Some of you might recognize my screen name as the math guy that doesn't "get" physics. Well maybe you can help me straighten out my understanding of at least one law, and I'll just bother the forum for others as I learn them at my own pace. Wikipedia gives the definition of potential energy: Potential energy is energy stored within a physical system as a result of the position or configuration of the different parts of that system. So depending on the way objects are placed, we can "store" energy. Okay, seems fair enough to me. Then take this example. I'm going to "store" some elastic potential energy into a spring. Consider an idealized room, where we can analyze just the potential energy without other forces getting in the way. Well, almost all forces. You'll see in a second. I place a spring on the floor of this room and it points straight up. I place two heavy books on top of it, in such a way that one book would cause the spring to bounce back but two books is enough mass to keep it in a compressed state. With me so far? Now, I leave the room and let it sit there for years, no.. decades. The metal of the spring now begins to rust. It is no longer elastic, it's coils are now welded together by oxidation. I return and lift the first book from the spring and it does not move. The energy is now gone. My question is, where is the energy that was stored? We know by the conservation of energy that energy is neither created or destroyed in a closed system. Granted, when I entered and left the room I broke that closed-ness but I hope I'm correct that during the period of time when the spring began to rust that the stiffness of the spring became less and potential energy was lost. So what exactly is going on? Is the idealization of elastic potential energy that I'm given too simple to take into account this case? If so, then for what it's worth, I don't really "understand" the law, just a simplified version that's only applicable in special cases, correct? How do you extend this definition to include such cases?