The video shows an animated billiards experiment in which a cue ball strikes a glued-in-place eight-ball. Which of the following explains why the momentum of the eight-ball is conserved?

Choose 1

The collision is elastic.
The cue ball rebounds with the same speed it had coming in.
The eight-ball is an isolated system.
The "glue force" cancels the collision force.

Part B

Which statement must be true for the momentum of a system to be conserved?

Choose 1

The net external force on the system is zero.

The internal forces sum to zero.

There are no external forces acting on the system.

The net external force on the system is non-zero.