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.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.