I was involved in a discussion on Quora about going faster than the speed of light and I pointed out that $E=mc^2$ implies that the faster the mass goes the more energy you would obtain from it. One of the responders said that the energy obtained only pertained to the rest mass (which begs the question, when is the mass at rest, when “all” mass is moving). I’m asking which is true and why, if so, doesn’t the amount of energy obtainable from a mass doesn’t increase with the speed of that mass?
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1$\begingroup$ Possible duplicates: physics.stackexchange.com/q/133376/2451 , physics.stackexchange.com/q/143652/2451 and links therein. $\endgroup$– Qmechanic ♦Commented Dec 24, 2021 at 10:13
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You are correct. A mass in motion has a kinetic energy that adds to the "rest energy". But energy is relativistic. In the frame of reference moving with the mass, the energy hasn't increased. In that way, your opponent is correct.
