Gravity - an accelerating frame paradox

[ZirkMan]

I suggest you read some more / take some more physics classes. What you wrote makes no sense. Newton's third law is equivalent to conservation of momentum, not energy. Conservation of momentum follows from Newton's third law, and Newton's third law follows from conservation of momentum. The derivation is in practically every text for the sophomore/junior level class classical mechanics taken by almost all physics majors.

See my #59 reply to DaleSpam.

Regarding your earlier notion that the normal force is equal but opposite to the gravitational force: It isn't. From the perspective of an inertial observer, the forces acting on a person standing still on the surface of the Earth are the normal force and gravitation.

So is there a gravitation force for the inertial observer? I believe what DaleSpam says here is correct (and I was wrong when I thought there is a fictitious force in that frame):

A free-falling frame is inertial. There are no fictitious forces in it, only the real forces.

Regarding your interpretation of the equivalence principle: Please re-read post #43.

I tried to sum up of how I understood it in post #44. You haven't replied on that so I am not sure if I understood it correctly.

 

[D H]

See my #59 reply to DaleSpam.

Already discussed in post #60.

[QUOTE}So is there a gravitation force for the inertial observer? I believe what DaleSpam says here is correct (and I was wrong when I thought there is a fictitious force in that frame)[/QUOTE]
I should have been clearer that the inertial observer in post #55 was looking at things from a Newtonian rather than relativistic perspective.

 

[ZirkMan]

And you had the gall to accuse me of using non-standard definitions of the conservation laws?

I didn't say they are non-standard! I was just trying to say that for that particular illustration the distinction between energy and momentum conservation is not necessary.

The problem here is that you are trying to learn to run (general relativity) when you can't even crawl (non-calculus Newtonian mechanics).

Sorry this will be a little bit metaphysical but we have come to this point.

I know I still have to learn a lot even more about the very basic notions and their relations. Because what I often see is that lot of people don't see that there are way too many basic physics notions that are defined for a very specific purpose to illustrate some aspect of a well known and physically real phenomenon in a special situation. And once that definition is done the special aspect is often forced to apply on situations where this aspect is not important at all and what is worse it takes an independent life and many proclaim its existence independent from the real phenomenon from which it was derived (because it has a definition of its own). Don't get me wrong, exact definitions are necessary for a rigorous debate when we know what we are talking about (I don't deny that!). Even worse than "independent aspects"are definitions of "virtual notions" that are part of models that explain some aspect of the physical world but don't have a connection to any physically real entity (such as speed or maybe even spacetime). So you see, yeah, I have to learn a lot. But it seems to me I have to unlearn a lot as well in order to understand what is this physical theory really trying to say.

 

[nismaratwork]

I didn't say they are non-standard! I was just trying to say that for that particular illustration the distinction between energy and momentum conservation is not necessary.

Sorry this will be a little bit metaphysical but we have come to this point.

I know I still have to learn a lot even more about the very basic notions and their relations. Because what I often see is that lot of people don't see that there are way too many basic physics notions that are defined for a very specific purpose to illustrate some aspect of a well known and physically real phenomenon in a special situation. And once that definition is done the special aspect is often forced to apply on situations where this aspect is not important at all and what is worse it takes an independent life and many proclaim its existence independent from the real phenomenon from which it was derived (because it has a definition of its own). Don't get me wrong, exact definitions are necessary for a rigorous debate when we know what we are talking about (I don't deny that!). Even worse than "independent aspects"are definitions of "virtual notions" that are part of models that explain some aspect of the physical world but don't have a connection to any physically real entity (such as speed or maybe even spacetime). So you see, yeah, I have to learn a lot. But it seems to me I have to unlearn a lot as well in order to understand what is this physical theory really trying to say.

You have enough to learn that what you just said is a meaningless rant. Stop typing... Start reading.

 

[Dale]

This is exactly the situation I have a problem to understand. Where does this "unbalanced upwards-pointing real contact force" come from? Does it arise as a "barrier force" preventing me following a geodesic motion?

Do you understand where it comes from in the situation where the astronaut is on a rocket accelerating in deep space? By the equivalence principle it comes from the same thing here, the ground is accelerating upwards and carries the astronaut upwards with it, accelerating him upwards via the EM contact force.

 

[Dale]

one should not forget that at the end of the day it is only energy that is being transferred (momentum being just a specific expression of it).

This is simply incorrect, as I have already shown. Please stop repeating this erroneous assertion or any variations thereof, you simply need to let go of this incorrect concept. You cannot replace a vector conservation law (momentum) with a scalar conservation law (energy). It is logically and mathematically impossible.

 

[D H]

But it seems to me I have to unlearn a lot as well in order to understand what is this physical theory really trying to say.

I would say that you have a lot of basic physics to learn before you should even start delving into general relativity. For example,

Thanks for a clarification. I have regarded the momentum only as mass x velocity not as a vector unit.

Mass times velocity is a conserved quantity -- but only if you interpret velocity as a vectorial quantity. Mass times speed is not a conserved quantity.

 

[nismaratwork]

I don't see why you two (DaleSpam and D H) should waste your time this way. He has 3-4 solid pages of links and explanations to peruse, and before that he has basic NM to learn. I honestly think you've gone beyond polite and kind; does this thread continuing serve any purpose except to frustrate you?

 

[D H]

Please don't tell me it's wrong

You got it wrong.

You are missing the main point of Einstein's thought experiment. There is no way that a person inside the windowless elevator car ( spacecraft in modern parlance) using local experiments can distinguish between a quiescent (thrusters not firing) spacecraft in deep space versus a quiescent spacecraft in orbit about some planet. Yet Newtonian mechanics says a frame fixed with respect to the spacecraft is an inertial frame in the first case but is not inertial in the second. There similarly is no way via local experiments to distinguish between the spacecraft being deep space, this time with thrusters firing, versus being in a quiescent sitting still on the ground. Yet once again Newtonian mechanics says these are very different conditions. This suggested to Einstein that something was remiss with the Newtonian concept of an inertial frame. One shouldn't have to look outside the window or rely on reports by an external observer to determine whether or not the local environment is behaving inertially.

 

[ZirkMan]

You got it wrong.

Alas, premature joy. That insight fails to explain other many important aspect that I have not included. Sorry, I really don't want to frustrate you anymore (enjoy nismaratwork), I heed your advice and will report back only after I have learned everything that you have recommended or mentioned.

 

[nismaratwork]

Alas, premature joy. That insight fails to explain other many important aspect that I have not included. Sorry, I really don't want to frustrate you anymore (enjoy nismaratwork), I heed your advice and will report back only after I have learned everything that you have recommended or mentioned.

A wise choice...

[PLAIN]http://www.smileyshut.com/smileys/new/emot67.gif