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Author Topic: Does gravity pull or push?  (Read 32294 times)

Offline percepts

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Does gravity pull or push?
« on: 23/02/2014 22:12:19 »
If I jump off the ground gravity causes me to fall back to the ground. But is it pulling me down from below or pushing me down from above?
« Last Edit: 08/03/2014 15:02:26 by chris »


 

Offline evan_au

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Re: does gravity pull or push
« Reply #1 on: 24/02/2014 08:07:29 »
There is a gravitational attraction between every atom in your body and every atom in the Earth.

For practical purposes, this can be approximated by an attraction between the center of gravity of the Earth (which is near the center of the Earth), and your center of gravity (which is usually somewhere near your belly button).

So I would suggest that "The Earth is pulling you down" is the better description.

PS: Every action has an opposite and equal reaction (as described by Newton), so you could also say that "You are pulling the Earth up" - but the distance that the Earth moves is invisibly small compared to your very visible fall.
 

Offline Colmik

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Re: does gravity pull or push
« Reply #2 on: 02/03/2014 19:48:22 »
Disclaimer - I'm not a physicist or a scientist.  I'm a Nuffin.

Einsteinian laws of motion add only a very small correction to the Newtonian view of how bodies move - for example, all orbital paths of satellites and deep space missions are calculated using Newton's laws.  I have read (or heard on the TV, perhaps) that Newton was unhappy himself with the idea that "gravity sucks".  He said that every body maintains its state of rest or of motion  unless acted upon by a force - and Newton envisaged that force as a pushing force.  We had to wait for Einstein to put a different slant on it.  To Einstein, the structure of space-time is warped in the vicinity of a massive object, and the more massive it is, the more space-time is warped.  By massive, I mean "having mass", not "big".  A balloon may be huge, but not very massive.

Heat moves from a hotter environment to a cooler one - it moves to the position of less potential.  Charge does too - it moves from a higher to a lower potential (voltage), and my understanding is that objects move from a position of greater to lesser gravitational potential.  I know that the following example uses gravity to explain gravity, but if you imagine the forces acting on a ball on an inclined plane, the ball rolls down because its centre of gravity overhangs the point at which the ball touches the inclined plane, and when the force applied to the ball by the plane is resolved into horizontal and vertical vectors, you see that there is a horizontal force applied to the ball (as well as most of its weight being opposed by the force that prevents it from falling through the plane).  It's difficult to explain this without a drawing, but I hope you see what I mean.  Thus, the ball does not roll down the plane because it is being attracted to the bottom of it, but because it is being pushed down the plane.  It's hard to envisage how a gravitational potential difference can push massive bodies together, but for better or worse, that is how I understand it. 

We are all familiar with the explanation of gravity in which a cannon ball is placed on a trampoline, causing a dent. If a marble is now placed in its vicinity, it rolls towards the cannon ball, or if given an orbital velocity, will orbit the cannon ball.  Unfortunately, that example also explains gravity by assuming gravity.  If there wasn't any, the cannon ball wouldn't make any dent in the trampoline, and the model wouldn't demonstrate anything.  The marble, of course, finds itself on an inclined plane, so this model uses exactly the same maths as the ball on the inclined plane does.

When a hydrogen-filled balloon rises, it does not do so because "hydrogen is lighter than air", it does so because atmospheric pressure is greater on the bottom of the balloon than on the top of it, and because the hydrogen inside of it does not push down on the bottom of the balloon as hard as atmospheric pressure is pushing up, the balloon goes up.  Rockets move because there is a continuous explosion of gasses going on inside them, which pushes out in all directions - but while it is pushing on the top of the rocket (the pointy end), there's a hole at the bottom of the rocket, so there's nothing to push on there, so the rocket moves in the direction of the pointy end where the force is.  All forces push, so it seems probable that gravity does too.
 

Offline RD

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Re: does gravity pull or push
« Reply #3 on: 02/03/2014 20:07:25 »
... All forces push ...

Don't you have magnet's on your fridge door ?, (even if you don't you probably have magnets pulling the door closed).
« Last Edit: 02/03/2014 20:11:23 by RD »
 

Offline yor_on

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Re: does gravity pull or push
« Reply #4 on: 05/03/2014 19:11:44 »
Every time you're free falling you're more or less following a geodesic path. the universe you live in move you through three room dimensions and one time dimension. Being at rest with Earth you're still 'moving' in time. The geodesic is not pushing you, and I'm not sure you can speak about it as a pull either? Both words assume a force acting upon you, don't they? A geodesic is where no forces are acting on you, and that one you can test by using a scale (accelerometer). In a geodesic there should be no 'forces' acting upon you. While being 'at rest' with Earth though, you're accelerating constantly and uniformly at one approximate gravity through time :). And that one you also can test with any scale.
 

Offline Colmik

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Re: Does gravity pull or push?
« Reply #5 on: 08/04/2014 23:28:28 »
Hi yor-on, you said " The geodesic is not pushing you, and I'm not sure you can speak about it as a pull either? Both words assume a force acting upon you, don't they? [...] In a geodesic there should be no 'forces' acting upon you."  Well, ignoring the almost infinitesimal corrections by Einstein to Newtonian mechanics, we can say that Newton still holds sway.  He said that any body will remain in its state of rest or linear motion unless acted upon by a force.  While you're standing in that room, try jumping, and see if some sort of force doesn't slow your upward velocity, reverse it, and bring you back to Earth before you hit the ceiling.  The question I was addressing was whether gravity is a pulling or a pushing force.  Newton explained it as a pulling force, but he was uncomfortable with that because he envisaged all forces as pushing forces.  He had not had the insight that Einstein brought us.  In my understanding of Einstein's model of gravity as an entity of space-time and mass, it is possible for gravity to push objects possessing mass towards each other.
 

Offline JP

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Re: Does gravity pull or push?
« Reply #6 on: 09/04/2014 01:07:44 »
This is presenting a false choice.  The truth is that gravity (in general relativity) is modeled geometrically and that different objects can do a lot more than a simple "push" or "pull" might indicate.  However, you can say that it extended gravity beyond only an inverse-square "pull force," which it is in Newton's model.
 

Offline zunimtn

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Re: Does gravity pull or push?
« Reply #7 on: 12/04/2014 13:24:45 »
I'm just a physics enthusiast, but I understand that gravity is neither push nor pull.  To be a push, gravity would seem to have to be due to something "away" from the planet so it could "push" you toward it.   And in freefall, relaxing in the gravitational field of earth, you feel no force.  I understand gravity to be the gradient of space and time itself.  That mass causes space and time to be directional toward the (and all) mass.  Not a force but a directional gradient.  You only feel force when you resist it.  The weight you feel standing on the earth is because its surface is not allowing you to move with the gradient all the way to the center.   
 

Offline JP

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Re: Does gravity pull or push?
« Reply #8 on: 12/04/2014 14:14:07 »
You have to be very careful about "gravity is not a force," because of course it is a force.  But we have an explanation for how it works that is different from the other fundamental forces.  They are carried by tiny particles that exchange momentum, while gravity is explained as curvature of space-time.  It is likely that gravity is also carried by particles, but we don't have a working theory for that yet.
 

Offline yor_on

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Re: Does gravity pull or push?
« Reply #9 on: 13/04/2014 03:10:54 »
Yes, 'force' is tricky to define. And we have inertia that now presumably is explained by the Higgs particle / field. Then there is gravity as a result of the SpaceTime geometry (in GR) http://www.einstein-online.info/elementary/generalRT/GeomGravity

Assuming both to be correct we now spit inertia from gravity, although that should be a oversimplification as one is defined in the quantum regime, the other macroscopically and geometrically.

and when it comes to earths gravity I think it might be correct to state it as exerting a force on you, accelerating at one constant uniform gravity, hindering your 'free fall'.
 

Offline evan_au

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Re: Does gravity pull or push?
« Reply #10 on: 13/04/2014 04:58:04 »
Quote from: zunimtn
in freefall, relaxing in the gravitational field of earth, you feel no force

This is because in freefall, every atom of your body feels a gravitational force proportional to that atom's mass. This means that every atom of your body undergoes the same acceleration towards the center of the Earth.

So the reason you (as an organism) don't feel any force is because every atom feels the force.
 

Offline yor_on

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Re: Does gravity pull or push?
« Reply #11 on: 13/04/2014 13:40:52 »
The point with a free fall, to me then, is the question if that could be used to define a 'absolute frame' from some abstract viewpoint. If I stop assuming motion, then we still have a equivalent 'uniform motion' as defined locally. And yes, you can turn it around I guess and argue that it is those standing on earth accelerating to some ideal center. On the other hand, if we define gravity from some free particle, perfectly spherical and with a evenly distributed mass, no other gravity. Then the direction seems to be inward to a center. So discussing particles acting, and being acted upon, by other particles might be able to be resolved into such a 'force', or direction. It's just that I want to keep it as simple as I can. Also suspect that Einstein was considering gravity, or micro gravity, as something involved in making particles when he and Rosen thought up the EinsteinRosen bridge? Can't be sure of the last one though? http://www.ias.edu/about/publications/ias-letter/articles/2013-fall/maldacena-entanglement
=

"If we define gravity from some free particle, perfectly spherical and with a evenly distributed mass, no other gravity. Then the direction seems to be inward to a center."

This one is in fact very close to me scaling 'gravity'.  And if we take this one, one step further, we will find that scaling gravity 'disappear' it. And that one is about what you expect to find, assuming a constant scaling. Will there be a 'final bit' defining the laws we have, or not? It's about whether the universe consist of discreteness, or a smooth continuum? I think it consists of both myself, as I do expect a 'bit' even if not measurable. But it won't stop you from scaling, so I have to assume the logic giving us this 'bit' must exist, even past it.
« Last Edit: 13/04/2014 15:56:41 by yor_on »
 

Offline yor_on

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Re: Does gravity pull or push?
« Reply #12 on: 13/04/2014 14:50:07 »
What's interesting with this paper is in fact its last sentences.

"There are several interesting lessons regarding this picture of geometry emerging from entanglement. Perhaps the deepest one is that the peculiar and strange property of quantum mechanical entanglement is behind the beautiful continuity of spacetime. In other words, the solid and reliable structure of spacetime is due to the ghostly features of entanglement. As we entangle two systems with many degrees of freedom, it seems possible to generate a geometric connection between them, even though there is no direct interaction between the two systems."
==

There are also subsequent questions created by assuming a entanglement creating a geometry. In how many steps can you 'keep it continue'? That one definitely connect to what ideas one might have of a arrow too. Depending on your definitions of time as being seamlessly continuous, or discrete. Also depending on your definition of what propagation, and a arrow, might be. As I connect a arrow to 'c', locally equivalent, I also have to consider the 'container' definition we have of that 'commonly same universe¨' we exist in. In that one we define 'time' as a property existing everywhere, but observer dependently. In my definition you have only one type of arrow that exist, and that is 'c'. 'c' creates the repeatable experiments we build physics from. Although it does not state how it can exist. You can choose between 'propagation', aka all types of 'motion' for that one, or something not 'propagating' at all, or both co-existing. Then there is the question of if one entanglement can lead from one, to many? Is there a limit for those connections? Are they then the same on some abstract  'property' level? That one is also a question of how you define 'time' and a arrow. And what would then make them differ? Because it's not only change we see, we also see differences.

Because that is exactly what bothers me, in my own thoughts. How and why (locally definable) frames of reference can interact, and exist. Those 'locally equivalent frames of reference' are what create our definition of 'repeatable experiments' in my thoughts. Also it could be seen as a question of symmetries possibly? And complexity created out of simplicity? Creating a entropy? I don't know, but if it was so that entanglements is a part of the answer then it takes me one step further.

You can also think of it this way. You have two choices, one is the macroscopic definition in where we must find 'c' as a propagation creating observer dependencies that are real. No matter if you define SpaceTime as unlimited or not, or how you like to treat its 'dimensions', it's still a mindconcept of a 'commonly same container, with observer dependencies'.

The other is QM, that one you will reach by scaling. Einsteins 'locality' principles are valid for QM too, by that referring to the concept of 'frames of reference'. Being 'at rest' can be aplied macroscopically and is applied that way. But a frame of reference treated from a QM concept becomes a question of if a frame of reference also is possible to define to some 'bit concept'. and I think you can do that. As I also define repeatable experiments as coming from (locally defined) frames of references equivalence, and then sets 'c' as equivalent a arrow (everything locally defined) Planck scale becomes a simple foundation for a 'bit concept'. That does not state that Planck scale is a limit, how can anything be a limit? But I would expect physics to change passing that scale.

And this goes for gravity too. Gravity is a relation between frames of reference, in a atom we find forces interacting, those frames existing inside that atom can both be seen macroscopically as a atom being at rest with itself, as well as if those frames creates a gravity. And that is fairly metaphysical, but logical if you use my view :)
=

Looking at it from local, scaled up 'frames of reference', dimensions becomes questionable, and if they are, so are 'motion', including 'propagation'.  Which then makes it imperative to define why frames of reference can interact, how they connect. Defined macroscopically there are no 'singular' frames of reference, Einsteins universe is you locally, measuring on another frame of reference. That's the only way to reach our definitions of a measurement, always 'you' measuring, using a (ideally defined) local clock and ruler.
=

You might add that by assuming a local clock and ruler, real and useful, we do introduce the concept of one singular frame, from which we measure another. So even though you need two frames for a actual measurement, you will use one 'local ideal frame' as your gold standard, defining the other from. That's also the ground for a repeatable experiment.

So, singular frames is what we use, to measure from, and we assume them equivalent, if equivalently created (as in how one define a equivalence of experiments, proving it as a repeatable experiment). It's a sort of principle combined with the idea that this principle should be valid anywhere in this universe that you can measure, and subsequently report, on.
« Last Edit: 13/04/2014 21:57:21 by yor_on »
 

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Re: Does gravity pull or push?
« Reply #12 on: 13/04/2014 14:50:07 »

 

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