what is energy?

it is the capacity to do work. it is measured in kilo caloie.

 In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems. Since work is defined as a forceacting through a distance (a length of space), energy is always equivalent to the ability to exert pulls or pushes against the basic forces of nature, along a path of a certain length.

2. A feeling of possessing such strength and vitality