This 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).

• Point Particle Dynamics

• system
• force
• impulse
• momentum
• velocity

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.

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