  We present a simple example in which the importance of the inertial effects
of stress is evident. The system is an insulating solid narrow disc whose faces
are uniformly charged with charges of equal magnitude and opposite signs. The
motion of the system in two different directions is considered. It is shown how
the contributions to energy and momentum of the stress that develops inside the
body to balance the electrostatic forces have to be added to the
electromagnetic contributions to obtain the results predicted by the
relativistic equivalence of mass and energy.
