动量守恒定律的适用条件是:一个系统不受外力或者所受外力的合力为零.当研究对象满足上述条件时,系统的动量整体守恒(即在各个方向上动量都守恒)且严格守恒.但是,在应用动量守恒定律处理实际问题时,常会遇到并不满足上述条件的三种特殊情形,下面分别进行讨论. The applicable condition of the law of conservation of momentum is that a system is free from external force or the resultant force of external force is zero. When the research object satisfies the above conditions, the momentum of the system is overall conserved (that is, momentum is conserved in all directions) and strictly conserved. When applying the law of conservation of momentum to deal with practical problems, we often encounter three special situations that do not satisfy the above conditions.