The concept of force, as we have seen, defines a culture. In the previous columns of this series (Physics Today, October 2004, page 11, and Physics Today, December 2004, page 10.) I’ve indicated how F = ma acquires meaning through interpretation of—that is, additional assumptions about—F. This body of interpretation is a sort of folklore. It contains both approximations that we can derive, under appropriate conditions, from modern foundations, and also rough generalizations (such as “laws” of friction and of elastic behavior) abstracted from experience.
In the course of that discussion it became clear that there is also a smaller, but nontrivial, culture around m. Indeed, the conservation of m for ordinary matter provides an excellent, instructive example of an emergent law. It captures in a simple statement an important consequence of broad regularities whose basis in modern fundamentals is robust...
