任何一个质点系中都存在着一个称为质心的特殊点,它的运动与内力无关,只取决于外力.当需将质点组处理成一个质点时,它的质量就是质点组的总质量,当需要确定质心的运动时,就设想把质点组所受的全部外力集中作用在质心上,对于质心的运动有以下二级结论:当一个系统受到合外力为零时,系统的总动量守 There is a special point called a centroid in any particle system. Its motion has nothing to do with internal forces but depends on external forces. When the mass group needs to be processed into a mass point, its mass is the total mass of the mass group. When it is necessary to determine the movement of the centroid, it is assumed that all the external forces exerted on the particle group are concentrated on the center of mass. For the movement of the mass center, there are two secondary conclusions: when a system is subjected to zero external forces, the total momentum of the system is maintained.