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Post by carnyx on Apr 5, 2011 17:01:52 GMT 1
More insights?
Despite your claims to literary exactiutude, you can't even get the poster right, let alone what is said. Why can't you READ properly?
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Post by speakertoanimals on Apr 5, 2011 18:09:23 GMT 1
Who said I couldn't refer to two posters in one post............. Get back to the PROBLEM, rather than wasting all this space trying to have a go at me! Or perhaps (given your penchant for cut and paste), I could save you all some time, just use this: STA, you're talking sense, its all insight, and your english is entertaining (ditto spelling)............... Back to the problem. There is definitely a misapprehension being repeated here about the significance of centre of gravity. It is NOt the case that the gravitational force on a test body due to some other collection of bodies always has to point towards the centre of mass of the collection -- it is just the case that for a distant test body, the approximate gravitational field is that of a point body placed at the centre of gravity. For a body within the Earth-Moon system, the actual gravitational field and allowed orbits are a tad complicated: www.nasa.gov/centers/ames/research/2008/Lissauer.htmlthe diagram is for the sun & earth, but earth moon vaguely similar. Talking about the centre of mass doesn't capture in any way the actual detail here! So, the earth and moon orbit each other about their common centre of mass. An object placed at the Lagrange points L4 or L5 insights in the same plane as the earth and moon, and will be stable, and in effect will orbit always behind or ahead of the moon. This isn't the Lagrange case. It would be a body trying to orbit somehow in a plane perpendicular to the line joining the earth to the moon. but since the moon is orbiting the earth anyway (or common centre of mass, but since that is within the body of the earth, let's just say earth for shorthand), then there isn't a fixed plane in which this body could be! Instead, the only plane worth considering is the plane of the earth-moon system itself, for stuff that is really effected by both. Wrong. stuff can apparently orbit about the Lagrange points, but they don't orbit about the CofG, but about the Lagrange point! The L4 and L5 Lagrange points in the sun-jupiter system are where the trojan asteroids hang out -- orbit WITH jupiter, either sixty degrees ahead or behind. Back to the rugby ball. As I said previously, just using Gauss law, we can't rule out a non-zero gravitational field inside. We can say that the centre point on the axis is a fixed point, just by symmetry, but we can't say if it is stable or unstable. Just that if youi placed a mass exactly there, it would feel no net gravitational force. And the foci? No reason (apart from their use in basic 2D geometry) to assume that they play any special role. They are just points on the axis of symmetry, and all we can say so far is that for points on the axis, gravitational force is going to be along the axis. Another problem to think about is the grav field within a circular ring -- contrary to what you might expect, once you are off centre in the plane of the ring, the gravitational force is towards the ring. Hence why some of the constructs in the Ringworld series By Larry Niven wouldn't actually work..................
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Post by Progenitor A on Apr 5, 2011 18:41:25 GMT 1
;D What STA is saying is that this insightful circumlocution is actually CORRECT .......... No. Why not? simple. to be in a closed orbit, the force on the test body must always be towards the centre of its orbit. {A prize of 2 weeks in the Sands Barbados for anyone that can make sense of the next sentence] ;D!.......So, suppose we have body A, then one side of the orbit of the test body, interior of its orbital ellipse/circle whatever, then other side of orbit, then body B. [Hahahaha.....!] ;DLets call if A L (left) C R (right) and then B. ;D When at L, if force is to be inwards then grav effect of B must be greater than grav effect of A, which pulls in opposite direction. Hence when at R, we are closer to B, and further from A, hence if the pull of B won at L, it will win by even more at R. Hence we cannot set it up so net gravitational forces is towards the centre at both sides. [Here is a pair of knitting needles = help yourself! ;D] Hence we can't have an orbiot that lyes wholly between A and B, we can only have an orbit where at least one of the bodies lyes WITHIN the orbit[Oh my god.......! .....usual wordy praise of others (Love that Hence......!) And now that she has looked up Wiki (and quoted almost verbatim from it) ;D, she hopes, that by quoting Lagrange points we will not notice that she is contradicting herself ! ;D For the Langrange points are the very points that Buckley and I were referrring to without mentioning them! Those are the very points at which a third body is stable following the isoNewtonian lines of force of the earths and moon's gravity! Using these Langrange points is is possible for a light third mass to rotate around the mutual CofG of the earth and moon But we already knew that didn't we? The only person that contested that was STA And now she has (once again) contradicted herself ;D What a [snip] contributor! ;D
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Post by carnyx on Apr 5, 2011 22:41:56 GMT 1
@sta
Er.. common courtesy to posters, who differ more widely in all respects than you could even guess at.
Er... from what has been discussed on this thread so far, I reckon there is no problem in undertanding that problem .. perhaps only in your head.
And with regard to your rugby ball comments, as I had said I intuit that there may be two stable points of zero net force, and these will be at the foci.
I did ask you as a purported academic physicist, if any of your colleagues or students could have anything to say .. but you reacted rather than answer this request. What is wrong with the answer 'no'?
The fact you lack the resources to tackle the problem in the first place speaks volumes .. let alone your second failure.
So STA, you have contributed nothing of note to this thread, but have obviously sought to disrupt it.
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Post by buckleymanor1 on Apr 6, 2011 0:32:55 GMT 1
OK.this is not a lagrange case. So there isn't a fixed plane because the moon is orbiting the earth which makes this position not possible. Well what the difference between an object orbiting the moon which is not fixed it orbits the earth. Or one orbiting the earth which also is not fixed it orbits the sun. Or any object orbiting anything non of them are fixed. Why is it a neccesity for there to be a fixed plane for something to orbit effectively I am not sure if one exists.
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Post by carnyx on Apr 6, 2011 8:01:40 GMT 1
buckleymanor1
I have not been following this part of the thread, but within the solar system we have the sun with planets obriting in their planes, and moons orbiting these planets. And then we have the bizzare behaviour of Saturn's great disc, where the movement of objects within it are still not understood'
However, I too have been intrigued by the observation that orbiting objects will tend to stay in a plane. It is easier to see why objects being rotated on an axis are forced to stay in a plane .. lile glassblowers making a disc ... or how man's invention of the wheel really does work ..
ects .
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Post by Progenitor A on Apr 6, 2011 8:31:54 GMT 1
Hi Carynx, thanks for your thoughts on the G field inside the rugby ball – some very useful stuff there that I will come back to. I have considered what you have said and think we can logically derive the field without resorting to mathematics But first some clearing up of accumulated miscomprehensions leading to false and contradictory conclusions (some might say rubbish!)(with some lovely archives for the ‘STA Science’ thread) First we had Eamon making this contribution which is a fair enough start but leads to a false conclusion: What happens to the floating ball bearing? Isn't that obvious? It is repelled from the hole with a force inversely proportional to the square of the distance (if the distance is large compared to the size of the hole). Which is why I asked this question Perhaps that is 'obvious' But in which direction is it impelled to travel and where will it finish, will it come to rest?? But Eamon unfortunately decided to leave the discussion on this false note Never mind, in jumps STA to take up the cudgel and make exactly the same egregious mistake: Hence drilling a hole in a sphere, as someone already said, can be considered the SAME as removing the sphere entirely (since net gravitational effect zero inside anyway!), and just adding a small NEGATIVE mass on the point where you drilled the hole. Hence you'd be repelled from that point until you just hit the inner surface opposite. You wouldn't orbit about the centre of mass, that particular approximation does not apply when you are INSIDE the set of gravitating masses. I think that she might have seized on this because it looks an ‘intellectual’ approach, but unfortunately it leads to the wrong conclusion! Being repelled from a ‘negative’ mass is not the same as being attracted to a positive mass as a short pause for consideration will show Indeed when the absurdity of invoking ‘negative mass’ is pointed out ,this is her reply: Not actual repulsive gravity, just easiest to compute net effect by starting froms somehting you do know (zero field inside uniform sphere), and working from that. Hence net effect or removing mass is the same as replacing that removed mass by negative mass, which gives the correct sign for net gravitational effect. Which is, of course, nonsense, and is repeated in her next post: When you drill a hole, the gravitational effect of that bit of mass was previously cancelled by the effect of the REST of the sphere. Hence, as has been said, the net effect of the hole is the SAME as removing the rest of the sphere (since the whole sphere had zero net effect anyway), and replacing the 'hole' by a chunk of negative mass. As long as you are inside the sphere, that gives the same net effect. Of course ‘replacing the 'hole' by a chunk of negative mass’ does not give the same effect at all! Judge for yourself whether a physicist is speaking! Then Buckley asks this question (which has already implicitly been answered, but I must admit I did not realise it myself at the time) So would it be possible for a test mass to orbit in between the earth and moon, say in a horizontal plane to the earth. . And this is where the replies become astonishing in their opacity and gobbledegook. Here is STA replying in prize mode: Contradicting all her previous argument! Quite astonishing! But what is really astonishoing is the argument she uses to ‘justify’ this contradiction, a prime example of garrulous nonsensical gobbledegook! Why not? simple. to be in a closed orbit, the force on the test body must always be towards the centre of its orbit. So, suppose we have body A, then one side of the orbit of the test body, interior of its orbital ellipse/circle whatever, then other side of orbit, then body B. Lets call if A L (left) C R (right) and then B. When at L, if force is to be inwards then grav effect of B must be greater than grav effect of A, which pulls in opposite direction. Hence when at R, we are closer to B, and further from A, hence if the pull of B won at L, it will win by even more at R. Hence we cannot set it up so net gravitational forces is towards the centre at both sides. Hence we can't have an orbiot that insights wholly between A and B, we can only have an orbit where at least one of the bodies lyes WITHIN the orbit. Then Carnyx brings in the rugby ball internal G field and here is STA on that invoking Gauss’s laws – presumably to impress except that there is an evident piece of nonsense – here it is: this is Gauss law for gravity. …. Okay, so inside a sphere, ANY closed surface lying wholly inside the sphere has zero enclosed mass. This so self-evidently wrong that the whole of her reasoning is false and can be disregarded because her reasoning rests on this solecism This is pointed out to her… Okay, so inside a sphere, ANY closed surface lying wholly inside the sphere has zero enclosed mass. This quite reasonable statement about a circle within a sphere enclosing mass is met with this rather interesting response from STA A circle isn't a surface! And as Gauss law states, it consider the flux through a closed surface, and I specifically mentioned a closed surface lying wholly within the sphere (spherical shell). There you are, a circle drawn on the interior of a hollow sphere does not enclose a surface. Can you believe a self-proclaimed physicist making such a statement? Now the following post is interesting for here she contradicts herself again. For you wil remember that she has already said that it is not possible to orbit the mutual moon-earth CofG, and has found some impressive literature here to contradict herself! www.nasa.gov/centers/ames/research/2008/Lissauer.htmlthe diagram is for the sun & earth, but earth moon vaguely similar. Talking about the centre of mass doesn't capture in any way the actual detail here! So, the earth and moon orbit each other about their common centre of mass. An object placed at the Lagrange points L4 or L5 insights in the same plane as the earth and moon, and will be stable, and in effect will orbit always behind or ahead of the moon. This isn't the Lagrange case. It would be a body trying to orbit somehow in a plane perpendicular to the line joining the earth to the moon. but since the moon is orbiting the earth anyway (or common centre of mass, but since that is within the body of the earth, let's just say earth for shorthand), then there isn't a fixed plane in which this body could be! Instead, the only plane worth considering is the plane of the earth-moon system itself, for stuff that is really effected by both. Wrong. stuff can apparently orbit about the Lagrange points, but they don't orbit about the CofG, but about the Lagrange point! The L4 and L5 Lagrange points in the sun-jupiter system are where the trojan asteroids hang out -- orbit WITH jupiter, either sixty degrees ahead or behind. ;D
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Post by Progenitor A on Apr 6, 2011 9:03:02 GMT 1
Now Carnyx, with some of the rubbish swept out of the way,back to your interesting thoughts on the internal G field of a rugby ball.
Here are my thoughts based upon your reasoning and I think that they are logically consistent: Firstly imagine the ball sawn in half and consider only the central half-ellipse Draw a tangent to any point on this curve and then a line perpendicular to that tangent and you will find (I think) that the gravitational field of any point will extend to the Centre, the centre forms the CofG and any external mass will be attracted to that.
Of course at the CofG we have zero G. So we have line of G force diverging from the centre.
Rotate that half ellipse around its axis of symmetry and we have a half-rugby ball with a gravitational field directed from the centre
Now bring the other half-ball and join it to the half-ball we have just considered.
It is self evident that at the plane of the centre we have equal and opposite G fields at all points, therefore we have a circle -the fattest part of the ball - with zero G field, but we do have a G field in the other parts of the ball directed away from the centre
Now consider that fattest part of the ball again. It is a circle with the widest diameter and the rest of the ball can be considered as other circles of progressively decreasing diameter.
Now as you have said, a circle is similar to a two-dimensional sphere. If we consider such a circle made of a very dense, very thin material , an extremely thin shell cut from a sphere, then it is self evident that radially there is a zero G field within the ring!
This applies to all the rings of diminishing radius that make up the ball!
So there is zero radial G field throughout the ball
There is a longitudinal field starting at zero for the centre of the ball and 'flowing'' toward the ends of the ball
So all around the inside the ball there is zero radial G field toward the surface with the central circle have no G field and any ball bearings in the ball would congregate at each end!
Thanks for the lead!
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Post by eamonnshute on Apr 6, 2011 9:36:36 GMT 1
But Eamon unfortunately decided to leave the discussion on this false note Being repelled from a ‘negative’ mass is not the same as being attracted to a positive mass as a short pause for consideration will show That is not true. Being repelled from the small negative mass alone gives exactly the same force as being attracted by the holed sphere, on the inside. This gives a hyperbolic path to a particle inside the sphere. So the problem is solved, that is why I left the discussion.
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Post by Progenitor A on Apr 6, 2011 9:42:25 GMT 1
But Eamon unfortunately decided to leave the discussion on this false note Being repelled from a ‘negative’ mass is not the same as being attracted to a positive mass as a short pause for consideration will show That is not true. Being repelled from the small negative mass alone gives exactly the same force as being attracted by the holed sphere, on the inside. This gives a hyperbolic path to a particle inside the sphere. So the problem is solved, that is why I left the discussion. Think again Eamon A small mass will be repelled radially away from such a repulsive force and if it is offset from the line passing through the hole and the centre of the sphere, it will not come to rest opposite the hole
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Post by eamonnshute on Apr 6, 2011 9:47:48 GMT 1
Think again Eamon A small mass will be repelled radially away from such a repulsive force and if it is offset from the line passing through the hole and the centre of the sphere, it will not come to rest opposite the hole I didn't say it would. If it starts from rest it will travel directly away from the hole till it hits the sphere. If it starts off-axis it will move further off-axis.
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Post by Progenitor A on Apr 6, 2011 9:51:52 GMT 1
Think again Eamon A small mass will be repelled radially away from such a repulsive force and if it is offset from the line passing through the hole and the centre of the sphere, it will not come to rest opposite the hole I didn't say it would. If it starts from rest it will travel directly away from the hole till it hits the sphere. If it starts off-axis it will move further off-axis. Yes that is true But that is not what happens - ther is no 'negative gravity' - the particle is attracted to the point opposite the hole (on a line through the centre of the sphere), no matter what its position in the sphere may be A 'negative gravity' does not give the same force - the directions differ Consider
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Post by eamonnshute on Apr 6, 2011 10:18:17 GMT 1
A 'negative gravity' does not give the same force - the directions differ Consider I have proved that it does. If you disagree and think the two situations give different results then prove it.
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Post by Progenitor A on Apr 6, 2011 10:21:40 GMT 1
A 'negative gravity' does not give the same force - the directions differ Consider I have proved that it does. If you disagree and think the two situations give different results then prove it. You have proved nothing You have hypothesised a non-existent gravity that does not exist where the hole is created! The whole hypothesis is absurd If we remove mass it does not create negative gravity It simply eliminates to zero the gravity that was there before we removed the mass You are effectively saying that 1-1=-1! To remind you, 1-1=0 Think again!
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Post by eamonnshute on Apr 6, 2011 10:34:26 GMT 1
I am saying that 1 + (-1)= 0; ie a positive mass plus a negative mass is equivalent to no mass,ie the hole.
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