The electromagnetic force does not know the true distance, here the ruler is more of a theoretical construct. Perhaps it helps, if you think about it as moving

The electromagnetic force does not know the true distance, here the ruler is more of a theoretical construct. Perhaps it helps, if you think about it as moving the underlying coordinates vs moving the particle. (There is no difference between the two pictures in the theory only the relation between coordinates and atoms appear. Sort of, at least.) So, if you apply a force to an atom, that means that the second derivative of its position changes. On the other hand, if we imagine for a moment that the atom is stationary and the coordinates change, then the second derivative of its position will change and it will therefore feel a force, gravity.