因为总有外力,作用于系统。
The net force is ultimately the sum of the external force and all the internal forces.合力是最终的总和,外力与所有内力总和。
Momentum is conserved, because there was no net external force, so now kinetic energy is also conserved.动量是守恒的,因为它不受外力,因此动能也是守恒的。