I’m currently involved in a debate with a few folks regarding the validity of the Conservation of Momentum principle. One demonstration of its failure was dropping a book. The book begins with no momentum, gains momentum, then looses it and again has none (when it hits the floor). Of course, a rebuttal to this demonstration was to note that, even though we cannot readily detect it, the earth is in fact simultaneously being drawn up towards the book. This results in a net zero momentum for all points in time and momentum is conserved. However, this rebuttal assumes some kind of frame of reference independent of the Earth and the book. What if the reference is the Earth? For the book then, we see a positive and negative change in momentum. For the Earth, however, there is no change in momentum as there is obviously no change in velocity for the Earth relative to the Earth. This means there is a production and destruction of momentum within the book! Interesting… All of this raises questions regarding valid frames of reference. Certainly something needs to be said regarding taking the book and Earth as the system and the frame of reference as the Earth – is that kind of system definition “legal” ? If it is, there’s certainly a problem… I welcome input.