对于一元流动，控制体的动量改变由两部分组成。For univariate flows, the momentum change of the control body consists of two parts. 过程不稳定而引起的动量改变量The change in momentum caused by an unstable process 由I-I’位置移至II-II’位置后相应的动量改变量The change in momentum corresponding to the change in position from i-i ’to ii-ii’，即系统由位移增加的动量与减少的动量之差That is, the difference between the momentum increased by the displacement and the momentum decreased by the displacement