The object or the group of objects whose motion is being described using a model.

System versus Environment

In each physics problem, some object or objects will have their motion described by a chosen 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. The environment will influence the system only through interactions that generate forces, work or torques on the system.

System Specification

In solving a mechanics problem, the system is specified by stating which objects will constitute the system and how those objects will be idealized. In introductory mechanics, objects can always be idealized to meet the criteria for inclusion in one of the following four categories:

  • Point Particle: An object that has no internal structure, and no physical size. Also commonly called a point mass.
  • Rigid Body: An extended object which does not change shape.
  • Massless Object: An object that is treated as having no mass.{
  • Infinitely Massive Object: Ideally, an object which is the result of letting the mass approach the limiting value of infinity. In practice, an object whose mass so far exceeds those of any others in the system that its mass may, for practical purposes, be taken as that limiting case.

Some problems solved using angular momentum may involve a person or object changing shape to alter their/its moment of inertia. In this case, the person or object will often be idealized twice, as two different rigid bodies (the "before configuration" and the "after configuration"). This is permissible because a conservation process does not require detailed information about the intermediate stages where the person or object is actually changing shape.

If the intermediate stages of a shape-changing process are of interest, a common practice is to idealize the relevant person or object as a collection of point particles and rigid bodies. A person's arm, for example, might be idealized as two thin cylinders attached together (one for the upper arm, one for the forearm).