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It is important to note that the principle of conservation of momentum is not a new law of nature — it has been shown in this section of Understanding Physics to follow automatically from Newton's laws of motion. Care should be taken in its application, however, and the following two important points should always be kept in mind.

(1) The physical quantity that is conserved is the total momentum of the system of particles involved

  (recall that total momentum is a vector quantity)

(2) Momentum is only conserved in the absence of a net external force on the system.

The principle should become more concrete after some examples have been studied (Sections 6.4 and 6.5).