The inertia of stress
  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. 
Rodrigo Medina