In my previous column (Physics Today, Physics Today 0031-9228 5710200411 https://doi.org/10.1063/1.1825251 October 2004, page 11 ), I discussed how assumptions about F and m give substance to the spirit of F = ma. I called this set of assumptions the culture of force. I mentioned that several elements of the culture, though often presented as “laws,” appear rather strange from the perspective of modern physics. Here I discuss how, and under what circumstances, some of those assumptions emerge as consequences of modern fundamentals—or don’t!

Ironically, it is the most primitive element of the culture of force—the zeroth law, conservation of mass—that bears the subtlest relationship to modern fundamentals.

Is the conservation of mass as used in classical mechanics a consequence of the conservation of energy in special relativity? Superficially, the case might appear straightforward. In special relativity we learn that the mass of a...

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