In what cases can the law of conservation of momentum be applied?

The momentum conservation law can be applied if:
– external forces do not act on the bodies of the system;
– external forces act on the bodies of the system, but their sum is equal to zero:
– the sum of the external forces is not equal to zero, but the sum of the projections of the forces in some direction is equal to zero;
– external forces are much less than internal forces.

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