4.3.B—Describe how the selection of a system determines whether the momentum of that system changes

Describe how the selection of a system determines whether the momentum of that system changes.

• Momentum is conserved in all interactions.
• If the net external force on the selected system is zero, the total momentum of the system is constant.
• If the net external force on the selected system is nonzero, momentum is transferred between the system and the environment. BOUNDARY STATEMENT AP Physics C: Mechanics only expects students to quantitatively analyze collisions and interactions in one or two dimensions. Three-dimensional collisions may be analyzed qualitatively.