I encountered a problem which goes like this: Two charges of mass m and charge q are initially positioned far from each other. Now one is projected towards the other with velocity v . Find the closest distance of approach. We( in class) solved the problem using energy conservation but I had this doubt later: Since the charges are moving, the electric field is not static in nature , so how can we define potential of such a field and use it in energy equation? And then I had this another doubt which we encounter more often in gravitation where two masses come towards each other by their mutual attraction, since gravitation is also similar to coulomb's law can we say we cannot gravitational field for a moving body?