 Once we have established our basic law for the conservation of energy and we simplified it for our mechanics course where we're not looking at the heat, we can look a bit more at the details of it. What are the forms of energy that I could have in a mechanics class? In mechanics class we can have energy due to the movement, which is kinetic energy, kinetic energy, and we have potential energy due to gravity in most cases, gravitational. If you have a more advanced course you might have a spring potential energy. Let's stick with the simplest one. Then this equation translates in your final kinetic energy plus your final potential energy. So your total final energy is your kinetic energy initial plus your potential energy initial plus the change which came through work. And this is the basic formula that you can use for all your simple mechanical conservation of energy loss. So we have what we have before, what we have after is what we have before plus the change just that now we have two forms of energy and it looks like energy can be converted from one to the other. So even if we have no work coming in from the outside we could convert some potential energy into kinetic energy but the total amount of energy will always remain constant. That's what we mean by energy is conserved. It cannot be made up or destroyed. It can only be transferred from one object to another or transferred to transform from one form of energy into the other.