I was looking at some sliding doors and thinking/reviewing about the concept of work. So I have a few questions In order to close or open it, gravity does no work since the door moves on horizontal and not vertically. So far I understand it. 1)If I move it back and forth and return to the same place, again no work is involved. Except if we consider loss of energy due to friction. But that energy is converted in heat... So in the end is there a net work or not? 2)If I open it and close it, it's stopped then moves and stops again, so the initial kinetic energy is zero and final kinetic energy is zero, therefore total work should be zero. But what if we take heat into consideration? 3)What about those little wheels? Do they store some kind of potential energy? 4)If I pick an object, like some dumbell and keep holding it, the total work on it due to gravity is zero. Eventually I'll start to get tired, lose some calories, and sweat a little bit so my arms muscles are turning energy into heat, rising body temperature and so on. So, are they doing work?