I was just going through the derivation for the equivalent spring constant of two springs (of different stiffness) connected in series, and I realized that there's one thing I don't understand. In the process of this derivation, it is necessary to write down the net force acting on the point connecting the two springs, and then set this equal to zero. This seems to imply that the two forces are the action-reaction pair dictated by Newton's Third Law. My question is: why is this the case? Given that the connecting point between two springs does accelerate, why do we set the net force on this point equal to zero?