A: (PH) In addtion to the external force, if the charge is accelerating there is a radiation reaction force on the charge. Both forces must be taken into account to calculate the acceleration. Alternative formulation: F=dp/dt, but you need to account for the momentum carried away by the EM field, whose 3rd law action-reaction counterpart is the radiation reaction force. I'm trying to find a pedagogical reference where this is explained. So far, no luck.