Re: New version of a relativity FAQ

harry wrote:
"Tom Roberts" <tjroberts137@xxxxxxxxxxxxx> wrote in message news:yEr8k.19594$co7.11474@xxxxxxxxxxxxxxxxxxxxxxx
The underlying equation, valid in all cases of Newtonian mechanics, is F=dp/dt (there's a similar equation for SR and GR).

The IDENTICAL equation in SR (valid in all cases) was probably part of nearly every physics course in the eighties.

It is not "identical" -- the symbols have different meanings (3-vectors => 4-vectors, time coord. => proper time).


Taking some piece of the time derivative, christening it as a "force" and moving it to the left hand side and re-labeling that side with a new type of "f" does nothing useful, and is practically guaranteed to confuse people [#]. Note that in EVERY case the "force" involved is not directly measurable (measuring a force directly requires a two-terminal device, but such "forces" never have a second reference point).

Interesting! I haven't thought about that, but at first sight that is only true for gravity.

Not true. Consider trying to "measure" the "centrifugal force" on the popcorn stand next to a rotating carousel, in the carousel's coordinates. That "force" is truly enormous (mega Newtons or more), but the booth attendant is completely unaware of it. It is, of course, a pure artifact of the rotating coordinates.

Or more to the point, how could one possibly measure "v*dm/dt" as a FORCE? To what would one connect the two terminals of the sensor?


Tom Roberts
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