Question

When you put two charged objects that are free to move near one another, you can calculate the force the exists between them, but it only lasts for one instant. Why?

Answer 1

Two charged objects either repel or attract each other (depending on the signs of their charges). Being free to move, they either start approaching each other or start receding from each other. In any case, the distance between the charges starts changing immediately, and since the electric force between them is inversely proportional to the square of this distance, it also starts changing immediately, so that its initial value will last only for an instant.