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While reading a textbook on Physics, I came across this :

Mass is a universal constant. It does not depend upon the position of the body on the Universe but it changes with speed of the body.

It's just two contrasting statements with no further explanation. Maybe the author left it to our curiosity but I can't even find out how mass changes with time. Thanks P.S : This is not a homework question.

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marked as duplicate by Gert, John Rennie, Emilio Pisanty, Kyle Kanos, ACuriousMind Nov 10 '15 at 16:54

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    $\begingroup$ This question makes no sense at all. Mass is not a universal constant. Mass does not change with speed either but momentum does change relativistically. An object can lose mass over time in various ways, although in most cases no (or very little) mass is actually lost (destroyed). $\endgroup$ – Gert Nov 10 '15 at 16:19
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    $\begingroup$ What textbook did this come from (author & title)? $\endgroup$ – Kyle Kanos Nov 10 '15 at 16:21
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    $\begingroup$ While I wouldn't normally advocate burning books I might be willing to make an exception in this case. $\endgroup$ – John Rennie Nov 10 '15 at 16:23
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    $\begingroup$ yes, @Gert is right. The amount of matter doesn't change with speed, it just takes more energy than required to move that amount contributing to "apparent mass increase" (am I right?). $\endgroup$ – prakhar londhe Nov 10 '15 at 16:49
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    $\begingroup$ @HGSur: that statement contains neither the author nor the title that I had requested. $\endgroup$ – Kyle Kanos Nov 10 '15 at 17:34
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The textbook writer is referring to the concept of relativistic mass, which is the idea that accelerating a body tends to become harder and harder as its speed approaches the speed of light. This is sometimes thought of in terms of an increase in the object's mass as the speed increases.

However, you should think of this as a deprecated concept that most modern physicists consider unnecessary and misleading. Nowadays, we prefer to think that the force-acceleration momentum needs to be rephrased to something along the lines of $$ \mathbf F=\frac{\mathrm d}{\mathrm dt}\left[\frac{m_0\mathbf v}{\sqrt{1-v^2/c^2}}\right], $$ with an invariant mass $m_0$, than to introduce a variable "relativistic mass" $m_R=m_0/\sqrt{1-v^2/c^2}$ in an attempt to clean up that relationship and make it look more classical.

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