are eight other planets with perfect orbits in our solar system as well.

I understand the effect of gravity and orbiting objects, but I just can't understand these stable orbits. Is there another factor in this formula or am I missing something??

Tom

Wont the Earth gradually move away from the Sun due to tidal forces? Just as the Moon is moving away from the Earth.

We had a similar question at SSSF and, I believe, that over the life of the Earth it has moved a few centimetres away from the Sun.

yes, Joeblow, that's what I initially meant!

I have been reading about all this a very very long time ago and the only thing I remembered was that nobody really knew and that it was quite a mystery ... we'll waite for some more replies cause there are some really clever people out here and I always appreciate their effort to try to explain physics stuff to us dummies :)

so Crisp, Q, James R, Hamster, etc.: give it a shot!

According to theories of solar system development, there were HEAPS of planetary bodies earlier. We the best of what's left.

Hi all,

c'est moi,

It really depends on how far you want the explanation to go. One way (the most basic and elementary way) to explain why earth (or any other planet) does not leave its orbit is because all forces compensate. There simply is no force that can drive us towards the sun or away.

A more decent explanation would be one where you investigate the stability of the solutions for earth's orbit. For just the system earth + sun there is no problem: the solution can easily be proved to be stable (this is a textbook example). When you take in all other planets, then there's a whole different story to tell.

For example, simply the system sun-earth-moon is still not solved. It involves chaos theory, and it was Poincaré who was the first to study this problem in a consistent mathematical way. I believe a short while ago the first results about the stability of earth's orbit were in, and if I remember correctly the answer didn't predict much good: the solar system would be unstable.

Luckily however, chaos only exhibits itself after a long time (you might want to look up Lyapunov exponents for our solar system, these tell you roughly how long it will take before chaotic behaviour can be observed). And now I come to think of it, I believe the time required before chaos would come into play in our solar system, was again longer than the lifetime of our sun.

Bye!

Crisp

The Earth's orbit is not "perfect". Nor is is particularly special. If you set something into motion anywhere in the solar system, it will orbit the sun. Depending on its initial energy, that orbit will be either parabolic, hyperbolic or elliptical. All the planets have elliptical orbits. Earth's orbit is nearly circular, but not quite. It is a slightly eccentric ellipse. Why is it nearly circular? The answer is that the angular momentum of the Earth was effectively set at the origin of the solar system, and remains approximately constant. However, the energy has varied due to collisions and so on. The minimum energy configuration for a planet with constant orbital angular momentum is a circlular orbit.

I do not think that there is an accepted theory which explains how the solar system evolved and why the planets are in their current stable orbits.

It is my understanding that a correct theory starts with a rotating mass of gas contracting due to gravity. As this mass contracts, it spins faster due to conservation of angular momentum, with most of the mass ending up in the sun.

At various stages of contraction, the rotational velocity of mass at particular distances from the center exactly balanced the gravitational force tending to pull that mass to the center.

All of the mass at each of these special distances initially formed a ring, which later collected into a planet.

I think (am not sure) that the above is a general description of what is believed to have happened. I do not think that anybody has been able to come up with a well worked out theory and good equations to back up the overall concept.

It is my guess that a super computer given a proper set of initial conditions might be able to provide a simulation leading to the solar system as it is today, based on a theory somewhat like that described. We might not have enough computing power to do such a simulation.

If we had the computing power and a knowledge of the proper equations, it could still be a formidable task. The equations might not be time reversable, requiring us to guess at the initial conditions. You might have to make thousands (millions, billions, more?) of guesses before you hit on a set of conditions leading to a stable solar system. For each guess you would have to run a simulation that could take a lot of computer time.

The above is my SWAG, based on various articles I have read and partially understood. If somebody knowledgable in this subject disagrees, I will not argue hard for my views.

BTW: For those not familiar with technical jargon, A WAG is a Wild Assed Guess, and a SWAG is a Sophisticated Wild Assed Guess.

Dinosaur,

Such simulations already exist, and they produce solar systems similar to ours when you run them. We know all the relevant equations. Simple Newtonian physics is sufficient.

A slightly more philosophical approach.. :)

Our solar system and the stable orbit of Earth is probably a very improbable configuration for solar systems in general (correct me if I'm wrong here). But that doesn't mean that there is some theory we have missed out on that should explain this, for it doesn't matter if its very improbable.

The reason is simple. We could not be here to ask ourselves these questions if the orbit of Earth was elliptic or somehow unstable, because we would not exist. So its very logical that Earth has an almost circular orbit and its nothing to ponder about.

Just a thought.

It really depends on how far you want the explanation to go. One way (the most basic and elementary way) to explain why earth (or any other planet) does not leave its orbit is because all forces compensate. There simply is no force that can drive us towards the sun or away.

hum, but my thought is following: a satellite which orbits around the earth is really "falling" all the time around it.
BUT, if it gets to close to the earth it falls ON the earth; IF it gets to far, the attraction of the earth is not strong enough anymore and will not orbit the earth anymore ............. do you see where I'm going?? I project this line of thinking to the sun: We are orbiting the sun as well as for example Pluto ... and Pluto is so so so much further. If you replace the sun here with the earth and those two planets with two sattelites, then I think you will all understand what I mean. Or Pluto shouldn't be orbiting the sun and the earth should fall on the sun, or the earth orbits the sun and Pluto not (and flies away or just does nothing). Maybe I'm just too stupid - that's the other possibility!

cest moi

It would make sense that if the Earth loses a bit of energy due to drag as Crisp mentioned , then it is entirely possible that the Earth IS gradually moving closer to the Sun. But remember, the Sun is also losing energy and therefore losing mass. If it gradually loses mass, it's gravitational influence on the planets lessens.

I'm not saying that these losses of energy are in equilibrium, but it would help somewhat to equalize the orbits and conserve angular momentum.

yes (Q), that might explain the earth

but let us take Pluto and the Earth together ...

cest moi

Earth and Pluto?

We need to apply Keplers Laws of Planetary Motion. In other words, orbital mechanics, which requires some calculus.

Basically, for an object to remain in a stable orbit, the centripetal forces must balance the gravitational forces of the object orbited. Once orbital velocity has been achieved, the orbit should remain stable. Anomolies such as drag and the change in landscape of the orbited object will influence the orbit over time and a correction in velocity may be required. This change in velocity is known as Delta V.

"Basically, for an object to remain in a stable orbit, the centripetal forces must balance the gravitational forces of the object orbited."

I don't know what a "centripetal force" exactly means.

Centripetal force is the perpendicular force applied to an object in motion forcing the object to follow a circular path. The force is constant in that the object is always accelerating and is always directed towards the center of the curvature.

Centripetal literally means 'center-seeking.'

Okay, but then the centripetal force and the gravitational force point BOTH towards the sun in this case. They can't be cancelling each other out like you suggested.

If gravity is *ever attracting* and the other force is making it go around a certain point, then I still don't get it why we are not smashing against the sun and why pluto is still nicely orbitting around the sun instead of drifting of.

After all, with the earth it is easy to see that once a satellite is too far from earth it won't be orbitting earth anymore or once it is too close it will simply fall on earth.

But maybe I'm just getting stupid here ... when an object turns around a point very rapidly, it tends flying away from the centre no? Is that what you mean with the centripetal force? So the force of wanting to fly away cause of the speed and the force of gravity wanting to keep it, must be a little in balance and cancel each other out so that the object's orbit is stable.

Does this mean that cause gravity is much weaker far away where Pluto is, Pluto's orbiting speed must also be much lower?
and the speed of mercurius must be very very high ---> does in the evolution of the solar system it must have been spinning very very hard, much harder than the others

Whoops, missed that c’est moi ASKED for hamster input. Can’t let such an opportunity pass. Let’s see…

Crisp already covered two-body Newtonian solutions and the lack of general solutions for more than two bodies. Also covered the chaotic nature of solar system dynamics. Yep, the Earth “could” be the next object to impact Jupiter. (If “could” were stretched to cover such low probability events.)

James R. gives reasons why relatively stable circular orbits arise. And he refers to existing solar system simulators.

Dinosaur provides a very good SWAG. This hamster suspects both James R. and Dinosaur are correct. Simulations do exist that model the present solar system configuration very well. And predict future orbital positions very well. And some model solar system generation. However this hamster guesses Dinosaur is also correct that no existing simulation captures the full complexity of the birth of the solar system. (Given the difficulty of the task and the vagueness of the hamster statement how could it be wrong. Hehe.) Nor does any existing simulation begin to include the vast number of celestial bodies in the solar system.

Itchy brings up the good philosophical point that the requirements for life have potentially filtered out many possible stellar system configurations. So the Solar System could be very unusual. Science might wrongly favor explanations and models that predict the Solar System as a likely outcome. (This should be resolved as distant stellar systems are observed.)

Sorry C’est moi. Nothing this hamster can add.

well you came too late :p :p

cest moi

Okay, but then the centripetal force and the gravitational force point BOTH towards the sun in this case. They can't be cancelling each other out like you suggested.

You're getting a little confused. Newtons first law states that an object will remain in motion unless acted upon by an unbalanced force. In other words, a body in motion will continue along a straight line unless acted upon by a force. The gravitational attraction is an unbalanced force that is accelerating the orbiting object towards the center of curvature.

Hence we have centripetal forces AND gravitational attraction, and they need to balance to create a stable orbit.

I hope this helps.

I'm really not getting it :o

okay, i know the law of Inertia

"In other words, a body in motion will continue along a straight line unless acted upon by a force. The gravitational attraction is an unbalanced force that is accelerating the orbiting object towards the center of curvature."

towards the centre -----> the earth is attracted towards the sun
again: what is keeping it from not going completely towards the sun? okay, let us say gravity is too weak cause we're too far, then why aren't we drifting of --> cause of the huge speed? (centrifugal force) well, gravity is just strong enough to counter that. Then again, for mercurius, as it so so much more close to the sun it must have a much greater orbital speed otherwise cause of gravity which is much stronger so close, it would smash against the sun ...... that's my reasoning so far

what is keeping it from not going completely towards the sun?

Because the body in motion will tend to want to continue along a straight line.

I dont understand,

isnt your question similiar to:

why doesnt an electron fall into nucleas while orbiting it?

...


bye!

If your to look at the full picture, it's not just the Earth spinning, while moving around the sun. There's all the other planets, the moon, Astroids, debris, meteors etc.

All of those things have a Gravity, and in space the larger the body or denser the mass, the greater the gravity.
Different bodies are drawn to and push away from one another, doing a kind of dance.

I mean I could mention here that you have Astrologer's that mention of planetry alignments and how it effects your mood. Well I might not be a great believer in it, but I do believe that they have a point that an alignment of planets can make an orbit change in shape, from circular to eliptical, and possible enducing velocity changes.

The Eliptical orbit, is made from such velocity changes, where the earth could be brought near the sun, and the near it get's it increase in veolcity enough to break free from the suns gravity and slingshot itself around.

In fact the whole slingshot usage of bodies is renound as a lifesaver in space. Especially for a nearly doomed spaceflight.

There is only one force to speak of on a planet in orbit: gravity.

Gravity <b>is</b> a centripetal force. It is a force which pulls the planet towards the sun (at the "centre" of the orbit).

There is no force balance happening in an orbit. If all the forces on a planet balanced, there would be no net force and so the planet would travel in a straight line with constant speed, according to Newton's first law. What happens instead is that gravity accelerates the planet, continually changing the direction of its velocity, so that it follows an elliptical path around the sun. The net force on the planet is the gravity force.

"What happens instead is that gravity accelerates the planet, continually changing the direction of its velocity, so that it follows an elliptical path around the sun."

so the velocity is due to gravity

BUT, as I said before, isn't it because this speed that the earth stays where it is??

and aren't you wrong when you say "acceleration", if that would be true, we would have a very unpleasant stay here on earth ...

and how does it come gravity "accelerates" (i really don't think that's correct, would be falling of my chair if that happens) the earth and making it *change from direction* --> can't we see the force of gravity as a perpendicular force on the sun towards the earth, like an arm reaching out for the earth, or does that "arm" turn along with the spin of the sun.....

"The net force on the planet is the gravity force."

ok.

me is mucho confused

James R

There is no force balance happening in an orbit. If all the forces on a planet balanced, there would be no net force and so the planet would travel in a straight line with constant speed, according to Newton's first law.

Although I agree with you're explanation in that gravity is the only net force acting as the centripetal force on the orbiting body, I don't agree there is no balance of forces. If we take the Earth-Moon system for example, the force of gravity of the orbiting body, the Moon, must also be taken into account as another force acting on the system. This force needs to balance out the centripetal force of gravity from the orbited body for the orbiting body to remain stable.

Aside from that, your explanation is spot on.

cest moi

When no work is done upon an object by external forces, the potential and kinetic energies of the object remain constant. In other words, if the body is moving at a constant speed, the centripetal force doesn't alter the total mechanical energy, in this case the kinetic energy. The centripetal force can accelerate the object by changing its direction, but it can't change its speed. There would need to be a force acting in the direction or opposite direction of the motion of the object in order to change its speed and direction.

Remember, speed refers to how fast an object is moving (scalar) and velocity refers to the rate at which an object changes its position (vector).

I have an analogy which you can try yourself which demonstartes very clearly why we maintain our current orbit.

Get a foot or two of string, and tie a metal nut to one end. Dangle it from your hand, and start it swinging around in a circle, maybe a foot in diametre. Keep it at that speed/diametre. Notice that it's path is a balance between gravity (pulling it toward the centre, or toward hanging straight down), and the force of motion which tends to make it want to go up and out.

For a larger circle (orbit), apply more speed, and watch the circle grow. Again, it will maintain a stable orbit in its new diametres by finding equilibrium between gravity pulling it inward and the force of motion pulling it outward.

For any given weight of the metal nut (planet), there are different balances of speed and diametre which will result in stable orbits, or equilibrium.

I hope this helps people picture easier how planetary orbits work.

okay (Q)! centripetal force changes only direction

after all these replies, I am still finding myself lost

Maybe someone could sum it up with an example (not a dummie like me):

Imagine I throw a globe the size of the earth in our solar system:

...oooo
..oooooo............ooo
.ooooooo..........oooo.. .^
..oooooo........... ooo... .|
...oooo....................... | (gets thrown from this direction)

...SUN..............GLOBE

what will happen and why?

I still don't get why gravity causes things to orbit instead of just pulling it completely towards the mass who produces the gravity. When I jump of my house I get smacked on the earth ... I don't orbit it ... actually it seems I don't even understand why satellites orbit the earth.

I guess you were writing that post while I posted my analogy. The end of Page Two.

yeah, i was busy with that

BUT if you look up you'll see that I've exactly told already what you are telling now! (or is it on page 1)
I said exactly the same thing but nobody answered directly to it ... instead the centripetal force etc. was mentioned and I forgot all about what I had said myself

that's how I said it:

"Then again, for mercurius, as it so so much more close to the sun it must have a much greater orbital speed otherwise cause of gravity which is much stronger so close, it would smash against the sun ...... that's my reasoning so far"

but you say it better :D

but I think there's something wrong with the reasoning ... it's far too simple!

It IS simple. I find that everything is about equilibrium. Planetary movements, weather patterns, sociology, psychology, galactic spin, the whole damn lot. Newton was right. :)

okay, but let us waite for some replies of some of the smart guys around here ...

Long time no see you. ;)

Ok, I'm not the smart guy you wish for :D, but i like to post my thought here.

I think we should calculate all of forces in several position of planet orbits. Let's take a look at mass of two objects (sun and earth) as gravity source (gravity depends on it's mass isn't it?). Then we can calculate the vector forces between them, to check is it equilibrioum in certain positions (snapshots of dynamic movement). If it does, so it solved. No further question.

But if we ask WHAT cause the initial movement creates centifugal force which precisely balance to the gravity... I think no one really sure, since the big bang theory itself; why/how it's happen still in debates among several scientist.

C'est moi:

<i>so the velocity [of an orbiting planet] is due to gravity</i>

No. Not at all. The velocity is just something the planet happens to have had from the time of the formation of the solar system. Gravity acts at right angles to the velocity (for a circular orbit). Therefore, the gravitational force does not change the <b>speed</b> (i.e. the magnitude of the velocity), but it does change the direction of motion. A change in the direction of motion is a change of velocity, and a change of velocity is an acceleration. So gravity is correctly said to accelerate the Earth in its orbit around the sun.

We do not feel the acceleration because we are accelerated by the sun's gravity at the same rate that the rest of the Earth is accelerated. Both the Earth and the people on it are in free fall towards the sun at all times. We don't move closer to the sun, though, because our pre-existing velocity makes us go round. We continually fall towards the sun, but "miss".

Think about throwing a ball horizontally off a high tower. If you drop it (zero horizontal velocity), it simply falls to the ground. If you throw it outwards (some horizontal velocity), it hits the ground some distance away. If you could throw the ball very fast, as it fell towards the ground, the ground would curve away from it (due to the Earth's curvature). The ball would never hit the ground, although it would be continually pulled downwards by gravity. In other words, it would be in orbit, for the same reasons that the Earth orbits the sun.


(Q):

<i>If we take the Earth-Moon system for example, the force of gravity of the orbiting body, the Moon, must also be taken into account as another force acting on the system.</i>

The force of gravity due to the Moon acting on the Earth is equal and opposite to the force of gravity due to the Earth acting on the Moon (see Newton's third law). In fact, the Earth and Moon orbit their common centre of mass. However, the centre of mass is a point inside the Earth, so it is more or less correct to say that the Moon orbits the Earth.

My point was that there is no "outward" force on the Moon. The only force on the Moon, from an inertial point of view, is the centripetal gravitational force from the Earth. Thus, there is no
"balance of forces" if you look only at the Moon, for example.

Well, I'm drunk and my nut on a string analogy still makes sense. I think maybe it is a good one. :)

Okay James R, so what I and then Adam said was thus correct (except that I thought the speed was due to gravity)

so mercurius has a much higher speed than the earth and pluto must be the slowest ...

and any object that enters the solar system and that has no balance between its initial speed and the force of gravity will or drift away from our solar system or be attracted towards the sun and burned

In fact, the Earth and Moon orbit their common centre of mass. However, the centre of mass is a point inside the Earth, so it is more or less correct to say that the Moon orbits the Earth.

A very good point. The Sun-Earth systems center of mass is a point somewhere between the two bodies.

But I don't think it's that cut-and-dried. The orbit of the Earth around the Sun is more complex. It requires the use of the 2-body problem (with moon - 3 body problem) and if all other forces of gravity were to be taken into account, the n-body problem is used for more accuracy. These calculations will take all gravitational forces into account although the forces are miniscule and will have little effect on the orbit.

cest moi

so mercurius has a much higher speed than the earth and pluto must be the slowest ...

and any object that enters the solar system and that has no balance between its initial speed and the force of gravity will or drift away from our solar system or be attracted towards the sun and burned

You're starting to get the idea.

I love it when things get more clear :) :)

Since the mass of Sun is higher than earth,the systems(2 body problem)C.G will be outside the earth's geometric centre...
good point Q.


bye!

It is more correct to say that the center of mass of two spherical objects is between their centers, rather than between them. In the case of the Earth & Sun, the center of mass is deep inside the Sun. I am not sure, but think that the same is true for all of the planets in the solar system.

The Moon, Earth, & Sun interactions are unique in the solar system. The Sun’s gravitational force on the Moon is slightly over twice the force of the Earth on the Moon..

The average distance of the Moon from the earth is about 239,000 Miles (384,500 km).

At about 160,000 miles (259,400 km) from the earth, the attraction of the Earth is equal to the attraction of the Sun.

Jovian satellites and Jupiter sometimes move in the opposite direction with respect to the Sun. The Moon always moves in the same direction as the Earth with respect to the Sun.

If you plot the orbit of the Moon with respect to the Sun, it is a wavy curve. If you plot the orbits of the Jovian satellites with respect to the Sun, they have loops.From the above, it seems technically correct to say that the Moon is a small planet of the Sun, with an orbit highly perturbed by the Earth.

dinosaur

It is more correct to say that the center of mass of two spherical objects is between their centers, rather than between them. In the case of the Earth & Sun, the center of mass is deep inside the Sun. I am not sure, but think that the same is true for all of the planets in the solar system.

Thanks for the clarification. The center of mass is deep inside the Sun for all the planets.

Gosh!is Sun's mass so much higher?:confused:!!??
curious...


bye!

zion

http://sprott.physics.wisc.edu/lectures/seasons/sld017.htm

Zion,

Yes, the mass of the sun is really gigantic. The answer is on The Nine Planets (a site that should be bookmarked by everyone ;)):

Mass comparison:
http://www.seds.org/nineplanets/nineplanets/data1.html

Note that even Jupiter's mass is only 1/1000 (!) of the sun.

The Nine Planets:
http://www.seds.org/nineplanets/nineplanets/nineplanets.html

Bye!

Crisp

The mass of the sun accounts for 99% of the mass of the solar system.

Gee,Thanks all for info.:cool:



bye!

Q, you mention LaGrange points. From what I gather these are points where gravity cancels out. why don't you shotgun a little info on the subject out since we're sorta talking about that kind of thing.

http://www.physics.montana.edu/faculty/cornish/Lpoints.gif