Applicable conditions for the law of conservation of momentum
1. The system is not subject to external force or the combined external force is zero;
2. Although the combined external force received by the system is not zero, when the internal force of the system is much greater than the external force, such as collisions and explosions, the momentum of the system can be regarded as approximately conserved;
3. In general, the system does not meet any of the above conditions, then the total momentum of the system is not conserved. But if the system meets any of the above conditions in a certain direction, the system's momentum is conserved in that direction.