The instructor draw a random diagram showing an arbitrary number of interacting (e.g. gravitationally) entities (e.g. stars, asteroids, etc.) represented as numbered dots. The student is asked to perform the following tasks:

- Draw a system boundary such that the system experiences a net external force. Name each entity that contributes to this net external force. Will the momentum of this chosen system be conserved? How do you know?
- Draw a different system boundary such that the system experiences no net external force. Will the momentum of this chosen system be conserved? How do you know?

The diagram may be as random as you wish, and certainly need not be the same for every student.

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