高中物理教材指出:“当系统不受外力作用或所受外力的合力为零,这个系统的动量就保持不变。”这里,教材对动量守恒的条件的阐述是十分明确的;但由于系统不受外力作用的情况是一种理想情况,即使是系统所受合外力为零的情形实际应用中也是极少的。所以在处理实际问题时,如果系统 High school physics textbooks point out that: “When the system is not affected by external forces or the external forces involved are zero, the momentum of this system will remain unchanged.” Here, the elaboration of conditions for the conservation of momentum in the textbook is very clear; but The situation that the system is not affected by external forces is an ideal situation, and even if the external forces involved in the system are zero, the actual application is very rare. So when dealing with practical problems, if the system