well i am trying to understand the basic of mass and i got myself into a deadlock of questions firstly the definition of mass as i know is :quantitative measure of an object's resistance to the change of its speed. assumption:particle have charge but no mass and they just fall into the significant field of each other,there is nothing they interact with except each other and a force F. now consider a situation where a particle with charge say -q comes into significant field influence of another particle with charge +Q,it then follow a particular path P,suppose a force F is used to the bring a change in path of -q,the force F does so in two ways:at first it tries to change the path of -q by change it's state of motion(either by accelerating or deceleration him) ,thus the resistance offered to F relative to him appeared because of the electromagnetic interaction between the charges. In the second case the force F tries to change the trajectory of -q but by not changing it's state of motion(maintaining it's velocity),in this case also the resistance offered was due to the electromagnetic interaction. so according to the definition of mass the resistance offered in first scenario should add to mass but not second,though it can be clearly seen that the resistance offered is because of the electromagnetic interaction under action. SO WHAT IS WRONG WITH THE PICTURE DEPICTED?