  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.
