• For any interaction involving a system that experiences no external forces, the total momentum before the interaction is equal to the total momentum after the interaction

During an interaction between two objects in a system that experiences no external forces, the change in momentum of one object is equal in magnitude but opposite in direction to the change in momentum of the other object: $m_1\overrightarrow v_{i1} + m_2 \overrightarrow v_{i2} = m_1\overrightarrow v_{f1} + m_2 \overrightarrow v_{f2}$

• Interactions within a system can be categorized as collisions, in which two or more objects come together, or explosions, in which a single object or collection of objects separates