Common Forces
Core Sentence / 核心句
A force is an interaction, so every force label should identify both the interaction type and the object exerting it.
中文理解：力不是孤立物体自带的属性；分析时要问“谁对研究对象施加什么力”。
Force Map / 常见力
- weight: ，由 Earth 施加；
- normal force: perpendicular to a contact surface；
- friction: opposes relative sliding or its tendency；
- tension: pulls along a taut string or cable；
- spring force: restoring interaction from deformation；
- resistive force: opposes motion relative to a fluid。
Watch Out / 易错点
Normal force 不自动等于 ；static friction 不自动等于最大值；摩擦力反对接触面的相对滑动，不必反对质心速度。