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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

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.

h3. 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*|point particle]: {excerpt-include:point particle|nopanel=true}
* [*Rigid Body*|rigid body]: {excerpt-include:rigid body|nopanel=true}
* [*Massless Object*|massless object]: {excerpt-include:massless object|nopanel=true}
* [*Infinitely Massive Object*|infinitely massive object]: {excerpt-include:infinitely massive object|nopanel=true}

{note}Some problems involving [angular momentum|Angular Momentum and External Torque about a Single Axis] 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 modeled _twice_, as two different [rigid bodies] (the "before configuration" and "after configuration").  This is permissible because a conservation process does not involve the intermediate stages where the person or object is actually changing shape. {note}
{info}If the intermediate stages of a shape-changing process are of interest, a common practice is to model the relevant object as a _collection_ of point particles and rigid bodies.  A person's arm, for example, might be modeled as two thin cylinders attached together (one for the upper arm, one for the forearm).{info}

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