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{excerpt}The object or the group of objects whose motion is being described using a [model|model].  Aand the environment influences the system only through interactions that generate forces.  More generally, the part of the universe upon which attention is concentrated, thereby creating an internal and external region - the later called the environment.  Systems are abstractions of reality that have structure, variables to describe their state, and structural and functional relationships among their constituents. 
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h3. System and Environment

h4. Definitions

In each physics problem, some object or objects will have their motion described by a chosen [model|model].  That object or objects then form the system that the [model] applies to.  There will usually be other objects that influence the motion of the system, but whose motion is not modeled.  Those other objects constitute the [environment|environment].  The [environment|environment] will influence the system only through interactions that generate [forces|force], [work|work] or [torques|torque] on the system.


h4. Examples


h5. Block on Inclined Plane

In a typical inclined plane problem where a box slides down the plane, the box would be the logical system to consider.  The plane and the earth will be the [environment|environment].  The [environment|environment] affects the system through [interactions|interaction].  In this example, the earth affects the system through the force of [gravity|gravity (near-earth)], and the plane affects the system through [forces|force] like the [normal force|normal force] and (possibly) [friction|friction].

h5. Atwood's Machine

In a typical [Atwood's Machine] problem where two boxes are held together by a rope that passes over a massless pulley, the boxes will generally be treated as _two separate systems_.  The relevant [model] ([Point Particle Dynamics]) is most easily applied to the two boxes separately.  For each box, the relevant pieces of the [environment|environment] are the rope ([tension|tension]) and the earth ([gravity|gravity (near-earth)]). If the pulley is massive, it may be treated as a third system with the [tension|tension (near-earth)] from the ropes as an [interaction|interaction] (in this case represented as [torques|torque (single-axis)]).

h3. System Specification

In solving a mechanics problem, the system is specified by stating which objects will constitute the system, whether those objects will be treated as [point particles|point particle], [rigid bodies|rigid body], or [massless objects|massless object].  Note that the environment is not part of the system.  The environment is described through its [interactions|interaction].
{note}Some problems (like the Atwood's Machine described above) may require more than one system to be considered.
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