If we continue to add load to the spring, we experience a problem: the extension is no longer proportional to the force and the graph looks like this. At this point, we have stretched the spring beyond the limit of proportionality. If we take away the force it will not return to its original length. This is Hooke's Law: the extension of the spring is directly proportional to the load applied to it providing that the limit of proportionality is not exceeded.