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Gravity questions!


Relative

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In the subject of Gravity in Physics, it is said that all Physical bodies are attracted to one another by gravity, all matter attracted matter!.

 

It is also explained that the Earth wants to travel linear, but is held by gravity in its orbit around the Sun.

 

post-87986-0-07989600-1412255304_thumb.jpg

 

 

 

My question is, what Force, stops expansion or contraction of the orbit, the ? mark on the diagram , shows the area I am talking about?

 

 

 

 

 

 

 

 

 

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Newton's first law: the Earth will continue in a straight line unless acted on by a force. This would carry it away from the Sun.

 

Newton's second law: the Earth will be accelerated towards the Earth by the force of gravity.

 

http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion

 

When the continuous acceleration towards the sun (2nd law) is balanced by the movement away from the sun (1st law) then it will be in a stable orbit. It will stay in that orbit unless you provide another force to either speed it up, or slow it down. After which it will find another stable orbit where its new speed balances the force of gravity.

 

orb.jpg

From here: http://www.astro.wisc.edu/~mab/education/astro103/lectures/l2/l2.html

 

PhyFig2b.gif

From: http://hsc.csu.edu.au/physics/core/space/9_2_2/922net.html#net7

 

More here: http://astro.unl.edu/naap/pos/pos_background2.html

Even more detail here: http://www.physicsclassroom.com/class/circles/Lesson-4/Circular-Motion-Principles-for-Satellites

Edited by Strange
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Thank you Strange that is how I understand current gravity explanation, but it still does not answer my question. I have downloaded your diagram and done a slight edit to ask the same question, what force is involved to make matter F=ma from my red arrow point, and directional with the arrow, what force drives the planet ''forward''?

 

post-87986-0-98474400-1412289081.jpg

 

 

 

 

 

 

 

 

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There is no force in that direction (as you can tell, because it doesn't accelerate in that direction). The acceleration, and therefore the force, is towards the Sun.

I do not understand , how can the force be towards the Sun, gravity pulls us to the sun, and pulls the sun towards earth, The orbit around the Sun at X velocity, there must be some pushing force from my arrow point?

I hit a ball around a pole on a string, the force has come from me, so what force makes the orbital velocity if not f=ma?

Edited by Relative
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I hit a ball around a pole on a string

 

You apply an initial force to start it moving but you don't have to supply a force to keep it moving. Newton's 1st law, remember. (Well, OK, there is air resistance so it will slow down and you would have to keep hitting it. But there is no air in space to slow the Earth down.)

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You apply an initial force to start it moving but you don't have to supply a force to keep it moving. Newton's 1st law, remember. (Well, OK, there is air resistance so it will slow down and you would have to keep hitting it. But there is no air in space to slow the Earth down.)

I thought Newtons first law was going to be replied, so what initial force started it moving then?

 

 

And if there is force of gravity, does the gravity attraction not cause a ''friction'', a gravity drag has such taking away energy from the motion?

 

 

 

The Earth in the diagram, has force applied to it from gravity, the force trying to make The Earth travel linear movement also towards the Sun, if the Sun is pulling the Earth , that surely would take away energy out of the motion?

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I thought Newtons first law was going to be replied, so what initial force started it moving then?

 

The rotation of the cloud of gas and dust that the solar system formed from.

 

And if there is force of gravity, does the gravity attraction not cause a ''friction'', a gravity drag has such taking away energy from the motion?

 

No.

 

The Earth in the diagram, has force applied to it from gravity, the force trying to make The Earth travel linear movement also towards the Sun, if the Sun is pulling the Earth , that surely would take away energy out of the motion?

 

No. Just changes the direction.

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To reiterate what others have said: motion at a constant velocity requires no force; that's the natural state of motion.

 

The orbit maintains an equilibrium because the force that keeps the earth from moving in a straight line (gravity) varies with distance. There is a steady-state condition achieved, depending on the speed.

 

For a circular orbit, GMm/r2 = mv2/r (gravitational force is centripetal)

so v2 = GM/r

i.e. there is a unique solution for v given r, or r given v

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To reiterate what others have said: motion at a constant velocity requires no force; that's the natural state of motion.

 

The orbit maintains an equilibrium because the force that keeps the earth from moving in a straight line (gravity) varies with distance. There is a steady-state condition achieved, depending on the speed.

 

For a circular orbit, GMm/r2 = mv2/r (gravitational force is centripetal)

so v2 = GM/r

i.e. there is a unique solution for v given r, or r given v

Yes , I understand this, I have always understood this from when I first studied the subject material, and I still say there is something missing.

 

 

 

I understand Newtons first law of motion, and something that is accelerated in space will maintain velocity until it hits something or is slowed down by other force.

 

 

 

I spin a magnet around a piece of wood making an orbit, at velocity 5 rpm, revolutions per minute,

 

 

I then spin another magnet around another magnet making an orbit, also at 5 rpm,

 

 

Are you telling me that I need the same force to make 5 rpm although the magnets will be attracted to each other or repel each other, taking away energy from the cycle?

 

 

 

Something that pulls another something has force yes?

And if there is force of gravity, does the gravity attraction not cause a ''friction'', a gravity drag has such taking away energy from the motion? <-- why?

Because something that pulls has a stopping force.

you explained to me in another thread about Fn, why is the diagram got no Fn on it?

post-87986-0-27331700-1412331936_thumb.jpg

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Because something that pulls has a stopping force

 

Only if it acts in (or against) the direction of motion. If it acts at right angles to the direction of motion (which is what gravity does to an orbiting body) then it will not change the speed.

 

Remember, in F=ma both F and a are "vector" quantities.In other words, they have a direction as well as a size. The acceleration is in the same direction as the force: towards the Sun. (Not along the line the Earth is moving).

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Only if it acts in (or against) the direction of motion. If it acts at right angles to the direction of motion (which is what gravity does to an orbiting body) then it will not change the speed.

 

Remember, in F=ma both F and a are "vector" quantities.In other words, they have a direction as well as a size. The acceleration is in the same direction as the force: towards the Sun. (Not along the line the Earth is moving).

You say the planet wants travel linear, away from the sun, a straight line, I understand the gravity part well, that mass is attracted to mass, so with no centrifugal added, we would make contact and collide with the sun yes?

 

Gravity pulls, centripetal, how can the pulling not slow it down, the static friction of the forces involved?

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Yes , I understand this, I have always understood this from when I first studied the subject material, and I still say there is something missing.

 

 

 

I understand Newtons first law of motion, and something that is accelerated in space will maintain velocity until it hits something or is slowed down by other force.

 

 

 

I spin a magnet around a piece of wood making an orbit, at velocity 5 rpm, revolutions per minute,

 

 

I then spin another magnet around another magnet making an orbit, also at 5 rpm,

 

 

Are you telling me that I need the same force to make 5 rpm although the magnets will be attracted to each other or repel each other, taking away energy from the cycle?

 

 

 

No.

 

I never said ANYTHING about magnets. They were not part of anything you've said in this thread thus far, so you never said anything about magnets. Thus, I could not be telling you this.

 

It's not the first time you've implied or stated I said something which bears no resemblance to anything I'd actually said. (I never said energy has force, for example)

Stop doing that.

 

Gravity pulls, centripetal, how can the pulling not slow it down, the static friction of the forces involved?

 

The pull is perpendicular to the motion for a circular orbit. It can't change the energy of the object; it does no work.

 

In an elliptical orbit, there is some slowing but also some speeding, depending on where you are in the orbit. But the orbit remains an ellipse.

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No.

 

I never said ANYTHING about magnets. They were not part of anything you've said in this thread thus far, so you never said anything about magnets. Thus, I could not be telling you this.

 

It's not the first time you've implied or stated I said something which bears no resemblance to anything I'd actually said. (I never said energy has force, for example)

Stop doing that.

sorry and i did not mean you mentioned magnets,

 

 

You say about Fn, this is what I am trying to show and ask about. I understand the ground pushing back now, I get what you are saying.

 

post-87986-0-26502500-1412335540_thumb.jpg

 

If fn is used on earth why not considered in space by invisible forces?

and venus reached its curie point, the magnetic field would not even break the surface.

Edited by Relative
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You say the planet wants travel linear, away from the sun, a straight line, I understand the gravity part well, that mass is attracted to mass, so with no centrifugal added, we would make contact and collide with the sun yes?

 

Yes. If the Earth did not orbit the Sun it would fall straight towards it in the same way an apple falls to Earth from a tree. (Do you understand the significance of that comparison?)

 

Gravity pulls, centripetal, how can the pulling not slow it down, the static friction of the forces involved?

 

Friction requires a mechanical contact between two surfaces. So if you slide a pencil across a table, there is friction because gravity pulls the pencil down and it "sticks" to the table.

 

There is nothing to cause (significant) friction in space. (There is a tiny amount of gas, and there are tiny tidal effects - but they can be ignored as they are so small.)

 

A force, by itself, does not cause friction. I'm not sure why you think it does. Unless you are trying to extend our everyday experience on Earth where, for example, a ball or a bullet will be stopped by air-resistance (friction) and will fall to Earth. If you throw a ball in space it will keep going until it hits something.

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sorry and i did not mean you mentioned magnets,

 

 

You say about Fn, this is what I am trying to show and ask about. I understand the ground pushing back now, I get what you are saying.

 

attachicon.gifr.jpg

 

If fn is used on earth why not considered in space by invisible forces?

and venus reached its curie point, the magnetic field would not even break the surface.

 

 

How about this: as this thread is about gravity and orbits, we stick to discussing gravity and orbits? When you have a good handle on that, only then add in complications.

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If you throw a ball in space it will keep going until it hits something.

 

For example, the Voyager probes are now outside the solar system. They are no longer firing any thrusters so there is no force causing heir continued motion. But they will keep going, possibly forever (space is big - they are quite unlikely to hit anything).

 

They will not keep going in a straight line - because of gravity they will end up in some sort of orbit around the galaxy. But they won't come to a stop.

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If it is the centrifugal force of a planet that stops it falling into the sun, When in a planets history was this force first brought into being, Is it possible that planets formed further away from our sun/star, Until our sun gravity started acting upon said planet giving its initial speed to maintain their closer stable orbits?

 

 

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If it is the centrifugal force of a planet that stops it falling into the sun, When in a planets history was this force first brought into being, Is it possible that planets formed further away from our sun/star, Until our sun gravity started acting upon said planet giving its initial speed to maintain their closer stable orbits?

 

If you are in an inertial frame, which makes this analysis much easier, there is no centrifugal force.

 

The sun's gravity didn't "turn on" all of the sudden, so a scenario of "gravity started acting upon said planet" isn't consistent with physics.

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Yes. If the Earth did not orbit the Sun it would fall straight towards it in the same way an apple falls to Earth from a tree. (Do you understand the significance of that comparison?)

 

 

Friction requires a mechanical contact between two surfaces. So if you slide a pencil across a table, there is friction because gravity pulls the pencil down and it "sticks" to the table.

 

There is nothing to cause (significant) friction in space. (There is a tiny amount of gas, and there are tiny tidal effects - but they can be ignored as they are so small.)

 

. I'm not sure why you think it does. Unless you are trying to extend our everyday experience on Earth where, for example, a ball or a bullet will be stopped by air-resistance (friction) and wA force, by itself, does not cause frictionill fall to Earth. If you throw a ball in space it will keep going until it hits something.

Thank you and I do understand friction,'' A force, by itself, does not cause friction'', I am not saying a force by itself , I am saying two forces acting upon each other.

 

 

Can two forces create friction?

 

 

I am sorry , I feel I can't even ask the questions I need to ask with diagrams, in fear of thread closure.

 

sunshaker-''If it is the centrifugal force of a planet that stops it falling into the sun, When in a planets history was this force first brought into being, Is it possible that planets formed further away from our sun/star, Until our sun gravity started acting upon said planet giving its initial speed to maintain their closer stable orbits? ''

 

 

My original thoughts a while back when considering gravity, but after further consideration I changed my mind,

I am sorry mods I really need to ask about my homework diagram,

 

 

You consider centrifugal force has the force involved , why are the other forces not accounted for in stopping the collision of the sun and the earth?

 

 

post-87986-0-86148300-1412377506_thumb.jpg

 

 

 

 

''Magnetic lines do not cross each other. it is natural phenomenon.

Answer

Magnetic 'lines of force' do not actually exist. They simply represent a 'model' which is used to explain the behaviour of a magnetic field through the use of something we can easily understand. In this case, one of the conditions for this model to apply is that these imaginary lines of force cannot intersect.

 

The lines do not cross because the field can not have two values at one point. There is a basic equation that says that the lines always form closed loops:

div B = 0, one of Maxwell's equations describing a property of the magnetic flux density B.''

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Can two forces create friction?

 

No. All you get is the sum of the two forces.

 

 

why are the other forces not accounted for in stopping the collision of the sun and the earth?

 

I assume you mean things like the magnetic fields, light pressure from the sun, etc.

 

They are not taken into account because they are so small they have no measurable effect.

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!

Moderator Note

 

thread locked

 

Relative - you must have read the previous modtip; this thread was about gravity with respect to orbital mechanics.

 

This is not the forum to be bringing in speculative ideas and frankly what I consider to be arrant nonsense. Within the main fora please restrict yourself to ideas based on understood mainstream science - not guesswork and word-salad.

 

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