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Introduction to the Model

Description and Assumptions


This model is generally applicable (assuming knowledge of the external forces and system constituents). The model 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 such as collisions (impulse will be easier to determine than force).

Learning Objectives

Students will be assumed to understand this model who can:

Relevant Definitions


S.I.M. Structure of the Model

Compatible Systems

The system must be effectively composed of point particles, though rigid bodies may be treated as point particles with positions specified by the center of mass positions of the rigid body when this model is used.

Relevant Interactions

Only external forces need be considered, since internal forces do not change the system's momentum.

Laws of Change

Mathematical Representation
Differential Form


Integral Form


Diagrammatic Representations

Relevant Examples

Examples Involving Constant Momentum
Examples Involving Impulse
Examples Involving 1-D Collisions
Examples Involving 2-D Collisions
Examples Involving Elastic Collisions
Examples Involving Totally Inelastic Collisions
Examples Involving Continuous Momentum Flux
All Examples Using this Model


Click on the image to learn a handy rule of thumb for pool shots.




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