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Mass as a form of Energy in a simple example

A major consequence of special relativity, expressed in the relation $E_0 = m c^2$, is that the total energy content of an object at rest, including its thermal motion and binding energy among its constituents, is a measure of its inertia, i.e. its mass. This relation was first stated by Einstein. He showed that, in order to be consistent with the principles of special relativity, there must be a loss of inertia in a block that emits two pulses of electromagnetic radiation. A pedagogical difficulty with this example is that radiation is a purely relativistic phenomenon, and so the connection with the examples one learns in introductory Mechanics courses is not simple. Here we use a more familiar example of masses and springs, where the non-relativistic limit can be easily found and where the potential energy is clearly shown to be part of the mass of the bound system.