# Absolute Motion

## 42 posts in this topic

I was unsure of what term exactly to use because other people get so hung up on "wait, centrifugal force, is it that that doesn't exist, or is it centripetal force, or centrifugal acceleration..."

So I just said centripetal acceleration because that is what we always use in our college physics courses.

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There is a gravitational effect of the Sun (and the Moon) exerting a torque on the equatorial bulge of the Earth. This effect is the primary cause of the precession of the equinox.

So, to see if I understand you, this "torque" energy is a force that operates in an opposite direction from the pull of gravity, as a result of the planet being confined to its orbit, rather than continuing off indefinitely on a straight path into space, like the weights in Mr. Jordan's example?

Not quite. Most people think that the Earth's axis of rotation has some fixed direction in space, but in fact it is slowly precessing. The axis traces out a cone in space that takes almost 26,000 years to complete. Right now we call the star Polaris the "North Star," but in half the period of precession, some 13,000 years, the star Vega will become our "North Star." (If you are willing to wait another 13,000 years then Polaris will again have the honor.)

This precession can be understood (with simplifications) as a consequence of a torque exerted on the Earth's equatorial bulge by gravitational effects of the Sun and the Moon. A "torque" is not a force, but rather a force acting through a distance. (Sometimes this is called a "moment" or a "couple.") If you have forces acting on different portions of an object they can result in a net torque causing the object to rotate. In the case I highlighted the gravitational potential of the Sun and the Moon is greater on the nearer part of the equatorial bulge than on the part further away. These small differences in forces cause a net torque that is responsible for the precession.

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If a man is holding weights and spinning, the weights are enclasped within his hands, and so force is exerted inwardly, from the hand upon the far side of the weight. If the weights were made of more fragile material, like putty, then if the person were spinning fast enough, the putty would mash in on itself and maybe be left with an imprint of the hand.

But in the case of a weight that is not enclasped, such as a ball on a string, the force is a pulling, rather than pressing force, exerted on the near side of the ball against the string. On the far side of the ball, there is no force pressing inward upon the ball. If the ball were made of putty, then if the ball were spinning fast enough, it would stretch out-- the opposite result of the putty that is enclasped.

So, does gravity enclasp a planet, or is it more like a ball on a string? If the earth were orbiting the sun much faster, would it implode like a broken light-bulb and continue its orbit, or would it stretch and rip in half, with one part spiraling into the sun and the other part being flung out into deep space?

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Sorry if I am engaging in "Physics of Emergencies," but I can't think of any other way to pose the question I'm after!

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Don't these two say the same thing? (I'd say "force" instead of "acceleration" in Carlos's version, but otherwise...)

I am sure Stephen will correct me if I am wrong, but I believe he was responding to Bold Standard's summary that it was due to "centripetal force," which of course is the force inwards that keeps the particles in the Earth from continuing in their linear motion. Their inertia "resists" the centripetal force and causes the bulge.

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So, does gravity enclasp a planet, or is it more like a ball on a string?  If the earth were orbiting the sun much faster, would it implode like a broken light-bulb and continue its orbit, or would it stretch and rip in half, with one part spiraling into the sun and the other part being flung out into deep space?

I should have said: If the Earth were orbiting much faster, and gravity increased proportionately, such that the Earth could not simply spin itself out of orbit... Also, if the earth were less dense, say if it were a giant egg. That might make my question clearer.

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If a man is holding weights and spinning, the weights are enclasped within his hands, and so force is exerted inwardly, from the hand upon the far side of the weight.  If the weights were made of more fragile material, like putty, then if the person were spinning fast enough, the putty would mash in on itself and maybe be left with an imprint of the hand.

But in the case of a weight that is not enclasped, such as a ball on a string, the force is a pulling, rather than pressing force, exerted on the near side of the ball against the string.  On the far side of the ball, there is no force pressing inward upon the ball.  If the ball were made of putty, then if the ball were spinning fast enough, it would stretch out-- the opposite result of the putty that is enclasped.

So, does gravity enclasp a planet, or is it more like a ball on a string?  If the earth were orbiting the sun much faster, would it implode like a broken light-bulb and continue its orbit, or would it stretch and rip in half, with one part spiraling into the sun and the other part being flung out into deep space?

Not that in either case the force is directed inward, pulling or pushing the weight toward the center. In no case is there ever any force pushing the weight outward from the center (or in any other direction but inwards). This doesn't answer your question, but I think it's what really matters.

To answer your question, I'd say it's like the ball on the string. For example, there's nothing "holding in" the matter on the perimiter of the earth. Gravity comes from the more interior matter "pulling" on that further out.

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To answer your question, I'd say it's like the ball on the string. For example, there's nothing "holding in" the matter on the perimiter of the earth. Gravity comes from the more interior matter "pulling" on that further out.

Oops, that's not a direct answer, though the principle is the same. The earth revolving around the sun is also like the ball on a string, again because gravity (here from the sun) "pulls" and there's nothing beyond the earth's orbital path "pushing."

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Not that in either case the force is directed inward, pulling or pushing the weight toward the center. In no case is there ever any force pushing the weight outward from the center (or in any other direction but inwards).

Hmm.. That seems to contradict what I've perceived. In the case of an object on a string, it would appear that the forces are pushing outward from the center, on the near side and the far side. I would assume that such a scenario would result in forces pushing inward from the adjacent sides toward the center, as a consequential effect of the object being stretched.

But in this situation, there would be two forces acting on the object. The transfer of weight would be distributed to the distant end of the object. If it were a hollow ball filled with beads, the beads would be pressed upon the far wall, rather than hovering in the center of the ball. I assume weight would be shifting from the near side towards the center, but then it would continue past the center to the far side.

The second force would be the grip of the string, pulling the near side of the object away from its center-- or at least, not allowing it to proceed in the direction of the object's momentum. I'm not sure if the force of the string upon the object is accurately described as a distribution of weight towards the path of the string, but at any rate, if my object is made of putty, as I have observed, it will stretch toward the path of the string on one side, and toward the path of its momentum on the opposite side, and eventually tear in half. That would seem to be the result of the opposite of forces pushing only towards the object's center.

The reason I'm posing my question this way is that, although I've never taken a course in or formally studied physics, in my imagination I conceive gravity as being an interaction between the molecules on the far side of the object being orbited (the sun) and the molecules on the near side of the orbiting object (the Earth), almost like an invisible rope in space, although the Earth is left free to rotate. But the thought occurred to me that gravity might be completely otherwise, such as a force that encircles an entire planet, keeping it encplasped. That would seem, by analogy to my putty on the string vs putty enclasped, to be examples of two opposite kinds of forces-- one in which weight is distributed away from the center of the Earth, and one in which it is distributed towards the center.

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Hmm..  That seems to contradict what I've perceived.  In the case of an object on a string, it would appear that the forces are pushing outward from the center, on the near side and the far side.  I would assume that such a scenario would result in forces pushing inward from the adjacent sides toward the center, as a consequential effect of the object being stretched.

Although it appears that way, in fact there is no force pushing outwards. Consider a completely linear example: you in a car rolling on a level surface. Now there are no forces pushing you forward, because you are coasting on your own momentum. But if you hit a tree, you will feel thrown forward against the seatbelt. This is not because all of a sudden there is a force pushing you forward, but because all of a sudden there is a backward force stopping the car and its seat belts, but not you. In other words, anything that counteracts your inertia gets felt by humans as a force pushing you in the direction you had been going, but that is a misperception.

In the case of traveling in a circle, inertia would keep you going straight--on a tangent away from the circle's path. But if you are going in a circle, some force must be making you turn to continue in a circular path and not the straight line you would otherwise travel in. This inwards centripetal force is what the string provides. There is no outward centrifugal force (in the sense physicists use "force"); there is only a tendency to continue in a straight line.

In your example of a ball on a string, the material connected to the string (the "near particles") will be subject to an inwards force, but the material opposite is not, strictly speaking, subjected to an outward force. It continues in a straight line, as best it can, while the inside gets "yanked" inwards.

The reason I'm posing my question this way is that, although I've never taken a course in or formally studied physics, in my imagination I conceive gravity as being an interaction between the molecules on the far side of the object being orbited (the sun) and the molecules on the near side of the orbiting object (the Earth), almost like an invisible rope in space, although the Earth is left free to rotate.  But the thought occurred to me that gravity might be completely otherwise, such as a force that encircles an entire planet, keeping it encplasped.  That would seem, by analogy to my putty on the string vs putty enclasped, to be examples of two opposite kinds of forces--  one in which weight is distributed away from the center of the Earth, and one in which it is distributed towards the center.

Neither of your scenarios is accurate. All particles have the same force simultaneously exerted on them, even those at the center of the Earth. (The difference in force exerted on the near particles and the far particles of the Earth is effectively zero.) Think of it this way instead: every particle in the Earth has a string connecting it individually to the sun, and the sum total of all those strings is the force of gravity from the sun. I hope it's clear that this scenario is neither like something "enclasping" the earth and pushing on the far particles, nor like something pulling only on the near particles.

Was that useful?

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Neither of your scenarios is accurate.  All particles have the same force simultaneously exerted on them, even those at the center of the Earth. (The difference in force exerted on the near particles and the far particles of the Earth is effectively zero.) Think of it this way instead: every particle in the Earth has a string connecting it

I have not been following this thread for a couple of days, but I did catch this. If I am misunderstanding the context, please correct me. As I understand the point made above, this is not correct. In fact, as I highlighted in an earlier post, it is tidal forces -- differential values in gravitational potential -- acting on the equatorial bulge that accounts for the torque responsible for the precession of the equinox. The gravitational potential of the Sun and the Moon is greater on the nearer part of the equatorial bulge than on the part further away.

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I have not been following this thread for a couple of days, but I did catch this. If I am misunderstanding the context, please correct me.

Well, you're a better judge of the context in this kind of discussion, so I will supply it and let you decide. I was responding primarily to Bold Standard's description of the two alternatives for modelling gravity's means of keeping the planet in orbit:

That would seem, by analogy to my putty on the string vs putty enclasped, to be examples of two opposite kinds of forces--  one in which weight is distributed away from the center of the Earth, and one in which it is distributed towards the center.

My intent was to demonstrate that both descriptions are wrong: Earth is not attached by a string to its near point that "pulls" that one point to the sun, thus stretching the near side, nor is it enveloped by a force that "pushes" it from the far side toward the sun, thus compressing the far side. (Have I summarized your two alternatives correctly, Bold Standard?)

That is why I suggested a better model than either of those two is a separate string connected from the sun to every "particle" in the Earth that pulls toward the sun (technically, each particle in the Earth would be connected to each particle in the sun, the Earth, and every other celestial body, but I don't think that matters for "first-order" understanding.) I grant that is not an advanced picture, but Bold Standard said he had never formally studied physics, so I was trying to continue in the spirit of his examples.

As I understand the point made above, this is not correct. In fact, as I highlighted in an earlier post, it is tidal forces -- differential values in gravitational potential -- acting on the equatorial bulge that accounts for the torque responsible for the precession of the equinox. The gravitational potential of the Sun and the Moon is greater on the nearer part of the equatorial bulge than on the part further away.

I concede this point since I'm out of my league. Perhaps you can't really treat the difference in gravitational force on the near side vs. the far side as effectively zero.

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Well, you're a better judge of the context in this kind of discussion, so I will supply it and let you decide.

The problem is there exist many gravitational theories, and unless you know the exact mechanism by which gravity actually works -- or, at least, have good scientific reasons for preferring one over another -- then these "explanations" are pure speculation. The idea of an attractive force is just the most common placeholder for the classical view of gravity, but many other alternatives were considered and developed after Newton. There exist all sorts of push-pull theories; look up, for instance, the LeSage theory where massive objects simply block the more uniform effects of a pervasive medium.

My own view is tied to Little's Theory of Elementary Waves, in which there is a complicated interaction of particles and waves which account for the curvature proposed in the spacetime of general relativity. But it does so as a particle-based theory through the interactions with graviton particles.

I Perhaps you can't really treat the difference in gravitational force on the near side vs. the far side as effectively zero.

That depends upon your purpose. As I already described, the difference in gravitational potential between the near and far parts of the equatorial bulge is sufficient to cause the axis of rotation to form a cone-shaped movement in space. Clearly the Earth cannot ignore this difference in potential. However, for most lab table experiments on Earth these differences are so minor as to be safely ignored. However, there are many types of experiments that must take cognizance of this effect, even small-sized experiments whose precision is such that they cannot be overlooked.

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Thanks a lot for all of the replies. I will have to put some thought into the idea of each molecule being individually attracted to the sun, rather than the whole Earth as a mass-- although I understand it is based on speculation and "only a theory."

The only remaining question I have is: if momentum is not properly referred to as a "force," what is it?

Now there are no forces pushing you forward, because you are coasting on your own momentum.

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I will have to put some thought into the idea of each molecule being individually attracted to the sun, rather than the whole Earth as a mass-- although I understand it is based on speculation and "only a theory."

No. It is worse than that. It is a myth. There is no mysterious attractive force reaching out between the Sun and the Earth. It is a lie told to school children, then repeated generation after generation. Might as well say that it is the handiwork of God.

The only remaining question I have is:  if momentum is not properly referred to as a "force," what is it?

In classical physics, simply put, momentum is mass times velocity, whereas force is mass times acceleration. These terms have a more complex meaning in modern physics.

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In classical physics, simply put, momentum is mass times velocity, whereas force is mass times acceleration. These terms have a more complex meaning in modern physics.

Bold Standard, another way of seeing the difference between momentum and force is that a force causes a change in momentum with respect to time.

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The only remaining question I have is:  if momentum is not properly referred to as a "force," what is it?

Momentum and force are indeed two different physical quantities; they are not measured in the same units. (As, for instance, mass and length are different physical quantities.)

I think of momentum as being a kind of "quantity of motion". Since it's the product of mass and velocity, one can see that if a body has either twice as much mass or twice as much velocity as another body, it will therefore have twice as much momentum. So momentum is a property of mass in motion; it's a way to quantify the motion.

Force, on the other hand, is easy to concretize as being what we exert when we stretch a spring. Or push a car to get it started. A net force that's applied over time to a body will cause the body's momentum to change (as Ed from OC pointed out). Push twice as hard, and the momentum will change twice as fast.