E = mc^2 is the energy contained within a particle of mass m, IF it is at rest (has zero velocity with respect to your frame). If it is moving, it&#x27;s energy is given by E = ymc^2, where y (gamma) is 1&#x2F;sqrt(1 - v^2&#x2F;c^2). This may sound like the particle has somehow &quot;gained&quot; or &quot;lost&quot; energy just by switching frames, but so has everything else. The differences in energy have been kept the same, which is all that really matters.<p>When deriving this formula for the energy it becomes clear that _any_ multiple of ymc^2 is conserved. We choose E = ymc^2 and not, say, E = 5ymc^2 because in the Newtonian limit (as speeds approach zero), the ymc^2 formula reduces to the well-known E = 1&#x2F;2 mv^2, which is the kinetic energy of an object obtained from classical mechanics.<p>I&#x27;d really recommend reading a mathematical treatment of special relativity, because you&#x27;ll be exactly clear on where this formula comes from.