Mechanical energy is the sum of all the forces acting on or in a system as a result of its chemical state (chemical-potential), relative position (potential) and state of motion (kinetic). Work, or a force acting upon an object, is the use of mechanical energy to affect a system.
An example of the total mechanical energy of a complex system is the human body. It converts chemical-potential energy in the form of food and drink into potential energy stored in the muscles. This energy can then transform into kinetic energy in order to move its mass around in order to perform work, such as walking or driving.