# 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:

1. Why Energy = Mechanical Energy + Internal Energy. Is it inferred from the definition?

2. Why there is only 2 main kinds of energy is potential energy and kinetic energy? And how do we classify energy?

• About second question. Potential energy measures what work system can do potentially. Imagine $1~kg$ brick laying on the roof of $100~m$ height building,- if would be falling from that roof to the ground,- could do about $1~kJ$ work upon touching ground. That's why it's potential energy, energy that's "in there" but not used still. Kinetic energy (btw, kinesis in Greek means "motion") means that some other form of energy was converted for speeding object. Usually it can be potential, but also can be internal/heat (from heat engine), etc. Nov 18, 2022 at 18:33

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.