动量守恒定律和角动量守恒定律辨析

Analysis on the Conservation Laws of Momentum and Angular Momentum

辨析了系统动量守恒和角动量守恒的适用条件,旨在加深学生对动量守恒定律和角动量守恒定律的理解.以3个例题为例,根据实际条件选择参数,计算了系统的内力和外力的数值,分析了系统所受合外力（外力力矩的矢量和）远小于系统内力（内力矩）的条件.结果表明,只定性地根据物体的质量很小就得出物体的重力远小于系统内力的结论是不充分的,质量很小的物体的重力是否远小于系统的内力还取决于碰撞前物体的速度;而外力力矩的矢量和是否远小于内力矩不仅取决于碰撞前物体的速度,还取决于碰撞的位置.

This paper analyzes the application conditions of the conservation laws of momentum and angular momentum, helping students to come to a better understanding of these two laws. By analyzing three examples, and choosing parameters, the values of internal force and resultant external force are calculated, and the conditions that resultant external force （resultant external moment） is far less than internal force （internal moment） are analyzed. The results show that it is imprecise to come to the conclusion that the gravity of the object with very small mass is far less than internal force, whether the gravity of object is far less than internal force also depends on the preceding collision velocity of the object. Whether the resultant external moment is far less than internal moment depends on not only preceding collision velocity of the object but also the collision location.