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The In mechancis, the system is the object or the group of objects whose motion is being modeled, and 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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In each physics problem, some object or objects will have their motion described by state variables like position, velocity, energy, that being 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
Block on Inclined Plane
In a typical inclined plane problem where a box slides down the plane, the box will would be the logical system that we to 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.
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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 are the rope (tension) and the earth (gravity). If the pulley is massive, it may be treated as a third system with the tension from the ropes as an interaction. However, the pulley, rope, and two boxes could be considered one system and described using the laws of angular momentum.
System Specification
In solving a mechanics 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, and stating the state variables that describe the constituents of the system. Note that the environment is not part of the system. The environment is described through its interactions.
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