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Do not let anybody tell you that mass is “equivalent” to energy. The rest energy is only part of the overall energy.
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The two sentences are not mutually exclusive, and mass is, in a sense, “equivalent” to energy. I'd prefer to use the phrase that mass is a form of energy, that in this way there is an equivalence between mass and energy - not the total energy, of course. just as a hamburger is a form of energy, but not the entire energy.
I think this is word-play, but still important because so many of these concepts can be confusing.
On another note, I saw this:
"The idea of plotting x versus t as we have done here is completely standard. It’s something you should have seen in 8th grade (if not before), and seen many times since then."