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I believe that a constant velocity moving object in empty space within it own frame of reference retains its rest mass as long as it is moving at constant velocity and an effective mass increase occurs due to inertia only during acceleration.

But of course I could be wrong?

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    $\begingroup$ Modern treatments of special relativity avoid using the concept of relativistic mass because it can be misleading and confusing. Please see physics.stackexchange.com/q/133376/123208 $\endgroup$
    – PM 2Ring
    Commented May 8, 2021 at 11:39
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    $\begingroup$ All SR effects are directly caused by relative velocity, not acceleration. The symbol $v$ appears in the Lorentz Transform, not $a$. $\endgroup$
    – m4r35n357
    Commented May 8, 2021 at 12:02
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    $\begingroup$ this answer of mine is related physics.stackexchange.com/questions/634523/… $\endgroup$
    – anna v
    Commented May 8, 2021 at 14:10
  • $\begingroup$ @m4r35n357 So if I travel in my car with a constant velocity v then I will weight more than when I am stationary? $\endgroup$
    – Markoul11
    Commented May 8, 2021 at 17:50

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Within its own reference frame, the object itself has zero velocity. Thus, it has to retain its own rest mass at all times in its frame.

In fact, that's how rest mass is defined. It is the mass measured in the frame of the object.

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Anything in its own reference frame views itself as being at rest so will always measure its rest mass only. when you view it from a different reference frame where you see the object has a velocity and you can apply the Lorentz transform for mass under special relativity
$$m=\frac{m_0}{\sqrt{1-\frac{v^2}{c^2}}}$$ to determine its relativistic mass and from there app;y $E=mc^2$ to find the energy of the particle.

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  • $\begingroup$ In my opinion the relativistic mass formula is an effective calculation of the total energy (including its kinetic energy) of an object and should not be translated as an actual increase of the mass of the object within its own frame of reference. Meaning, an object retains always its rest mass when moving at constant speed. $\endgroup$
    – Markoul11
    Commented May 8, 2021 at 18:06
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    $\begingroup$ That is exactly what I said it does not change in its own frame only when measured from a different object, that second object will see it moving with a velocity so will see its relativistic mass. I included the calculation because it is valid because just having mass is proportional to the energy that is why all the fundamental particle masses are given in energy units, so it is completely valid to use. $\endgroup$
    – Baxwell bolt
    Commented May 8, 2021 at 19:20
  • $\begingroup$ That's fine.Thanks. $\endgroup$
    – Markoul11
    Commented May 8, 2021 at 19:34

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