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h1. System

{excerpt}The object or the group of objects whose motion is being described using a [model|model].  {excerpt}

h3. System versus 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.
{note}

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