how does earth stay "in place" in space?

vandivx · Dec 11, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>this is a small follow up on a previous thread i made, i remember i was told the iss is in "constant free fall" if that's the case, then isn't earth in constant free fall in space ?, if not how does it stay in place (i don't mean rotation, i mean it's state of position) for example up down middle, on top of that why can't i feel the earth "move". Posted by science_newb</DIV>I would like to add something to the answers already supplied by others.  isn't earth in constant free fall in space? The correct way would be to say that the Earth is in a constant free fall not in space but relative to or towards Sun - bodies do not fall relative to space as such but always relative to other bodies to which they are gravitationally bound. In the same spirit, the ISS is in the constant free fall towards Earth, not in free fall as such, which by implication might then be taken as 'falling' relative to or in 'space' but that is wrong conclusion.    <div class="Discussion_UserSignature"> </div>

emperor_of_localgroup · Dec 14, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>  In contrast the gravitation instead acts to alter these inertial properties themselves so as to achieve the acceleration of the body in question without forcing it in any way. In short it is like the difference between hand pushing a recalcitrant stuck-in-place ox (oxen?) to make it go someplace instead of suspending a carrot on a stick and dangling it in front of its nose which it then naturally follows of its own will without any pushing being necessary.   Constructive criticism is welcomed (welcome?) Posted by vandivx</DIV>Vandivx, I always read your posts with great interest because they are different from normal textbook interpretations of physics. Most of the time I agree with your original observations and interpretations, but I'm neutral on your current post. I'm neither accepting it nor rejecting it because IMO centripetal force is not quite well understood even today.I do understand your points on this issue. You are saying mechanical rotation in a gravitational field, such as on earth, is different from planetary rotations because gravity is not a force but it changes inertial properties of objects it is acting on. This is where lies my hesitations.If you remember scientists are trying to simulate gravity in outer space by a rotating structure (may be in the shape of a drum). This tells us it doesn't matter whether the rotation is on earth (in a gravitational field) or in a gravity free space, there'll always be a centripetal force. Your post also raise a question, then what is 'force'? In my definition, anything with an 'acceleration' must have a 'force' behind it.  Our everyday experiences will validate this definition. That is why I think we can not completely drop centripetal force on planetary orbits, circular or elliptical, as long as there is a curvature in the path of the planet. May be we should call it 'centripetal gravity'.I also do not think there is any harm in equating 'ma=Gm1m2/r^2', as long as the magnitude of the two forces are equal, nature of the forces may be different. Even though you are saying the second part of the equation is 'not a force', but it has the same effects as a force, it causes acceleration.As far as your gravity changing inertial properties for object to move without a force like  moving a mule by luring it with a carrot  is concerned, I always had a difficulty in picturing a thought experiment. Let's take the earth in deep outer space void of all gravity, stop earth's rotation. Now we get an earth standing still. Now bring in the moon, which also doesn't have any rotation. Say, we are strong enough to place the moon at one point inside earth's gravitational fieldwhithout the slightest motion. Will the moon move towards the earth?? Or will the moon stay at the point where we placed it? I think GR says the moon will not move. Because  curvature of space is not a 'force'.Feel free to ridicule me. I have thick skin.   <div class="Discussion_UserSignature"> Earth is Boring </div>

emperor_of_localgroup · Dec 14, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>I believe electrons are actually deflected by magnetic fields where the trajectory is determined by the Lorentz force.  What you are referring to Columb's law where opposite charges attracts and like charges repel.  It is Columb's law that binds electrons to protons. Posted by derekmcd</DIV>Lorentz force? Jesus, it has been so long I even forgot what Lorentz force is. You can refresh my memory.  What I was referring to can be found in most college physics texts  in magnetism chapter. If an electron (any charged particle)enters a magnetic field, where electron path and direction of magnetic field make 90 degrees, the force on the electron is evB, e(charge), v(speed), B(magnetic field). This force forces (no pun) the electron to follow a circular path. This principle is also use, I think, in mass spectrogram.I have this weird thought the sun's gravity created planets orbits in similar way, and planets gravity created satellites' orbits. <div class="Discussion_UserSignature"> Earth is Boring </div>

Saiph · Dec 14, 2008

Van:  Okay, so I'm going to try and boil your post down into a short passage in order to see if I understand you...You say gravity isn't a traditional newtonian force, as it doesn't act upon an object to alter it's linear motion.  Instead it alters the properties of the body's inertia itself, so that instead of uniform linear motion, we experience 'circular' motion? I.e. the natrual unperturbed path of the body is no longer in a straight line, but a 'curved' path as seen from an outside observer?Then again, the way I've stated it isn't quite right, as we only have to look at the case of an object falling directly (and linearly) to earth for a counter example.  And you aren't claiming circular motion specifically.  So scratch that. That is certainly an interesting way of thinking about gravity.  I've always wondered about gravity itself, as it's working mechanisms act directly on space-time as opposed to the other forces.  Indeed I've pondered about the nature of inertia itself, and wondered if it's a consequence of how matter interacts with spacetime.   <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

DrRocket · Dec 14, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'> ... How so? In my books those two are certainly forces - the centrifugal force is the one-to-one equivalent of the inertial force (or the reaction force in Newton's laws) and the centripetal force is the one-to-one equivalent of the acting force F as in F=ma (or the action force as it is called in Newton's laws). These action and reaction forces are certainly forces if anything is.</DIV>Centrigugal force and inertial force are not really forces at all, but are merely the result of identifiable forces that cause accelerations.Let's take centrifugal force.  It arises when some massive object moves in an arc, and thereby changes direction.  Accleration is the time derivative of velocity, and both acceleratin and velocity are vector quantities.  So a change in the direction of velocity, even at constant speed, is a non-zero acceleration and to accomplish such and acceleration a force is required.  That is the content of the equation, for a constant mass, F = ma,  Note that the bolded quantities F and a are vectors.So let is look at the simple case of an object moving in a circle of radius R at constant speed.  Let w denote the (constant) -angular speed in radians/second.  Then the linear speed is w*R but the velocity is changing because the direction is constantly changing.  In circular coordinates, we will assume that the initial polar angle, P was 0 at time 0 so that in rectangular coordinates X = Rcos P and y = Rsin P and P(t) = w*t.    The velocity V is the derivative of the vector [X,Y]and so V = d/dt [Rcos wt, Rsin wt] = [-wR*sin(wt), wR*cos(wt)].  Note that the speed which is the magnitude of the vector V is just wR.  The acceleration a is the derivative of V , so a= [-w^2 *R*cos(wt), -w^2*R*sin(wt)] and the magnitude of a is R*w^2.  Using Newton's second law F = ma  we have F = -m(w^2)R[cos(wt),sin(wt] = -m(w^2)R[X,Y].  Note that F is a negative scalar times the position vector and hence is directed radially inward towards the center of the circle with magnitude m(w^2)R.  This is what is called "centrifugal force" but as you see is a logical consequence of the simple assumption of constant circular motion.  In the case of a planet the source is the gravitational force, in the case of a tether ball the source is a rope, and in the case of a motor cycle going around a turn the source is frictional force between the tires and the road.  Centrifugal force is a result of the kinematics, nothing more.Inertial force is also a fiction.  In fact inertia is not a force at all.  You will not find anywhere in mechanics a term in any equation labeled "inertia", only "mass".

They arise in any mechanical rotating sytem but only in it (as opposed to orbits under gravitational influence as explained below).  The terminology is not unfortunate or confusing at all if one keeps strictly to mechanical rotating systems, its purpose is simply to let us know that we talk about the special case of circular (rotating) accelerated motion as opposed to linear one. It is a motion distinct enough to warrant its own special terminology (i.e., acceleration can easily be maintained indefinitely as opposed to linear acceleration.</DIV> See above.  One can use the tereminology if one chooses, and it does perhaps  aid discussion.  But don't let the terminology obscure the fundamentals. Replying to: <DIV CLASS='Discussion_PostQuote'>~~~~~~~~~~~~~~~~~~~~~~~~~~~Any confusion arises from misapplication of these forces to gravitationally bound system but then the confusion is due to misapplication, not because the terms are not in some way not kosher. When it comes to orbits under gravitational influence as opposed to mechanical rotating sytem, these two forces - action and reaction Newtonian forces - are not present or acting and that is where some confusion may come from. The reason is that gravitation is not the classical mechanical Newtonian force. Where the mechanical force acts on the body directly in the push/pull fashion and alters its motion that way by working against and overcoming its inertial resistance, the gravitation is a force which acts directly on the body's inertial properties so to speak, altering them and thus generating the acceleration of the body without it being mechanically pushed/pulled around. That is also why it is (correctly) said that gravitation is (really) not a force, at least not in the sense of the classical Newtonian force.</DIV>Huh?  Gravity is easily formulated as a force, and that is precisely what is done in Newton's Theory of Universal Gravitation.  In fact the fundamental reason that Newton developed his theory of mechanics, his theory of gravity and calculus was to provide and explanation in terms of basic and far-reaching principles for Kepler's laws of planetary motion.  I have no idea where you got the idea that gravity is not a classical Newtonian force, but that idea is totally wrong both scientifically and historically. Replying to: <DIV CLASS='Discussion_PostQuote'>It might seem odd calling gravitational force non-Newtonian given that Newton was the one who 'discovered' it. That has to do with the fact that as he admited himself, he didn't understand how gravitation works, that is what is the machinery behind it. I call the gravitation non-Newtonian force to distinguish it from the classical Newtonian force acting in the F=ma formula. Perhaps there is a better way to make the distinction and I am open to ideas in that regard.</DIV>Gravity is as Newtonian as it gets.  Newton absolutely did understand, to a very high degree of approximation, how gravity works.  His understanding is still the basis for all modern calculation of satellite orbits as well as most orbits calulated in astrophysics.  In some highly specializeds circumstances Newtonian calculations are replaced by the more difficult more more accurate calculations of general relativity.  But Newton most assuredly did, by any reasonable definition, know HOW gravity works.What Newton did not understand is WHY gravity works the way it does.  Nobody does, Not then.  Not now.  He was originally hoping for an explanation of how gravity works similar the explanation of how a clock a works in terms of pendulums and gears.  No one has such an explanation.  That has nothing to do with the discussion as to whether gravity is a Newtonian force.  Replying to: <DIV CLASS='Discussion_PostQuote'>~~~~~~~~~~~~~~~~~~~~~~~~~ The gravitation doesn't work against the body's inertia in the sense of overcoming it but rather it alters the inertia of a body itself, so that the body moves in accelerated fashion on its own.</DIV>Actually gravity works on a bodies mass.  It is an experimental fact that the masses M and m that occur in the statement of universal gravitation F = G(Mm)/r^2 are the same masses that occur in F=ma.  Inertial mass and gravitational mass are the same, to as many decimal places as we have been able to measure. Replying to: <DIV CLASS='Discussion_PostQuote'> Centripetal (pulling) force takes no part in the Moons orbital motion (for example) simply because there is no centrifugal (pull resisting) force to begin with and vice versa - that's because gravitation is not a mechanical Newtonian force as pointed out and is really not a force at all in a strict sense. In a sense the gravitational 'force' is the very opposite of the mechanical Newtonian force - the Moon is in a freefall towards the Earth's surface which it approaches at the rate of 9.8 m/second squared and if there were no gravitation and we wanted to achieve the same result, we would have to do some hard pushing to make it move that way. Now if we were instead somehow able to adjust the Moon's inertial properties the same way that gravitation does it, then the Moon would have acceleration as its normal and natural motion without us having to push it. But then instead we would have hard time holding it still suspended in the space above the Earth. In short the Moon's natural motion with such tweaked inertial properties would then want to accelerate all the time in some direction (of course the inertia of the Moon could be put into imbalance only in a certain direction at a time in respect to its body) as opposed to the usual way of staying put, moving with whatever velocity it had, if there were no other bodies in close proximity to influence it gravitationally.</DIV>That is a rather confused and confusing explanation.  If you go back to my derivation of the acceleration and force required for simple circular motion then the force necessary to provide the necessary acceleratin is simply that of the gravity of the Earth.  And it is most certainly a Newtonian force.The Moon does experience an acceleration towards the center of motion, which is the center of the Earth.  But that acceleration is NOT 9.8 m/s^2 because the Moon is sufficiently far from the Earth that calculation of the gravitational acceleration is materially affected by the distance-squared term in the denominator of Newton's law of universal gravitation.  It is however, completely consistent with the w^2*R term in my derivation above.   Replying to: <DIV CLASS='Discussion_PostQuote'> ~~~~~~~~~~~~~~~~~~~~~~~~~~That's how gravitation works and that's why it behaves differently from the classical Newtonian forces (F=ma) and why the forces of action and reaction (or centri-petal/fugal when it comes to rotation) don't apply when it is the gravitation that acts.</DIV>No that is not how gravitation works in classical Newtonian theory.  It works precisely according the mathematics that I showed you. Replying to: <DIV CLASS='Discussion_PostQuote'> That is also the reason why in the GR the gravitation is not a force. In the classical Newtonian context also, the gravitation is not a force (should not be taken as such) if one wants to be strict with terminology employed, given that one is using today's insight into gravitation.</DIV>What is going on in general relativity is more subtle than what you seem to realize.In Newtonian mechanics an object in motion tends to stay in uniform linear motion unless acted upon by a force.  That is in fact why it takes an acceleration to cause a body to move in a circle.  I showed how to derive the necessary force, using Newtonian mechanics, in the case of uniform circular motion.It is important to recognize that Newtonian mechanics assumes that everything is happening against the backdrop of an ordinary Euclidean space.  In Euclidean space the shortest distance between two points is a straight line.  The geodesics of Euclidean space are straight lines.General relativity replaces the backdrop of Euclidean space with a non-Euclidean curved version of space -- a Lorentzian 4-manifold called space-time.  Mass causes curvature of space-time, and curvature results in geodesics that are not ordinary straight lines.Newton's law that objects follow geodesics unless acted upon by a force continues to hold in general relativity.  But those geodesics are no longer ordinary straight lines, but are affected by the curvature of space-time that results from the pressence of massive objects.  What we perceive as gravity in general relativity is simply bodies following geodesics in space-time (not just in space).  In that sense gravity is no longer a force in general relativity, but is rather a phenomena that results from motion in a curved space-time in the absence of force.One ought be very careful to clearly state whether one is working with Newtonian mechanics or general relativity and not confuse the two.  Either, in almost all (but not the most extreme) situations can produce extremely accurate predictions.  But if you mix the two inappropriately you get gibberish.Replying to: <DIV CLASS='Discussion_PostQuote'>Of course, in Newton's time there were no such insights and one equated ma=G[(mm)/r*r] without scruples. The danger is that confusion can develop with such usage which mixes the two kinds of forces freely. As explained, the F=ma force achieves acceleration by acting against the body's inertial properties - that is it seeks to overcome them. In contrast the gravitation instead acts to alter these inertial properties themselves so as to achieve the acceleration of the body in question without forcing it in any way. In short it is like the difference between hand pushing a recalcitrant stuck-in-place ox (oxen?) to make it go someplace instead of suspending a carrot on a stick and dangling it in front of its nose which it then naturally follows of its own will without any pushing being necessary. Alas we can only wish we were able to influence a body's inertia the same way we can influence the ox's because then all our problems would be solved. Like Sartre or some other philosopher said, quoting from memory of a quote - if a man has ten cows, all his problems are solved.   Constructive criticism is welcomed (welcome?) Posted by vandivx</DIV>You have more or less stated the case for general relativity, although it does not alter inertial properties of masses, but rather provides an altered view of space and time.  But you are trying to mix the Newtonian perspective and Einstein's relativistic perspective inappropriately.  The result is confusion.Newton's mechanics works extremely well in almost all common situations.  Einstein's general relativity works more broadly and more accurately.  But you should not and cannot accurately combime the perspectives of the two in the manner that you are doing.  They are distinct mathematical models, one simpler and accurate in almost all cases, the other more complex. more accurate, and more broadly applicable.  <div class="Discussion_UserSignature"> </div>

origin · Dec 14, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>The Moon does experience an acceleration towards the center of motion, which is the center of the Earth.    Posted by DrRocket</DIV> Good explanation Dr. Rocket - not that it will do any good, though.One nit pick - I believe the moon acclerates towards the center of gravity between the earth and the moon which I recall is inside the earth but not the center of the earth.   <div class="Discussion_UserSignature"> </div>

DrRocket · Dec 14, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Good explanation Dr. Rocket - not that it will do any good, though.One nit pick - I believe the moon acclerates towards the center of gravity between the earth and the moon which I recall is inside the earth but not the center of the earth.   Posted by origin</DIV>The acceleration is towards the center of the earth which is a result of Newton's Law of Universal Gravitation.  But the center of the Earth is in line with the center of mass of the two-body system  and the center of mass of the Moon, so "towards the center of the Earth" and "towards the center of mass of the system" are the same direction.  The actual orbit is centered at the center of mass, and both the Earth and the moon move about it.  For purposes of the calculation the usual approximation is made that the satellite is much less massive than the body around which it revolves, and this is very accurate for ordinary satellites.  With a body as large as the moon the orbital calculation is a bit more involved, and the calculation is not particularly illuminating from the perspective of how the force of gravity works and where "centrifugal force" originates.You are correct that the center of motion for the orbit of the Moon  and center of mass of the Earth are different points, but the distinction does not materially affect the physics that I was trying to illuminate. <div class="Discussion_UserSignature"> </div>

vandivx · Dec 21, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Van:  Okay, so I'm going to try and boil your post down into a short passage in order to see if I understand you...You say gravity isn't a traditional newtonian force, as it doesn't act upon an object to alter it's linear motion.  Instead it alters the properties of the body's inertia itself, so that instead of uniform linear motion, we experience 'circular' motion? I.e. the natrual unperturbed path of the body is no longer in a straight line, but a 'curved' path as seen from an outside observer?Then again, the way I've stated it isn't quite right, as we only have to look at the case of an object falling directly (and linearly) to earth for a counter example.  And you aren't claiming circular motion specifically.  So scratch that. That is certainly an interesting way of thinking about gravity.  I've always wondered about gravity itself, as it's working mechanisms act directly on space-time as opposed to the other forces.  Indeed I've pondered about the nature of inertia itself, and wondered if it's a consequence of how matter interacts with spacetime.   Posted by Saiph</DIV> Sorry for being silent so long, I flew to Europe for Christmas and also got influenza or something...  Your interpretation of my meaning is uneccessarily too involved and in a way it opens my eyes to difficulties people may have with my ideas and which I didn't forsee. That includes emperor_of_localgroup here. The mistake I made is giving as example the Earth orbiting body (the Moon) which causes confusion here. Also, forget curved space and even better let us suppose we are living in pre-Einsteinian time, so no curved space, no relativity theories, just plain Newtonian forces, mechanical and gravitational.  The idea was as follows: a weight of iron puts up an inertial resistance to being pushed by my hand along a linear path on a laboratory table top (idealized conditions - no friction etc as is usual in physics) . The force I exert with my hand to accelerate the body has to overcome the body's inertial force which acts against my pushing hand - the body puts up inertial resistance to being accelerated. Next I take the body in my hand and drop it from an initial rest state (relative to Earth) to the ground. As soon as I release it, the gravitation causes it to accelerate from rest (along a rectilinear path of course) untill it hits the ground. What I was saying was that the body which now accelerates while falling (straight) down does not do so because some external force is pulling it down or acting on it (like the force of gravitation) but rather the proximity of the large body of the Earth altered our body's inertial property in such a way that now it likes to accelerate on its own without being pushed or pulled. Its like the body's inertia is skewed by the proximity of the large body so that now it wants to keep changing its velocity where previously it tended to stick with whatever velocity it had. Newtonian gravitation works on the very heart of inertia of bodies - it is not overcoming the inertial resistance of bodies from 'outside' as we do when we push them around but it modifies (tweaks) the inertia itself from within the body so that the body then moves in accelerated fashion purely on its own. All that the big mass of the Earth does is the tweaking - the alteration of the 'balance' of body's inertia so to speak. In that funny analogy, it is like if an ox (or a mule or lama) wouldn't budge and stubornly refuse walking and if you wanted him to go anywhere, you'd have to push him there with his hoofs skidding on the ground untill you got the idea with that carrot which once suspended in front of his nose he would follow it walking after it without any other bidding. The Earth is dangling carrots so to speak in front of all bodies in its proximity, so that they all move towards it without bidding, that is without anything having to overcome their stubborn inertial resistance to have their motion changed. And when these bodies come into contact with the Earth's surface and rest on it, they still keep pushing against it as they try to reach those imaginary carrots...  The Moon's orbital motion and its constant free fall towards the Earth only provides more complication but changes nothing in principle. Also those centrifugal and centripetal forces are only present when we consider the 'external', that is mechanical pull on bodies when they are being rotated (suspended on a rope or when they are part of spinning solid body of which its parts can be viewed as rotating around its center). Such rotational motion is of course just a special case of linearly accelerated motion. That is, if we tied a body on a rope and pulled is along a straight path, we could call the force we exerted on the body via the rope the centripetal force and the inertial resistive force of the body could be called the centrifugal force. Its just that we reserve these terms only for cases of rotation but that's what those forces really are - the force we exert in push or pull is the 'centripetal' force and the force with which the body resists us on account of its inertia is the 'centrifugal' one. The point is though that where gravitation acts, those forces are not present because no forces are present. That's why the mechanical force exemplified by the F=ma shouldn't be equated with the gravitational force as a general principle. While it is true that it will hold 'numerically' as far as plain force accounting goes, it will cause only confusion when it comes to more involved case of rotational motion since the mechanical force implies those centri fugal/petal forces while no such forces are present where gravitation acts. But as far as those 'centri' forces themselves go, they are perfectly valid forces, same as acting and inertial forces are (of which they are just different names as I pointed out above). I must say I am encouraged that somebody doesn't find it complete waste of time reading what I post. Replying to: <DIV CLASS='Discussion_PostQuote'> I've always wondered about gravity itself, as it's working mechanisms act directly on space-time as opposed to the other forces.  Indeed I've pondered about the nature of inertia itself, and wondered if it's a consequence of how matter interacts with spacetime.   Posted by Saiph</DIV> Gravitation and inertia is my chief interest in physics and my interest is purely physical understanding of it as opposed to mathematical treatment of it. Of course inertia of matter could be said to be one side of a coin of which the other face is gravity of matter. Those two are intimately connected as is evident from the principle of equivalence of gravitational and inertial mass. IMO Einstein's major failing was that the phenomenon of inertia completely eluded him and full understanding of gravitation is not possible without understanding inertia. Of course what I posted above is not an explanation of gravitation and IMHO it really only provides somewhat different angle on what the textbooks say (except they keep silent on inertia). I could tell more but with gravitation one is always in a position that Newton also found himself in - when people came back to him saying that his theory didn't really explain how this mysterious force at a distance called gravitation works... <div class="Discussion_UserSignature"> </div>

DrRocket · Dec 22, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'> Sorry for being silent so long, I flew to Europe for Christmas and also got influenza or something...  Your interpretation of my meaning is uneccessarily too involved and in a way it opens my eyes to difficulties people may have with my ideas and which I didn't forsee. That includes emperor_of_localgroup here. The mistake I made is giving as example the Earth orbiting body (the Moon) which causes confusion here. Also, forget curved space and even better let us suppose we are living in pre-Einsteinian time, so no curved space, no relativity theories, just plain Newtonian forces, mechanical and gravitational.  The idea was as follows: a weight of iron puts up an inertial resistance to being pushed by my hand along a linear path on a laboratory table top (idealized conditions - no friction etc as is usual in physics) . The force I exert with my hand to accelerate the body has to overcome the body's inertial force which acts against my pushing hand - the body puts up inertial resistance to being accelerated. Next I take the body in my hand and drop it from an initial rest state (relative to Earth) to the ground. As soon as I release it, the gravitation causes it to accelerate from rest (along a rectilinear path of course) untill it hits the ground. What I was saying was that the body which now accelerates while falling (straight) down does not do so because some external force is pulling it down or acting on it (like the force of gravitation) but rather the proximity of the large body of the Earth altered our body's inertial property in such a way that now it likes to accelerate on its own without being pushed or pulled. Its like the body's inertia is skewed by the proximity of the large body so that now it wants to keep changing its velocity where previously it tended to stick with whatever velocity it had. Newtonian gravitation works on the very heart of inertia of bodies - it is not overcoming the inertial resistance of bodies from 'outside' as we do when we push them around but it modifies (tweaks) the inertia itself from within the body so that the body then moves in accelerated fashion purely on its own. All that the big mass of the Earth does is the tweaking - the alteration of the 'balance' of body's inertia so to speak. In that funny analogy, it is like if an ox (or a mule or lama) wouldn't budge and stubornly refuse walking and if you wanted him to go anywhere, you'd have to push him there with his hoofs skidding on the ground untill you got the idea with that carrot which once suspended in front of his nose he would follow it walking after it without any other bidding. The Earth is dangling carrots so to speak in front of all bodies in its proximity, so that they all move towards it without bidding, that is without anything having to overcome their stubborn inertial resistance to have their motion changed. And when these bodies come into contact with the Earth's surface and rest on it, they still keep pushing against it as they try to reach those imaginary carrots...  The Moon's orbital motion and its constant free fall towards the Earth only provides more complication but changes nothing in principle. Also those centrifugal and centripetal forces are only present when we consider the 'external', that is mechanical pull on bodies when they are being rotated (suspended on a rope or when they are part of spinning solid body of which its parts can be viewed as rotating around its center). Such rotational motion is of course just a special case of linearly accelerated motion. That is, if we tied a body on a rope and pulled is along a straight path, we could call the force we exerted on the body via the rope the centripetal force and the inertial resistive force of the body could be called the centrifugal force. Its just that we reserve these terms only for cases of rotation but that's what those forces really are - the force we exert in push or pull is the 'centripetal' force and the force with which the body resists us on account of its inertia is the 'centrifugal' one. The point is though that where gravitation acts, those forces are not present because no forces are present. That's why the mechanical force exemplified by the F=ma shouldn't be equated with the gravitational force as a general principle. While it is true that it will hold 'numerically' as far as plain force accounting goes, it will cause only confusion when it comes to more involved case of rotational motion since the mechanical force implies those centri fugal/petal forces while no such forces are present where gravitation acts. But as far as those 'centri' forces themselves go, they are perfectly valid forces, same as acting and inertial forces are (of which they are just different names as I pointed out above). I must say I am encouraged that somebody doesn't find it complete waste of time reading what I post.  Gravitation and inertia is my chief interest in physics and my interest is purely physical understanding of it as opposed to mathematical treatment of it. Of course inertia of matter could be said to be one side of a coin of which the other face is gravity of matter. Those two are intimately connected as is evident from the principle of equivalence of gravitational and inertial mass. IMO Einstein's major failing was that the phenomenon of inertia completely eluded him and full understanding of gravitation is not possible without understanding inertia. Of course what I posted above is not an explanation of gravitation and IMHO it really only provides somewhat different angle on what the textbooks say (except they keep silent on inertia). I could tell more but with gravitation one is always in a position that Newton also found himself in - when people came back to him saying that his theory didn't really explain how this mysterious force at a distance called gravitation works... Posted by vandivx</DIV>This simply defies classification.  It is not physics, as it flies in the face of known physics.  It is not speculation, as there is no attempt to be quantitiative  or to suggest any means whereby a real theory might be developed.  It is not philosophy, as there are no overarching principles.  It is not history, as it contradicts the known history of scientific discovery and badly misinterprets both Newton and Eiinstein.  It certainly is not mathematics, as there is no logic.  Is it perhaps a verbal hallucination, as it is long, disjointed and devoid of connection with reality ?   <div class="Discussion_UserSignature"> </div>

DrRocket · Dec 22, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>...  Indeed I've pondered about the nature of inertia itself, and wondered if it's a consequence of how matter interacts with spacetime.   Posted by Saiph</DIV>I don't think anyone really understands this at the level that you suggest, but you might want to take a look at the Higgs mechanism.  I think that is about as close as you are likely to get to a mechanistic explanation of inertia, and it remains to be proved.  http://en.wikipedia.org/wiki/Higgs_mechanism <div class="Discussion_UserSignature"> </div>

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how does earth stay "in place" in space?

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