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Aurino,
some quotes here sir,
"That is correct, but there is a subtle issue here. If an object that is, say, a mile about the
earth's surface has no weight, why is it that it suddenly becomes "heavy" when it touches the
surface? The important issue here is that "weight" is not caused by the earth's gravitational
field, for the field is just as strong one mile up the atmosphere as it is here on the surface.
It has to be something else."
actually earth's gravitational field is stronger at sea level than it is say at the top of Mt
Everest. so weight does vary according to distance from the center of gravity of a massive
object. rest mass remains the same (relativistic effects excluded) but weight varies.
The gravitational force, as explained by Newton's Law of Gravitation, is inversely proportional to
the square of the separation between the two masses (or the separation between the centers of
mass for the two objects).
"That is not correct; acceleration is not relative. According to modern physics, acceleration is absolute since it can be measured from its own frame of reference. If you are inside a rocket in space moving at constant speed you can't feel your movement; but if you suddenly ignite the engines, you will experience that "clutch to the seat" feeling, just like you do when you step on the gas pedal in your car.
I believe there are problems with that story, but that's how it's told to students of relativity."
you are correct in the following sense:
the case where one frame of referance is accelerated from one frame of referance that is not accelerated and then the accelerated frame is stopped and reaccelerated back to the accelerated
frame of referance the special theory of relativity can be used to calculate what goes on in the accelerated frame by the observer in the unaccelerated frame but the reverse is not true for the observer in the accelerated frame. an observer in the accelerated frame of reference needs to use general relativity to understand what is going on in the unaccelerated frame of referance since there have been several changes of inertial frames of referance for the accelerated observer.
the above is still in essence supposed to be relative (to be frank i don't fully understand why this is so) but it is supported by a researcher (whose paper i have but whose name i do not have on hand) who stated the following:
"Einstein, in reformulating gravity, was merely intellectually dissatisfied with the fact that
the mechanical and electromagnetic laws of nature were not the same in accelerating coordinate
systems—as they were in systems moving at a constant velocity. Einstein relates: “Could we build a relativistic physics valid in all coordinate systems; a physics in which there would be no place for absolute, but only relative motion?” In other words, Einstein is trying to extricate the “absolute” Now time aspect of acceleration from manmade coordinate systems."
from my own perspective with regards to the question of relativity with respect to an accelerated system consider the following:
a camera is fixed to a rocket that is accelerated from a launch pad. as the rocket accelerates
the camera records the view as the rocket accelerates away from the planet. that is one
interpretation of what the camera records, but one could also interpret the situation as the
camera recording the view as the earth accelerates away from the "stationary" rocket. now one can argue that the astronauts experience g's as a result of the acceleration of the rocket, but one could also argue that those g's are the result of pull of gravity of the earth as the earth pulls away from the "stationary" rocket. this i believe is another example of the equivalency principle of acceleration and gravity.
regards, tim
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