## Relativity by Albert Einstein

I've been reading the book Relativity by Einstein, but need some things cleared up. I have only studied physics in college (took both mechanics and topics pertaining to more smaller scale phenomenas) and have a vague familiarity with concepts of physics but would like to expand on this a bit more.

So here are my questions so far:

-I am assuming that every object has an absolute velocity; am I right?

-We measure kinetic energy with respect to a rigid body such as that of the earth. Do I possess kinetic energy at this moment in which the earth is moving? That is I am moving at a certain velocity even if I am at rest with respect to the earth. How can the measurement of kinetic energy be accurate then?

-I am not quite sure what Einstein was intending to explain near the beginning of the book with Gallaien coordinates.
 objects do not have an absolute velocity. Because kinetic energy depends on velocity the kinetic energy of other bodies will change when you change your velocity. I don't rember the book well enough to know why he used Galilean coordinates, but I do know that Galileo had earlier stated a version of relativity that did not depend on the speed of C being constant.

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 Originally posted by Meninger -We measure kinetic energy with respect to a rigid body such as that of the earth. Do I possess kinetic energy at this moment in which the earth is moving? That is I am moving at a certain velocity even if I am at rest with respect to the earth. How can the measurement of kinetic energy be accurate then?
If the reference body is the Earth then you have no
kinetic energy.
 Originally posted by Meninger -I am not quite sure what Einstein was intending to explain near the beginning of the book with Gallaien coordinates.
He was describing the evolution of coordinate systems.

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## Relativity by Albert Einstein

What I meant in absolute velocity is absolute speed and I am pretty sure that every object has to possess absolute speed; that is the reality of how fast the rigid body is traveling, or resting. What I still don't understand is how we can ascertain the speed of an object into the delta kinetic energy equation by measuring its speed relative to the earth which may be moving along the same dimensional line as the object. The equation for the change in kinetic energy deltaKE=m(vf^2- vi^2) is only valid for one relativistic subtraction of velocity (or speed since the velocity is squared). An object may start from rest, as seen by an observer, and move to six meters per, lets say, an hour. If the object was moving at a certain absolute speed (integrating the absolute speed of the earth, along the direction of the rigid body's travel) it would have a different change in kinetic energy even if it increased its velocity by six meters per hour. Forgive my ignorance, as I know I am missing something here; either that or I have disproved Einstein himself, which I know is not happening.
 Recognitions: Science Advisor Greetings ! Nope, no absolute speed and accordingly no absolute particuilar energy, though the total energy of a system in the same reference frame remains the same. (If Universal expansion and space itself are left out for the moment so we have a classical reference frame.) Live long and prosper.

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It's hard to believe you have been studying the book Relativity by Einstein when you say things like:
 I am assuming that every object has an absolute velocity; am I right?
and
 What I meant in absolute velocity is absolute speed and I am pretty sure that every object has to possess absolute speed; that is the reality of how fast the rigid body is traveling, or resting.
The fact that there is no such thing as an absolute velocity (or speed) is pretty much covered in the first chapter- that is, after all, the reason for the name "relativity". ALL motion is "relative" to some reference frame.

You also say
 We measure kinetic energy with respect to a rigid body such as that of the earth. Do I possess kinetic energy at this moment in which the earth is moving? That is I am moving at a certain velocity even if I am at rest with respect to the earth. How can the measurement of kinetic energy be accurate then?
You just said yourself that kinetic energy must be measured "with respect to a rigid" body, yet you say "Do I possess kinetic energy at this moment?" The question makes no sense until you specify "with respect to" a specific rigid body. If you mean "kinetic energy with respect to the earth", then, because you are not moving with respect to the earth, the answer is "no". If you mean "kinetic energy with respect to the sun", then the answer is "yes" because the earth (and you ) is moving with respect to the sun. However, that doesn't really mean anything because the question of "kinetic energy" only becomes important when it changes (and converts into some other kind of energy).
 Thanks for all of your explanations. I think that all of you have misinterpreted by what I meant by absolute speed. HallsofIvy, I have only read the first few pages of the book and I don't recall Einstein ever saying anything about non-existence of an absolute speed for an object. He did say that, according to the theory of relativity that the speed of light in reference to any object at any reference frame, is the same. The examples that you guys are using seems to contradict what he elucidates later. Here is what I thought a while ago; the thought that provoked my question in this forum. The fact that there are magnitudes of speed is self-evident. Since there are magnitudes of speed, and there is no such concept as what we can call a negative speed (since speed is magnitude) and that an object with a constant velocity is not varying in its speed (speed does not "happen" to the object, even in a relativistic point of view, an object has to possess its own speed even to be compared to other objects around it), I find it reasonable that an object moving in constant translatory motion at least possesses an absolute, inherent speed; although this perspective may change in respect to a system.

 Originally posted by Meninger -I am not quite sure what Einstein was intending to explain near the beginning of the book with Gallaien coordinates.
Galilien coordinate is from Newton's view to explain the space-time
and Einstein want to explain why the Galilien coordinate was wrong
and need to replace with Lorentz Transformation

 Originally posted by Meninger Here is what I thought a while ago; the thought that provoked my question in this forum. The fact that there are magnitudes of speed is self-evident. Since there are magnitudes of speed, and there is no such concept as what we can call a negative speed (since speed is magnitude) and that an object with a constant velocity is not varying in its speed (speed does not "happen" to the object, even in a relativistic point of view, an object has to possess its own speed even to be compared to other objects around it), I find it reasonable that an object moving in constant translatory motion at least possesses an absolute, inherent speed; although this perspective may change in respect to a system.
Sorry, i don't understand the logic in your conclusion, can you make it a clearer please ?
Thanks.