So, energy is simply a mathematical expression of how much work is done in the system? Since energy is not a "physical" object that has mass. It's not matter at all. If so, the laws of conservation makes sense, it can't be created nor destroyed. But I can't imagine how a mathematical expression be converted from one "form" to another. I tend to avoid using the term "energy" because I get confused a lot and can't interpret the idea at all. Yet I do use "Work" a lot. As W= Fd, its much easier. I treat energy as a number thats it. That can't be created/destroyed because it's a number that measures the work. Here is one of the questions that confuses me. What does it mean that energy is conserved as being the sum of PE and KE? Is energy a constant value in the universe? How should I deal with energy... As what exactly?