📚 A.4 Rigid Body Mechanics | A.4 刚体力学
A rigid body is an idealized solid object in which the distance between any two given points remains constant regardless of external forces. In this topic we study how forces cause rotation, how objects balance, and how energy and momentum are conserved in rotational motion. Mastering rigid body mechanics is essential for understanding everything from bridges and levers to spinning planets and figure skaters.
刚体是一种理想化的固体，其上任意两点间的距离在外力作用下保持不变。本主题研究力如何引起转动、物体如何平衡、以及转动运动中能量和动量如何守恒。掌握刚体力学对于理解从桥梁、杠杆到旋转行星和花样滑冰运动员等各种现象至关重要。
1. Torque – The Turning Effect of a Force | 力矩——力的转动效果
g an angle θ to the position vector, is given by the magnitude τ = F r sinθ. The SI unit of torque is the newton metre (N·m).
力矩 (τ) 是衡量力使物体绕轴转动能力的物理量。它不仅取决于力的大小，还取决于力到转轴的距离以及力的作用角度。力 F 作用在距支点 r 处，力与位置矢量的夹角为 θ，产生的力矩大小为 τ = F r sinθ。力矩的国际单位是牛顿·米 (N·m)。
τ = F r sinθ
e lever arm (θ = 0°), it produces no torque. The direction of torque is given by the right‑ ur thumb points along the axis of rotation, indicating the direction of the torque vector.
如果力垂直于力臂 (θ = 90°)，力矩最大：τ = F r。如果力平行于力臂 (θ = 0°)，不产生力矩。力矩
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