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{td:align=center|bgcolor=#F2F2F2}*[Model Hierarchy]*
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h2. Keys to Applicability
{excerpt}This model is [generally applicable|generally applicable model] (assuming knowledge of the external forces and system constituents), but is especially useful when describing the momentum of systems where external forces are absent (system momentum will be constant) or estimating the force in a process that occurs in a very short time interval as in collisions (impulse will be easier to determine than force).{excerpt}
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h2. Assumed Knowledge
h4. Prior Models
* [Point Particle Dynamics]
h4. Vocabulary
* [system]
* [force]
* [impulse]
* [momentum]
* [velocity]
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h2. Model Specification
h4. System Structure
*[Constituents|system constituent]:* System is composed of [Point particles|point particle].
{note}Rigid bodies may be treated as point particles with positions specified by the center of mass positions of the rigid bodies when this model is used.{note}
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*[Interactions|interaction]:* Only [external forces|external force] need be considered, since [internal forces|internal force] do not change the system's momentum.
\\
h4. Descriptors
*[Object Variables|object variable]:* None.
{note}This model does not require that constituent masses be constant. Thus, mass is a state variable for this model.{note}
*[State Variables|state variable]:* Mass (_m{_}{^}j^) and velocity (_v{_}{^}j^) for each object or momentum (_p{_}{^}j^) for each object inside the system.
*[Interaction Variables|interaction variable]:* External forces (_F{_}{~}ext,k~) or, alternately, impulses may be specified (_J{_}{~}ext,k~).
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h2. Model Equations
h4. Relationships Among State Variables
If not directly given, momenta can be obtained using the definition:
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