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Conservation of Momentum

The IMPULSE AND MOMENTUM law is certainly a rather simple transformation of Newton's SECOND LAW; in fact one may be tempted to think of it as a trivial restatement of the same thing. However, it is much simpler to use in many circumstances. The most useful application, surprisingly enough, is when there is no external force applied to the system and therefore no impulse and no change in momentum! In such cases the total momentum of the system does not change. We call this the LAW OF CONSERVATION OF MOMENTUM and use it much the same as Descartes and Huygens did in the days before Newton.^11.4

Momentum conservation goes beyond Newton's FIRST LAW, though it may appear to be the same idea. Suppose our "system" [trick word, that!] consists not of one object but of several. Then the "net" [another one!] momentum of the system is the vector sum of the momenta of its components. This is where the power of momentum conservation becomes apparent. As long as there are no external forces, there can be as many forces as we like between the component parts of the system without having the slightest effect on their combined momentum. Thus, to take a macabre but traditional example, if we lob a hand grenade through the air, just after it explodes (before any of the fragments hit anything) all its pieces taken together still have the same net momentum as before the explosion.

The LAW OF CONSERVATION OF MOMENTUM is particularly important in analyzing the collisions of elementary particles. Since such collisions are the only means we have for performing experiments on the forces between such particles, you can bet that every particle physicist is very happy to have such a powerful (and simple-to-use!) tool.