Purpose of the Hierarchy

The following hierarchical list of models has been developed and organized with several goals in mind:

• Each model must apply (approximately) to many situations
• The models should cover mechanics as completely as possible
• The models should be ranked hierarchically with most general on top
• Each model should have a descriptive name and be accompanied by its most frequently used formula 

Even these requirements create some difficulties, which we address as follows.  Firstly, to achieve perspective and to start at the top of the outline, we added a model for general energy conservation including thermal energy, even though this is usually considered part of Thermodynamics; Mechanics considers only the special case of Mechanical Energy, treating processes that generate heat as "Lost Mechanical Energy".  Arranging the many models into a hierarchy with only four principle models (Kinematics, Energy, Momentum, and Angular Momentum) properly stresses that there are only a few basic models in Mechanics and that many of the most used ones are simply special cases of these few; however it obscures the logical chain of proof and derivation of the laws of mechanics from only F = ma and the definitions of kinematics.  (This usually starts with kinematics and F = ma for point particles, then builds up and out to systems of particles, rigid bodies, momentum, angular momentum and mechanical energy.)  A further compromise concerns the equations we associate with each model.  It is a simple operation of calculus to express the laws of physics in either differential (v = dx/dt, ΣF = m d²x/dt² , ΣT = Iα), or integral form (E^final = E^initial + W^{nonConservative} ).  By presenting only the most frequently used form, we obscure this simplification for the benefit of helping students link titles and verbal concepts to the most frequently used equations. 

Hierarchy of Mechanics Models

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