A system is a collection of two or more objects. An isolated system is a system thatF is free from theinfluence of a net external force that alters the momentum of the system. There are two criteria for the presence of a net external force; it must be…

·         a force that originates from a source other than the two objects of the system

·         a force that is not balanced by other forces.

A system in which the only forces that contribute to the momentum change of an individual object are the forces acting between the objects themselves can be considered an isolated system.

Consider the collision of two balls on the billiards table. The collision occurs in an isolated system as long as friction is small enough that its influence upon the momentum of the billiard balls can be neglected. If so, then the only unbalanced forces acting upon the two balls are the contact forces that they apply to one another. These two forces are considered internal forces since they result from a source within the system – that source being the contact of the two balls. For such a collision, total system momentum is conserved.