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.