This paper applies the research method of fluid mechanics to analyzing such defective concepts as momentum, and puts forward a new definition and concept, such as the dense flow and dilute flow, free flow density, etc. 文章应用流体力学的研究方法,分析了几个有缺陷的概念,如动量等,并提出了新的定义和概念,如稠密流和稀薄流、自由流密度等。