Momentum theorem is one of the universal theorems of dynamics. The content is that the increment of momentum of an object is equal to the impulse of the combined external force, that is, ft = mδ V, that is, the vector sum of all external force impulses. Its definition is: if a system is free from external force or the vector sum of external force is zero, then the total momentum of this system remains unchanged. This conclusion is called the law of conservation of momentum.