system

The object or the group of objects whose motion is being considered.

System and Environment

Definitions

In each physics problem, some object or objects will have their motion modeled. 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.

Examples

Inclined Plane

In a typical inclined plane problem where a box slides down the plane, the box will be the system that we consider. The plane and the earth will be the environment. The environment affects the system through interactions. In this example, the earth affects the system through the force of gravity, and the plane affects the system through forces like the normal force and (possibly) friction.

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 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 are the rope (tension) and the earth (gravity).

System Specification

In solving a problem, the system is specified by stating which objects will constitute the system, and stating whether those objects will be treated as point particles or rigid bodies. Note that the environment is not part of the system. The environment is described through its interactions.