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{td:align=center|bgcolor=#F2F2F2}*[Model Hierarchy]*
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h2. Keys to Applicability
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).
* 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).
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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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