Imagine a system made of 4
pieces, each of mass *m*. Einstein pointed out that the
total mass of the system should be *Mc ^{2} = ℰ +
4mc^{2}*

Here ℰ is the total internal
energy of the system, the sum of the kinetic and potential
energies of the 4 particles. But for systems in nature, the
size of an atomic nucleus or larger, the total internal energy
of a bound system is **negative.** Notice that this means
that *M* is less than 4*m*! Physicists define the
binding energy of a system as *B = -ℰ*, so that *B*
is the work it would take to disassemble the system
completely. Example: suppose each particle
has a mass of 10 MeV/c^{2}, and the mass of the system
is measured to be 30 MeV/c^{2}. What is the
binding energy of the system? *ℰ = Mc ^{2} - 4mc^{2}* = 30
MeV - 40 MeV = -10 MeV. It would take 10 MeV of work to
pull the system apart.