二力杆指的是不计重力、两端可以自由转动的轻杆.我们知道,杆既可以发生拉伸或压缩形变,也可以发生弯曲或扭转形变,因此杆的弹力不一定沿杆的方向.但是,二力杆两端的弹力必定沿杆的方向,否则杆不能平衡. 例1 如图1,不计重力的带有光滑滑轮的细杆OB可绕O点在竖直平面内自由转动,跨过滑轮的细绳吊一重物 The second lever refers to a light rod that does not depend on gravity and can be freely rotated at both ends. We know that the rod can undergo either tensile or compressive deformation, and can also undergo bending or torsional deformation, so the elasticity of the rod does not necessarily follow the direction of the rod. However, the elastic force at both ends of the two-bar must be in the direction of the bar, otherwise the bar cannot be balanced. Example 1 As shown in Fig. 1, the OB with smooth pulley, excluding gravity, can rotate freely around the O point in the vertical plane. Pulley string hanging a heavy object