在动量守恒系统中,由于相互作用的内力大小相等、方向相反,所以系统中内力的矢量和为零。但一对相互作用的内力的总功并不一定为零。通过分析系统中相互作用的内力的性质、做功情况,就可以在题设条件内建立起系统的动能和某种性质的内力相 In the system of momentum conservation, the vector sum of the internal forces in the system is zero because the internal forces of the interaction are equal in size and opposite in direction. But the total work force of a pair of interacting internal forces is not necessarily zero. By analyzing the nature of the internal forces interacting in the system and doing the work, we can establish the kinetic energy of the system and the internal forces of a certain nature