What is energy and how it is classfied? 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?
 A: Strictly speaking, energy is capacity to do work for a given system. However, I think you are versed in this definition so I would like you to give you other intuition.
Typically, we often are interested in 'change in energy'. Absolute energy has to do very little with our lives. For example, our body's atoms has absolute energy $E=mc^2$, but you can't use it or transfer it in any way realistically possible. By doing work we are basically transferring energy to other mediums, in different forms.
A: *

*Yes, it can be inferred from the definition you gave but one needs more knowledge of physics to understand what it means: Typically in classical mechanics you consider an object as a point mass and define it's mechanical energy as E_kin=0.5mv^2. But that's not the whole story because the object can have a substructure, for example if you consider an atom it can move around which contributes to the mechanical energy but it also has internal energy levels (see for example https://en.wikipedia.org/wiki/Energy_level). Or you can consider a gas ensemble where it's center of mass velocity defines the mechanical energy and the internal energy is given by the velocity of the particles and their potential energy in the frame of center of mass (https://en.wikipedia.org/wiki/Internal_energy).


*There are only the types kinetic and potential energy because your system needs to be able to provide work as in the definition said. This can be only given by these two types.
I hope I could help you with my explanations. If you have more question do not hesitate to ask :)
