Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: mxplxxx on 14/10/2016 03:16:10
-
What is the force (of gravity) exerted by the sun on the earth. Is this enough to keep the earth in a circular orbit around the sun. What about Jupiter? How come Mars weighs less than the earth but is further from the sun? How come satellites stay in orbit and not plummet to earth due to the force of the earths gravity.
On a related topic, am I lighter when traveling in a plane?
On yet another related topic, is something weightless at the center of the earth. I read somewhere that the sun's fusion starts because of the gravity its center. Is this so? It seem to me the fusion is likely because of the particles near the center losing kinetic energy as they slow.
-
What is the force (of gravity) exerted by the sun on the earth.
If M is the mass of the Sun, m is the mass of the Earth, r is the distance from the Earth to the Sun, F is the force on the Earth by the Sun and F is magnitude of F then
F = GMm/r2
Is this enough to keep the earth in a circular orbit around the sun.
Yes. However the Earth isn't in a circular orbit around the Sun. The orbit is an ellipse with the Sun at one foci.
What about Jupiter?
What about it? The same kind of relationship holds.
How come Mars weighs less than the earth but is further from the sun?
What do you mean? The term weight has been defined in two separate ways in the physics literature. It can be said that if the only force on an object is a gravitational force then one definition defines its weight as equaling the gravitational force while the other definition defines it as being weightless (e.g. astronauts in the ISP are in free-fall and consider themselves to be weightless in their frame of reference.
How come satellites stay in orbit and not plummet to earth due to the force of the earths gravity.
Because their inertia tries to keep them moving in a straight line while the gravitational force tries to accelerate them to the Earth's center. The end result is the satellite falling around the Earth. Think about a cannonball being fired from a cannon which is pointed parallel to the Earth's surface. The faster the cannonball's speed the further out it will land. If the speed is large enough it will "fall" around the Earth.
On a related topic, am I lighter when traveling in a plane?
No. At least not significantly. Since planes fly above the Earth's surface the value of r is larger making F smaller. But the difference is so small that it can be ignored. In fact it'd be extremely hard to measure any difference in weight.
On yet another related topic, is something weightless at the center of the earth.
Yes.
I read somewhere that the sun's fusion starts because of the gravity its center. Is this so?
Not just at the center. Fusion occurs in a much larger region of the Sun than just at the Sun's center.
It seem to me the fusion is likely because of the particles near the center losing kinetic energy as they slow.
That's not the reason. Actually the opposite is true. The pressure inside the Sun is so high that the nuclei are basically "squeezed" together. This occurs because the nuclei have greater kinetic energy, enough to overcome the Coulomb barrier. There are also quantum mechanical considerations which play apart in the fusion of Helium nuclei. If you wish to learn more about all of this then I recommend looking it up on Wikipedia.
-
RE the center of the earth, it seems to me that a particle there will be be pulled apart by the force of gravity being exerted equally on all sides by all the other particles that make up the earth.
-
RE the center of the earth, it seems to me that a particle there will be be pulled apart by the force of gravity being exerted equally on all sides by all the other particles that make up the earth.
It is more likely to be compressed. Otherwise black hole formation couldn't happen.
-
RE the center of the earth, it seems to me that a particle there will be be pulled apart by the force of gravity being exerted equally on all sides by all the other particles that make up the earth.
It is more likely to be compressed. Otherwise black hole formation couldn't happen.
Thx for your contribution. "Compressed", how? Maybe a black hole is just absence of space into which space is pouring (maybe a new universe in progress) !
-
RE the center of the earth, it seems to me that a particle there will be be pulled apart by the force of gravity being exerted equally on all sides by all the other particles that make up the earth.
This is incorrect, Mr mxplxxx.
Isaac Newton showed that the gravitational forces from the distributed mass of the Earth exactly balance at the center of the Earth.
- So there will be no force pulling matter apart
- However, there is all that pressure from the surrounding mass of the Earth squashing down on you (as Jeffrey suggests)
- But if you were able to somehow build a little bubble at the center of the Earth, strong enough to resist all that heavy rock, your particle at the center of the Earth would float freely, with nothing compressing it, and nothing pulling it apart.
Of course, Isaac Newton had to invent a whole new branch of mathematics to come to that conclusion (his contemporary Gottfried Leibniz also developed calculus at about the same time).
-
Isaac Newton showed that the gravitational forces from the distributed mass of the Earth exactly balance at the center of the Earth.
Can you give me a reference for this please.
-
Isaac Newton showed that the gravitational forces from the distributed mass of the Earth exactly balance at the center of the Earth.
Can you give me a reference for this please.
See: https://en.wikipedia.org/wiki/Shell_theorem
-
Isaac Newton showed that the gravitational forces from the distributed mass of the Earth exactly balance at the center of the Earth.
Can you give me a reference for this please.
See: https://en.wikipedia.org/wiki/Shell_theorem
Thanks evan_au.
-
Isaac Newton showed that the gravitational forces from the distributed mass of the Earth exactly balance at the center of the Earth.
Can you give me a reference for this please.
See: https://en.wikipedia.org/wiki/Shell_theorem
I hadn't realised that the Yukawa potential could be related to gravity. I have something to play with.
-
So, it would seem that Newton's theories of gravity predict correct results but are not rooted in reality. The earth is not attracted to the sun via gravity, both warp the space (spacetime) around each other and travel in the lines of force that this warping generates (this seems to imply that gravity if just momentum acting along a line of force - is this a reasonable conclusion).
It could be argued that Newtons laws are digital simulations of the spacetime continuum and ditto for quantum theory. Possibly even these simulations also take place in our minds (and therefore that our minds are outside spacetime). A continuum, of course, can be approximated by splitting it up into points. The smaller the points, the closer the approximation.
-
(this seems to imply that gravity if just momentum acting along a line of force - is this a reasonable conclusion). What about the electromagnetic forces that spin the planets??
-
What about the electromagnetic forces that spin the planets?
The spin came from the rotation of the accretion disc, there is no continuing magnetic force making the planets spin or exerting force on the earth.
-
The WMAP results seem to suggest that we a missing a really large dark force. So if its not magnetic perhaps its electrostatic but our instruments cant sense it?
-
The WMAP results seem to suggest that we a missing a really large dark force.
This is significant on cosmic scales, but is (currently) insignificant on the scale of our Solar System or our galaxy.
This mysterious force has been dubbed Dark Energy. So it is no longer missing, but giving it a name doesn't make it any less mysterious.
So if its not magnetic perhaps its electrostatic but our instruments cant sense it?
We now have quite sensitive instruments that can detect electrostatic forces, so if electrostatics were significant at the level of the Solar system we would detect it.
If the electrostatics were only significant at cosmic scales (eg slightly more electrons than protons in the universe, for example) then we probably would not detect it on Earth.
However, this still does not explain the rate of cosmic inflation in the early universe, which we also infer from surveys like WMAP (https://en.wikipedia.org/wiki/Wilkinson_Microwave_Anisotropy_Probe#Objectives).
-
The earth is not attracted to the sun via gravity, both warp the space (spacetime) around each other and travel in the lines of force that this warping generates
Why then does quantum theory predict the graviton particle to explain gravity? The graviton is a force particle with a ridiculously small value. How can this puny particle mediate the (apparent) attraction of the sun and earth, particularly when the force it exerts is inversely propositional to the square of the distance between two bodie ?
-
The graviton is a force particle with a ridiculously small value.
...and Gravity is a force which is ridiculously weak (compared to electromagnetism or the strong nuclear force).
At this stage, the graviton is merely hypothetical, an analogy; there is no experimental evidence for it.
The sole claim to fame for string theory is that it combines general relativity and the graviton in a natural way; it's a pity it can't prove anything at the moment.
How can this puny particle mediate the (apparent) attraction of the sun and earth, particularly when the force it exerts is inversely propositional to the square of the distance between two bodies ?
In theory, it takes a large number of gravitons to mediate the attraction between Sun and Earth.
In principle, the mechanism is not radically different from photons which mediate the electromagnetic force, which also follows an inverse square law.
See: https://en.wikipedia.org/wiki/Graviton
-
(this seems to imply that gravity if just momentum acting along a line of force - is this a reasonable conclusion).
Where does the momentum come from when I jump up and down? It may be from our (near) absolute momentum. We are part of the Earth frame of reference but also part of many other frames of reference. A universe-level observer sees us whizzing around the universe as part of many structures (solar system etc.) under the influence of gravity. We are slowing down at one point or accelerating at another point or become stationary at yet another point and pretty much changing direction all the time. When we jump up and down I am guessing we temporarily leave our earth frame of reference and come under the influence of our absolute momentum. It is possible that it is this momentum that causes us to come back to earth.
It is worth noting that our absolute momentum and membership of a hierarchy of frames of reference (molecule, solar system, galaxy etc.) puts constraints on the way the universe can evolve structurally and energetically which may have gravity-related consequences.
-
We are part of the Earth frame of reference but also part of many other frames of reference.
As I understand it, there is a special place in Special Relativity for an "Inertial" frame of reference. Bodies in an inertial frame of reference (https://en.wikipedia.org/wiki/Inertial_frame_of_reference) will travel in a straight line at constant velocity, with no acceleration. Effectively when you are weightless, far from any source of gravity (mass).
- You, standing on the Earth, are not in an inertial frame of reference (not weightless; the path of a pendulum is slightly curved by the rotation of the Earth).
- The Earth, orbiting around the Sun is not in an inertial frame of reference (weightless, but the path is slightly curved by the mass of the Sun)
- The Sun, orbiting around the galaxy is not in an inertial frame of reference (weightless, but the path is even more subtly curved by the mass of the galaxy)
However, when you jump, you become weightless (for a short time).
- And if you consider a small volume around yourself, for the small amount of time while you are weightless, you can approximate this by an inertial frame of reference.
- For example, juggling becomes impossible, because the balls now travel in straight lines (relative to you).
When we jump up and down I am guessing we temporarily leave our earth frame of reference and come under the influence of our absolute momentum.
There is no such things as "absolute momentum". Your actual momentum will differ depending on which frame of reference it is measured in.
In the (almost) inertial frame of reference which is you jumping, you have zero momentum.
We are slowing down at one point or accelerating at another point or become stationary at yet another point and pretty much changing direction all the time.
General Relativity takes into account the effects of mass, and you can select frames of reference rather arbitrarily. But depending on what you are trying to calculate (and how accurately you are trying to calculate it), some frames of reference are easier to use than others.
-
The frame of reference used to be the centre of the earth, then the centre of the sun but now it is the centre of our galaxy in Sagittarius A* the magnetic black hole hub of our galaxy. How does relativity explain the moving of the stars around that hub without electromagnetism??
-
When we jump up and down I am guessing we temporarily leave our earth frame of reference and come under the influence of our absolute momentum.
There is no such things as "absolute momentum". Your actual momentum will differ depending on which frame of reference it is measured in.
I define absolute momentum as that momentum that is observable by the single observer at the level of the universe (god?).
-
About that pressure, and absence of gravity at 'the middle of the Earth', If you think of it the whole mass of Earth already are acting at you at the surface. Going to the middle won't add any more 'forces' acting on you than it already does at that surface. And it's definitely a added pressure down there, just the same as with water, or a atmosphere. As far as I get it you will find no 'gravitational forces' acting upon you though, except your own mass, all of this presuming a ideal spherical human, inside a ideal spherical Earth, not spinning and resting inside a otherwise empty, possibly spherical from some dimension or other, 'space'. Yep, now it's scientifically put.
=
a nicer sentence.
Welll if you insist, a spherical cow instead of that human, satisfied?
( Seems to be using Swenglish here instead of English :)
-
There is no such things as "absolute momentum". Your actual momentum will differ depending on which frame of reference it is measured in.
SOL is absolute isn't it?
The universe is organized in an abstraction hierarchy that is closely related to Einstein's Frames of reference. You are an abstraction. The only thing concrete about you is the elementary particles that you are currently composed of. As part of the Earth frame of reference , your momentum (or that of your particles) is mostly close to zero. Viewed from a galaxy frame of reference, you are part of a planet that is revolving on its axis and simultaneously zipping around a sun. And the solar system is whirling around the galaxy. At the universe frame of reference, all components of your momentum are visible and this is what I would call your absolute momentum.
The reason that light has an absolute momentum is because it belongs only to the Universe's frame of reference.
-
It depends on your frame of reference. That's what you compare that light too. A light leaving a 'gravity well' as earth can be defined to redshift relative me, coming into our 'gravity well' its supposed to 'blue shift'. A blue shifted 'photon' have a higher frequency, more 'impact' and 'energy', and therefore a higher momentum as I understands it
-
The 'energy' is observer dependent, and as there is no frame of reference in where light doesn't 'propagate' it becomes very tricky to define its momentum when 'at rest' with the observer. Einstein used a mirror and wondered if he would be able to see his reflection in it, if the mirror (at rest with him) would be moving at the 'speed of light'. Deciding that he should, he then went on to 'invent' SR :) well, sort of.
-
The 'energy' is observer dependent, and as there is no frame of reference in where light doesn't 'propagate' it becomes very tricky to define its momentum when 'at rest' with the observer. Einstein used a mirror and wondered if he would be able to see his reflection in it, if the mirror (at rest with him) would be moving at the 'speed of light'. Deciding that he should, he then went on to 'invent' SR :) well, sort of.
My understanding of a frame of reference is that it is an area of space in which all objects have the same momentum. There is only one area of space where this seems to hold true for light, being the universe itself. The inventors of this learning system we call reality seem to have decided on a type of finite state machine as the means to teach consequences. A finite state machine relates states and events. Events in reality are photons. A photon must be able convey the state of a fermion to another fermion in a different frame of reference. In other words, photons must be independent of frames of reference and consequently independent of time. I reckon in trying to understand reality we must always first try and understand why reality exists. More and more, it seems to me we are in a type of classroom:)
-
photons must be independent of frames of reference and consequently independent of time.
Please explain what you mean.
GPS satellites exist, regardless of what frame of reference you observe them from. They are independent of your frame of reference.
But they are not independent of time - in fact, their primary mission is to keep time, and broadcast it to all the GPS receivers on Earth.
-
Yes, I agree as photons have an energy of hf which is time dependant. Feynmans diagrams of QED are correct if you understand them in 3 dimensions. Electromagnetic light energy is 3D magnetic pulse volume of vibrating energy = root of current area{ Ix[current] times Iy[ right angle flux]} X Vz[voltage at right angles] X Cos .thi and can be massless in a magnetised space.
-
GPS satellites exist, regardless of what frame of reference you observe them from. They are independent of your frame of reference.
But they are not independent of time - in fact, their primary mission is to keep time, and broadcast it to all the GPS receivers on Earth.
Evan, about the GPS satellites that you mention. You seem to imply that they're in a "frame of reference" which consists only of the satellites, and the Earth.
And that their "primary mission" to keep time, and broadcast it to GPS receivers on Earth, can be accomplished without regard to any influences outside this "frame".
But is that true. I mean, aren't the satellites influenced by external gravitational "tugs" from the Moon, and the Sun? Shouldn't these "tugs" be taken into account, when the satellites broadcast their signals to Earth.
Or is it that they're just too weak to significantly affect the accuracy of the signals, and so are ignored for practical purposes.
-
photons must be independent of frames of reference and consequently independent of time.
Please explain what you mean.
I should have said "independent of frames of reference timeframes".
It is interesting to note that photons exist in only one frame of reference, that being the universe itself. To a photon, all other photons are at rest. Having a fixed momentum, it would seem they cannot take part in other frames of reference. Which leads to an interesting speculation, how do they interact at all with fermions?
-
aren't the satellites influenced by external gravitational "tugs" from the Moon, and the Sun? Shouldn't these "tugs" be taken into account, when the satellites broadcast their signals to Earth.
Or is it that they're just too weak to significantly affect the accuracy of the signals, and so are ignored for practical purposes.
The tugs do affect the satellites but the precaculated adjustment is fixed rather than dynamic so the satellites are monitored by an earth station which transmits adjustments to each satellite as necessary.
I read evan's comment as just noting that the satellites are in their own reference frame independent of our's or other's.
-
You seem to imply that (GPS satellites) in a "frame of reference" which consists only of the satellites, and the Earth.
GPS has 6 satellites chasing each other around a circular orbit. This means:
- Constant distance from the center of the Earth = constant depth in the Earth's gravitational well (about 20,000km).
- Constant orbital velocity around the Earth
- Constant relativistic time correction applied to each of the satellites
- Each of these 6 satellites is in the same frame of reference
However, the surface of the Earth:
- Is at a different depth in the Earth's gravitational well (about 6,370km)
- Is moving at a very different velocity
- And so the surface of the Earth is not in the same frame of reference as the satellites.
To get global coverage, additional satellites are placed in additional circular orbits, in 6 planes relative to the Earth's axis
- Because these planes are angled relative to each other, the satellites in different planes are in different frames of reference from each other.
their "primary mission" to keep time, and broadcast it to GPS receivers on Earth
So although the satellites are in a different frame of reference from the surface of the Earth, their time signals are preconfigured to be received with accurate time by GPS users on Earth's surface (or users in aeroplanes or Low Earth Orbit who are keeping Earth time).
can be accomplished without regard to any influences outside this "frame". I mean, aren't the satellites influenced by external gravitational "tugs" from the Moon, and the Sun?
Yes, the Sun and Moon and Earth's non-spherical shape do exert tugs on the satellites, which slowly distorts their orbits. This is why the orbit data transmitted by the satellites is typically updated about every 4 hours, so receivers on the ground know the precise orbit of every satellite.
- The Sun is 150 million km away, so it affects every satellite equally
- The Moon is much closer - at about 380,000km, it is 19 times farther away than the GPS satellites. But it has 1.2% of the mass of the Earth. Apply an inverse square law, and that is a very small perturbation in the gravitational well.
Shouldn't these "tugs" be taken into account, when the satellites broadcast their signals to Earth.
The Earth is at the same average distance from the Sun and the Moon as are the GPS satellites.
The Earth is in "free fall" around the Sun and the Earth/Moon center of gravity, as are the GPS satellites.
So I expect that these small tugs/relativistic corrections will average out over a 24 hour period.
-
You know mxplxxx. Sometimes I wish the same, as if we could define this universe as consisting of measurable 'objective' quanta. Frames of reference will in each case they are presented have a logically defined limit, and a understandable description, but they are not 'quanta'. I'm starting to look at 'properties' those days, not 'quanta' at all. Those might be 'objective' over a SpaceTime, they are after all an idea, not a 'touchable'.
As for a 'frame of reference means all objects have the same momentum'? We're in a same reference frame on Earth for example, will that give us the same momentum? I do see how you think though, and that's what I commented on above.
The closest I've come to 'quanta' is when I define it strictly locally. But that also means that I refuse to explain what 'connects' this universe.
-
I'm starting to look at 'properties' those days, not 'quanta' at all.
I would not give up on quanta. Just as in computers strings of bits that can assume values of 0 and 1 define the data and instructions that the computer operates on, quanta could do similarly for reality. Quanta have the advantage that they seem able to reflect on their own states, giving rise to emotion as a state. The number of states they can assume is also well in excess of 0 and 1 (think superposition of states). They can also annihilate each other. They also have momentum which is a state that relates time and energy, something vital for a system which teaches consequences. The properties of objects in reality may well arise out of configurations of quanta. The thing about quanta is that they are basically all the same (as per Planck's constant) except for their states making them ideal entities to do computational-type work.
-
There is no such things as "absolute momentum". Your actual momentum will differ depending on which frame of reference it is measured in.
SOL is absolute isn't it?
No, it is invariant. Absolute means that there is a single reference frame for which the everyone agree that light travels at c relative to. Invariant means that everyone measures the speed of light as being c relative to themselves regardless of what relative velocity that they have with respect to each other.
The universe is organized in an abstraction hierarchy that is closely related to Einstein's Frames of reference. You are an abstraction. The only thing concrete about you is the elementary particles that you are currently composed of. As part of the Earth frame of reference , your momentum (or that of your particles) is mostly close to zero. Viewed from a galaxy frame of reference, you are part of a planet that is revolving on its axis and simultaneously zipping around a sun. And the solar system is whirling around the galaxy. At the universe frame of reference, all components of your momentum are visible and this is what I would call your absolute momentum.
No such "universe" frame of reference exists.
The reason that light has an absolute momentum is because it belongs only to the Universe's frame of reference.
Light doesn't have an absolute momentum. For one, there is no frame of reference that the can be applied to the universe as a whole. For the other lights momentum is determined by its frequency. If you are holding a flashlight and measuring the momentum of the light coming from it you will get one value. If I am traveling towards you from the direction you are shining the light, I will measure the light as Doppler shifted to a higher frequency and its momentum as being greater.
-
What is the force exerted by the sun on the earth?
It's exactly the same as the force exerted by the earth on the sun!
"Every action has an opposite and equal reaction..."
-
Always a pleasure Evan, but ?
what about conservation laws?
Those are starting to turn my head. Although they do imply a 'container'. just not of the tangible kind. Then again :)
=
Don't read me wrong, we have some very clear. and knowledgeable minds, at TNS. Bored Chemist, Evan. and also those of you I don't mention. What I mention btw, is those I'm arguing with just now, nota bene. I'm f*ng human
and what they have, that you wont find anywhere else, is patience.
It's a good thing, taking time to think it out. Even when people have an opinion that strike you as being out of here.
I wouldn't be here, except for guys and gals as them
Ok RD and JP
So what? use that mind....
Don't look at others, be brave, and learn
-
I know, I'm starting to feel as if I'm in a cave, looking at shadows. So what can I fall back on?
Integrity, sober or not.
I'm here, and so are you
=
And if it's a cage?
Revolt
-
Maybe :)
But then we need a new mathematical definition mxplxxx. because when one question Boolean logic. as consisting of more than the theorem stands for, one also need to introduce a new mathematic.
-
I know, I'm starting to feel as if I'm in a cave, looking at shadows. So what can I fall back on?
Integrity, sober or not.
I'm here, and so are you
=
And if it's a cage?
Revolt
I find it better to think of it as an adventure playground. Even better it is not an it, it is an I. I am both reality and an observer of reality.