If we ignore non-mechanical processes like heat transfer, radiative losses, etc., then we arrive at a model involving only mechanical energy which changes due to the application (or extraction) of the work done by non-conservative forces The non-conservative forces can be external forces exerted on the system or internal forces resulting from the interactions between the elements inside the system.

The model is especially useful for systems where the non-conservative work is zero, in which case the mechanical energy of the system is constant. The most important cue for mechanical energy conservation is the dominance of gravity or spring forces (both conservative forces) in a problem. Since friction is a common source of non-conservative work, another important cue for problems in which mechancial energy is conserved is an explicit statement such as "frictionless surface" or "smooth track".

• Point Particle Dynamics

• system
• force
• work
• kinetic energy
• rotational kinetic energy
• [gravitational potential energy]
• [elastic potential energy]
• mechanical energy

One or more point particles or rigid bodies, plus any interactitons that can be accounted for as potential energies of the system.

All non-conservative forces that perform work on the system must be considered, including internal forces that perform such work. Conservative forces that are present should have their interaction represented by the associated potential energy rather than by the work.

• Initial-state final-state diagram.
• [Energy bar graph].