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Clarification on Gravity
Dr Matrix:
--- Quote from: sokarul on September 17, 2008, 07:40:02 AM ---I have no doubt there is upwards acceleration from the normal force, but the objects net coordinate acceleration is zero. And BTW acceleration and gravitation are only equivalent locally.
--- End quote ---
I agree that the person standing on the ground does not move relative to the ground, but they still feel like they're being pushed upwards by the ground. At no point does anyone feel like they're being pulled down by the ground/Earth beneath it (insomuch as at no point will an accelerometer give you a negative reading due to gravitational attraction, where positive is defined as 'up' - the lowest it will ever read is zero, which it will do when in free fall).
As for the equivalence principle being a local axiom only - fair point, but you're not going to notice any difference on a human scale without either ridiculously accurate accelerometers or massive tidal forces (such as near an event horizon). Neither of those conditions have been specified for this discussion.
sokarul:
--- Quote from: Matrix on September 17, 2008, 07:51:51 AM ---
I agree that the person standing on the ground does not move relative to the ground, but they still feel like they're being pushed upwards by the ground.
--- End quote ---
And the ground feels like it's being pushed down by the person.
--- Quote ---At no point does anyone feel like they're being pulled down by the ground/Earth beneath it (insomuch as at no point will an accelerometer give you a negative reading due to gravitational attraction, where positive is defined as 'up' - the lowest it will ever read is zero, which it will do when in free fall).
--- End quote ---
The normal force is present because of contact. Look up normal force and see how it arises. As for the accelerometer gravitation accelerates objects without a force.
--- Quote ---As for the equivalence principle being a local axiom only - fair point, but you're not going to notice any difference on a human scale without either ridiculously accurate accelerometers or massive tidal forces (such as near an event horizon). Neither of those conditions have been specified for this discussion.
--- End quote ---
I know, I just thought we were having a state the obvious contest.
--- Quote from: E.Jack on September 17, 2008, 07:51:26 AM ---
--- Quote from: sokarul on September 17, 2008, 07:40:02 AM ---I have no doubt there is upwards acceleration from the normal force, but the objects net coordinate acceleration is zero.
--- End quote ---
Right, coordinate acceleration. The object's physical acceleration is upward. Glad we finally cleared that.
--- End quote ---
Please research before you post again.
--- Quote ---So? That still doesn't disprove why they are different.
--- End quote ---
Unless you are not local.
Jack:
--- Quote from: sokarul on September 17, 2008, 08:02:58 AM ---Please research before you post again.
--- End quote ---
Find me a quote in GR where it says we are not undergoing a constant upward physical acceleration. I doubt you can.
--- Quote from: sokarul on September 17, 2008, 08:02:58 AM ---Unless you are not local.
--- End quote ---
I rest my case.
Parsifal:
--- Quote from: sokarul on September 17, 2008, 07:40:02 AM ---
--- Quote from: Osama bin Laden on September 16, 2008, 08:24:39 PM ---
--- Quote from: sokarul on September 16, 2008, 08:20:43 PM ---Moving clocks tick slower.
--- End quote ---
They do, but they don't appear to move slower.
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http://www.astrophysicsspectator.com/topics/generalrelativity/BlackHoleEventHorizon.html
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I don't need education on what goes on around a black hole, thanks. You said it yourself; the observer that experiences time dilation is the observer that experiences acceleration. What would happen if, at t=0, observer A just outside the event horizon of a black hole switches on his jet pack to hover where he is, while observer B allows herself to enter freefall along a path that will take her right alongside observer A, and just as she reaches him, he switches off his jetpack so that they are both inertial? Whose worldline will have been dilated when the two are in the same frame of reference, and therefore who has undergone acceleration?
--- Quote from: sokarul on September 17, 2008, 07:40:02 AM ---
--- Quote ---Because the surface is also accelerating up.
--- End quote ---
I don't argue for the FET.
Way to ignore the rest.
--- End quote ---
The surface accelerates up in both FET and RET.
Dr Matrix:
--- Quote from: sokarul on September 17, 2008, 08:02:58 AM ---And the ground feels like it's being pushed down by the person.
--- End quote ---
Or, equivalently, that it is being pushed upwards into the person.
--- Quote ---The normal force is present because of contact. Look up normal force and see how it arises. As for the accelerometer gravitation accelerates objects without a force.
...
I know, I just thought we were having a state the obvious contest.
--- End quote ---
We have no disagreement on the origin of the normal force, or on the nature of gravitation from what I can see here. I believe the issue was over whether the ground accelerating upwards is equivalent to gravitation, which it is. As I have mentioned before, though, don't take my word for it:
--- Quote from: Matrix on September 05, 2008, 01:21:43 AM ---
But don't just take my word for it:
--- Quote from: http://en.wikipedia.org/wiki/Equivalence_principle ---In the physics of relativity, the equivalence principle refers to several related concepts dealing with the equivalence of gravitational and inertial mass, and to Albert Einstein's assertion that the gravitational "force" as experienced locally while standing on a massive body (such as the Earth) is actually the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference.
--- End quote ---
--- Quote from: http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/grel.html ---Experiments performed in a uniformly accelerating reference frame with acceleration a are indistinguishable from the same experiments performed in a non-accelerating reference frame which is situated in a gravitational field where the acceleration of gravity = g = -a = intensity of gravity field.
--- End quote ---
--- Quote from: Albert Einstein, quoted in 'Gravity' by James B. Hartle ---"Then occurred to me the happiest thought of my life, in the following form. The gravitational field has only a relative existence... Because for an observer falling freely from the roof of a house there exists - at least in his immediate surroundings - no gravitational field....(in this consideration air resistance is, of course, ignored)."
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