Help me rationalize "conservation of energy" So, I understand conservation of energy, but.....I must be missing something, because in my mind, there are some problems with it. Let me explain by using an example. I'm sure you'll set me straight. Okay, let's say that I have a bow and arrow. I draw back the bow to its full extent, and shoot the arrow into a concrete wall. What has happened? 1) the chemical energy in my MUSCLES (which came from the food I ate, which was derived from......ultimately back to solar fusion, etc) was transferred to the BOW 2) The BOW was then charged with potential energy 3) When I released the BOW, most of the energy was transferred to the arrow thus charging it, the arrow, with KINETIC energy. 4) As the arrow is flying along its path, it is transferring small amounts of kinetic energy to the air molecules, and some work is being done in the form of moving air, thus overcoming its inertia, and generating relatively small amounts of kinetic energy there. 5) When the arrow smashes into the concrete wall, ALL of its remaining kinetic energy is used up doing work, such as destroying the structure of the arrow (let's assume it is smashed nearly into pulp), the sound of impact and damaging some smaller part of the concrete wall (small dent). So.....at this point, it seems to me that ALL of the energy that was ultimately in the moving arrow.....has now done work and is gone forever. How is this not so? Seems to me, that for practical purposes....once work is done, energy is in fact gone forever. Part II: I cut a branch from a tree, and allow it to dry out. I then set it on fire, thus breaking the hydrocarbon molecules (which were created by the energy ultimately from the fusion of H to He plus energy.....in our sun) into the components, or at least small parts (H, C, O, CO2, O2, etc). The heat I feel is just the energy that was once the molecular bonds, which no longer exist. Again, in my mind, ....I see work has been done, and those molecular bonds have been broken. The energy that was created from solar fusion has now performed its final possible level of "work"......and is now gone. Set me straight please. At the end of the day, I do know that mass is energy, and that mass can not be created nor destroyed.....but it seems there must be more to this. When the sun burns out......is the same amount of energy as 10 billion years prior present, even though work was done?? It seems there must be a difference between absolute energy (energy of mass) and something like practical energy, which can be used to do work. ?? Sorry for length. This has been bugging me for years, yet I know it to be a very basic concept.