It is a mechanics discussion regarding a problem of electrostatics. Couple of months ago I dealt with a problem:Given two electric dipoles, separated at a distance r and are perpendicular to each other.We are to find the torque exerted on each other. I took p1 as vertical and p2,right to p1,at a distance r,facing towards right. I used co-ordinate free form of E_dip which gave the torques... The torques were in the same direction and were not cancelling one another.It resulted from the fact that there were also dipole-forces exerted by one on the other---which I did not need to consider for the purpose of the problem.If one finds the torques due to both the factors---one arising from pxE and the other from (p.grad)E,the total torque on the system P1+P2 can be shown equal to zero.It is OK as the angular momentum of the system does not change. Everything is clear.What I want to know is that when I used co-ordinate free form,how can I be sure that I am working from an inertial system? Is my qustion clear?I am not referring to any particular co-ordinate system...but the end result is consistent with that as viewed from an inertial system... So,I think using a co-ordinate free form somehow makes it possible. can you try out something to make it clear?