Conservation of Linear Momentum
Core Sentence / 核心句
If the net external impulse on a chosen system is zero, its total linear momentum is conserved.
Watch Out / 易错点
守恒属于系统；所有 velocity 必须来自同一 inertial frame。
If the net external impulse on a chosen system is zero, its total linear momentum is conserved.
守恒属于系统；所有 velocity 必须来自同一 inertial frame。