物体在粗糙平面上滑动或有滑动趋势时,所受的摩擦力沿着接触面,弹力可能偏离物体的质(重)心,拉力或推力的作用线也未必通过物体的质心,……。可是,我们却常常把它们和重力一样,都画在物体的质心上。为什么可以这样做呢?下面简要讨论三种情况。 When an object slides on a rough surface or has a tendency to slide, the frictional force experienced along the contact surface may deviate from the mass (heavy) heart of the object, and the line of action of the pulling force or thrust may not necessarily pass through the mass center of the object. However, we often draw them like gravity on the center of mass of the object. Why can this be done? The following briefly discusses three situations.