According to Einstein’s equation “E = m c^2” an object that loses energy loses a VERY slight proportion of its mass. An object that, on the contrary, gains energy also gains a VERY slight increase in mass. Now, if an object (in space, or vacuum, say) moves or changes position, in general physics we say; the potential energy of the object has transformed into kinetic energy (because the object has moved). Thus it goes that the object lost part of its mass (or potential energy) as kinetic energy, where has the lost part gone or transformed into? Remember the law of conservation of energy states that energy lost = energy gained. Let me re-frame that: What happens to the K.E. after the object sets to rest? Is it re-transformed into potential energy so that the mass of the object returns the same? NOTE: I know I'm bullsh*tting, but.. what to do?