model

A model is a "simplified description of a complex entity or process".  It often highlights some feature of the modeled entity or process by blatantly ignoring others.  For example scale models of some particular airplane (e.g. XR-71) are faithful in appearance, but can't fly' other models of that same plane can fly but don't look much like an XR-71.  Other models are not tangible, e.g. computer models of a nuclear reactor designed to simulate its performance. 
According to David Hestenes, one of the founding fathers of modeling instruction,

"A model (in physics) is a representation of structure in a physical system and/or its properties." 

A closely related concept is that of a system - the portion of the physical or mental universe described by the model, thereby creating internal and external regions. 

A physics model will generally model only some of the structure in a physical system, for example the engine of a car can be regarded as a "heat engine" to turn heat into mechanical energy (work), or as the "powerplant" - a source of a certain amount of power that can accelerate the car.  The particularization of the model therefore relies critically on the selection of which

state variables will completely describe the system (in this model).  In the examples above these might be the volume of the space in the engine cylinder above the piston and the temperature and pressure of the gas in this volume (heat engine), or by the torque and revolutions per minute of the shaft exiting the engine (powerplant).   
Whatever the state variables, they are interrelated by the physical

interactions that will be considered in the model.  These are typically internal to the system or external, and if the latter completely determine the effects of the rest of the universe on the state variables of the system.  Generally the interactions may be represented as mathematical expressions that interrelate the state variables and their changes due to the interactions.    

Physical Model

A physical model will describe the system, the state of its constituents (including perhaps geometric and temporal structure), their internal interactions, external interactions, and the changes of state (i.e. behavior).  Models combine the definitions, concepts, procedures, interactions, laws of nature and other relationships that model some aspect of the physical world.  Models intermediate between laws of nature, which are relationships among abstract quantities, and experimental/experiential reality.   
A physical model is a mentally linked collection of physical laws, concepts, equations, and associated descriptions that relate to a particular common patterns found in nature.  Examples are motion with constant acceleration, harmonic motion, energy conservation, and applying SF = ma to a point particle.  A model consists of the following pieces:

1. the physical systems/situations where the model applies and vocabulary of involved objects, state variables, and agents(interactions)  involved.
2. specification of the independent and dependent (measurable) state variables that characterize the system and which the model interrelates
3. what idealizations and physical theories underlie the model and the resulting equations, representations
4. descriptions of the model and interpretation of its predictions as expressed in all various useful representations
5. the behavior/change in state, geometric, temporal, and interaction structure

Characteristics of Models: 

Name:  Each model must have a name 

Verbal Description:  A sentence or two

Motivation/Examples: Common physical situations where model good approximation

Assumptions and Limitations:

Preconcepts - concepts involved that should be known

PriorModels - models assumed known

New Vocabulary Separating Model from Environment - Structure of Models:
Internal constituents

External Agents

Interactions Considered

Assumptions and Approximations Key Descriptors Necessary

Internal objects

Description of State

Interaction(s), Agent(s) of Change Representations

Different ways to represent model Laws of Interaction

How does agent of change behave? Laws of Change

Often this is differential/integral Eq. (F=dp/dt)

And may be a special case (e.g. F=ma) 

A useful model fits many real situations to a good approximation.  Some useful models used by physicists to think about the physical include: motion with constant acceleration, the harmonic oscillator, the two level quantum system, Feynman diagrams and the Schwartzschield metric (which applies the law of General Relatively to find the warping of space-time due to a central sphere of matter).  A list of models of mechanics is available.