Suppose we have two boxes, A and B with different masses, the spring is fixed to block A. Then we take block B and press it against block A so that the spring compresses. Afterwards, we let go of both boxes. If we were to model this using the conservation of energy, then it is known that (KE of A)1 + (KE of B)1 + (PE of spring)1= (KE of A)2 + (KE of B)2 + (PE of spring)2 0 + 0 + (PE of spring)1= (KE of A)2 + (KE of B)2 + 0 But one thing is confusing me. That is, the potential energy of the spring. I know that upon release, it will do positive work on block B. But won't it also do positive work on block A? Doing an experiment, when I push two masses together with a spring,with the spring attached to one box (a pen spring) both boxes go flying both ways. Surely this affects the potential energy? So how do I account for this potential energy?Should it be double, or what?