Gravity

[warped space]

Ok I understand the subject of this topic is rather brief but that is irrelevant I have a question or possibly a topic for debate does gravity pull you towards the center of mass or does it pull you towards the object?

Now what I mean by that is better explained with a hypothetical example. Ok so you have a ring the ring is about the width of earth and stretches in a perfect circle around the sun but for this example assume the sun is not there and that the only thing is the ring now with mass there is gravity that isn't the question the question is that will gravity pull you towards the center of the ring where there is no mass or will it pull you towards the ring itself

[ajb]

An easier question that may help here is to consider a spherically symmetric shell instead of your finite cylinder. Look up Newton's shell theorem, which tells you that the Newtonian gravittaional field is zero within that shell.

 

You can show that inside an infintely long cylinder the Newtoian gravitaional field is zero. Also for a finite cylider the Newtonian gravitational field is zero along the equatorial plane within the cylinder. I am not sure of the form of the field off the equatorial plane, but I am sure this is known (just not by me).

[warped space]

Ok so what I'm gathering from that it pulls everything to the center until it gets there but because each individual particle has it's own mass and gravity the net gravitational pull is 0 in any direction because it is being pulled in every direction that makes sense but the question I ask now is gravitational relative change In time does that still apply just the same or is that essentially nullified

[ajb]

... I ask now is gravitational relative change In time does that still apply just the same or is that essentially nullified

I don't follow your question. Are you asking about how small perturbations propagate or something along those line?

[warped space]

Ok looking in, time is slower the closer you get to the center of gravity of an object but I could be wrong about that

[ajb]

Ok looking in, time is slower the closer you get to the center of gravity of an object but I could be wrong about that

Okay, so you are asking about gravitational time dilation.

 

The statement is

 

"Clocks at higher gravitational potentials run faster and clocks at lower gravitational potentials run slower."

 

So clocks which are far from the source of the gravitational potential run faster as compared to clocks closer to the source. So, I think what you said is okay, mod maybe the Shell theorem.

[timo]

Ok I understand the subject of this topic is rather brief but that is irrelevant I have a question or possibly a topic for debate does gravity pull you towards the center of mass or does it pull you towards the object?

It easy to show that gravity does not always pull to the center of mass of an object. Consider a test mass at location (0,0,0) and a pulling object consisting of essentially two equally-massive large masses located at (1,0,0) and (-3, 0, 0), respectively. A calculation will reveal that the gravitational pull is in positive x-direction, i.e. away from the center of mass at (-1, 0, 0) which lies in negative x-direction.

Edited August 10, 2014 by timo

[Strange]

Ok looking in, time is slower the closer you get to the center of gravity of an object but I could be wrong about that

 

If you were to make a tunnel through the center of the Earth, the graviational pull would descrease until it became zero at the center. But the gravitational potential would (I think) increase so a clock at the center would run slower than one at the surface.

[Enthalpy]

Every part of the massive objects attracts other massive objects. If the pbject is not a sphere, the resulting direction attraction needs not be towards its center of mass.

 

In addition to the nice example proposed by Timo, a very concrete case is Earth itself, which is a bit flattened. One consequence is that satellites on a nearly polar low orbit go more parallel to the north-south direction when they're nearer to the equatorial bulge, so that their orbital plane is not fixed but drifts slowly.

 

This is used for "sun-synchronous" orbits, for instance with 800km altitude and 98° inclination. Then, the orbital plane drifts by 1 turn every year, so that its direction is constant versus the Sun. The satellite passes a a constant hour every day (said a bit quickly), for instance 10h and 22h, at local solar time. Spot, Ers and many more satellite families use these orbits.

[imatfaal]

 

If you were to make a tunnel through the center of the Earth, the graviational pull would descrease until it became zero at the center. But the gravitational potential would (I think) increase so a clock at the center would run slower than one at the surface.

 

The lower the slower. As gravitational potential drops then clocks run slower. A fast ticking clock is situated at a great distance from the gravitational source. I think it would be a slow ticker as the gravitational potential is lower compared to anywhere outside the shell.

Gravitational potential is the negative of the work done through gravity per unit mass in moving the object in from infinity - thus it is zero at an infinite distance and drops as the object moves towards the massive gravitational source. At the centre of the symmetric shell it will be as low as it is going to get and the clocks will be slower than those a great distance from the object

[MigL]

Time dilation is based on the difference in gravitational potential. A signal, say an EM wave, which starts deeper in a potential well and climbs to a higher point in that well, must expend energy. This energy loss cannot result in a change of velocity, which is fixed at c, so it must lower its frequency/increase its wavelength. An increase in wavelength is, in effect, a longer time between signals. In other words time dilation.

 

The only way to determine if a certain arrangement of masses will produce time dilation at one point with respect to another , is to determine the potential at both points and calculate the difference.

[imatfaal]

Time dilation is based on the difference in gravitational potential. A signal, say an EM wave, which starts deeper in a potential well and climbs to a higher point in that well, must expend energy. This energy loss cannot result in a change of velocity, which is fixed at c, so it must lower its frequency/increase its wavelength. An increase in wavelength is, in effect, a longer time between signals. In other words time dilation.

 

The only way to determine if a certain arrangement of masses will produce time dilation at one point with respect to another , is to determine the potential at both points and calculate the difference.

 

 

It boils down to the same thing - but I think it is easier to think of the calculation as follows; if from a -->b you do work against gravity then b will be fast ticking, if you move from c-->d with gravity doing the work then d will be slow ticking. It removes the need of thinking of the need for an absolute value of potential - as you say in your first line it is only the difference that is important. Although I think if you actually want a figure rather than a simple determination of quality then I think you have to go the long way around.