Free-Body Diagrams
Core Sentence / 核心句
A free-body diagram isolates one chosen object and shows only the external forces acting on it.
中文理解：先选研究对象，再把外界对它的每个真实力画出来。
Procedure / 步骤
- isolate the object or system；
- identify every external interaction；
- draw one arrow for each actual force；
- label force type and agent；
- choose axes after seeing the geometry；
- resolve forces into components in equations, not as replacement force arrows。
Watch Out / 易错点
- velocity、acceleration、motion path 不是力；
- Newton’s third-law partner acts on the other object，不画在同一物体的 FBD 上；
- 分量是数学拆分，不是额外的力。