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 force acting 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. The total energy contained in an object is identified with its mass and energy (like mass) cannot be created or destroyed.