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Thread: Momentum and mass

  1. #1 Momentum and mass 
    Forum Professor leohopkins's Avatar
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    I have just been thinking....

    Lets take an object in space and start accelerating it, as we accelerate it the object takes on mass. Now, lets stop accelerating it, it keeps on going until another force acts upon it. I have the idea that as soon as an object has stopped accelerating and is not interacting with anything else, its mass returns to rest mass, regardless of its now (constant) momentum or speed.

    I have come to this conclusion because, if we take an object and want to measure its mesurment or momentum, we need a frame or points of reference in order to complete this task...Lets say we take this same object and accelerate it, during its initial acceleration period, it acquires energy and mass, granted. Now when the object is travelling at a certain speed; take everything else that exists in the universe and make it disappear. Can we now measure the speed or momentum of the object ? No ! Can we say in with certainty that the object is still moving. By frames of reference, no we cannot, by the initial conditions set up at the beginning of the experiment, yes we can. Why does the object keep travelling in that same direction and at the same speed ?

    Becasue there are no longer any forces acting up on it. It is balanced and stable. It has returned to its rest mass; regardless of momentum.


    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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  3. #2  
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    There's rest mass and there's relativistic mass, Leo. If you search the internet you'll find there's been a lot of debate about what mass is. The current prevailing view is that rest mass is the better definition, but I prefer relativistic mass because it offers a link between energy and mass. See MASS EXPLAINED for details. The trick is to make your massless travelling "particle" travel in a loop so it isn't going anywhere. Then the momentum is now inertia, and relativistic mass is now rest mass.


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  4. #3  
    Forum Isotope Zelos's Avatar
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    leo that is just alot of mike-stuff

    it will allways have its relativistic mass relative to any referens frame of motion its not depened upon if any force is acting on it, but relative to itself it will allways ahve restmass
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  5. #4  
    Forum Professor leohopkins's Avatar
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    Quote Originally Posted by Zelos
    leo that is just alot of mike-stuff

    it will allways have its relativistic mass relative to any referens frame of motion its not depened upon if any force is acting on it, but relative to itself it will allways ahve restmass
    Relative to any reference frame. Thats what you just said and that is my point. Take away all all of the other matter in the universe and you no longer have any frames of reference.

    And....im not saying it has no rest mass; of course it does, what im suggesting is that it ONLY has rest mass, regardless of momentum.
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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  6. #5  
    Forum Professor river_rat's Avatar
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    Quote Originally Posted by leohopkins
    Quote Originally Posted by Zelos
    leo that is just alot of mike-stuff

    it will allways have its relativistic mass relative to any referens frame of motion its not depened upon if any force is acting on it, but relative to itself it will allways ahve restmass
    Relative to any reference frame. Thats what you just said and that is my point. Take away all all of the other matter in the universe and you no longer have any frames of reference.

    And....im not saying it has no rest mass; of course it does, what im suggesting is that it ONLY has rest mass, regardless of momentum.
    Im at a loss here - how does taking all the matter out of the universe get rid off the uncountably infinite number of inertial reference frames out there?
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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  7. #6  
    Forum Professor leohopkins's Avatar
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    Where would your points of reference be ? If nothing else existed ?
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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  8. #7  
    Forum Professor river_rat's Avatar
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    Um - a frame of reference is a mathematical construct, it does not depend on anything else "existing" as the point of reference is you.
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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  9. #8  
    Forum Professor leohopkins's Avatar
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    I think you are missing my point a little.
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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  10. #9  
    Forum Professor river_rat's Avatar
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    um no - i think you are trying to describe a universal rest frame, which is impossible in SR.
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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