Final answer:
Momentum is conserved when two conditions are met: the mass of the system remains constant and no net external force acts on the system.
Step-by-step explanation:
Momentum is conserved when two conditions are met:
The mass of the system remains constant during the interaction, meaning that no objects are added or removed.
No net external force acts on the system during the interaction, which means that the total force acting on the system must be zero.
For example, if two billiard balls collide on a frictionless table, the momentum of the system (the two balls together) will be conserved as long as no external forces, such as friction or air resistance, are acting on them.
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