My physics book defines energy like this:
"Everything that works has energy, energy $E$ of a system is a function of state and its variation $\Delta E = W$ ($W$ is work done on system)".
After that, it said that energy of a system is sum of its mechanical energy and its internal energy. I have 2 questions about this:
Why Energy = Mechanical Energy + Internal Energy. Is it inferred from the definition?
Why there is only 2 main kinds of energy is potential energy and kinetic energy? And how do we classify energy?