Marlow wrote:
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.. . . In both Newtonian
physics and Einsteinian physics there exists a privileged class of frames
-- the inertial frames --
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I do not argue with the above, and what he says below gets a partial answer
to Marlow's question of mentioning inertial reference frames when first
encountering Newton's 1st and 2nd laws.
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and only particles not accelerating relative
to such frames can be said to be force free. The acid test, then, for a
*real* force is "Does it cause deviation from local inertial motion?" If
it does it is a force; if it doesn't it is not a force. And this holds in
both Newtonian and Einsteinian physics.
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The question I have is this: I'm watching a particle and its motion; how do
I know whether or not its motion is a deviation from local inertial motion?
Put more simply, "how do I know if the frame of reference I'm using for
measurements is inertial?", I have to know this before (and independently)
to decide if a measured acceleration indicates a force that passes your
acid test or not.
I can certainly say, my frame of reference is inertial with respect to the
star frame (let's rather say the far away galaxy frame); but that doesn't
resolve the question, as I don't know if the far away galaxy frame is truly
inertial or not.
It strikes me that Bowman's attitude towards forces makes the need for this
knowledge moot.