The momentum of the cluster as a whole would not change if there are no external forces on the system, 0 if the netsum, the total external force, is zero.
星云的动量,作为整体是不变的,若无外力作用于系统，如果外力总和为。
2
The net energy is the sum of the repulsive and the attractive.
净能量是排斥能，和引力能的总和。
3
And so the net, the sum of all forces on that object there is no net force on that wheel but there is a net torque, and that's why it's going to precess.