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non-inertial frames




A R Marlow wrote in part
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And it is definitely bad physics to try to apply Newton's laws in any frames
other than inertial frames; of course, you can
transform to any noninertrial frame you wish -- just don't expect that
Newton's laws will be valid in such frames (or Einstein's laws, for that
matter, either) with detectable corresponding forces, i. e., in the terms
introduced above, forces with detectable effects.
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I may be misunderstanding part of what A R is saying but it allows me the
chance to bring up a related topic.

Operationally one measures forces on a particle by measuring its
acceleration and then applying Newton's laws. This is implicitly true even
if you are doing it by balancing unknown forces against known forces,
because the balancing is really measuring a zero acceleration. I wouldn't
say that it is "bad" physics to do this in a non-inertial frame. One just
has to realize that if you do do it in a non-inertial frame you'll infer the
existence of forces for which there is no identifable physical agent that is
the cause. These are the so-called "fictitious" forces, centrifugal,
coriolus and azimuthal forces being the most famous.

This brings up a symantical objection that I have with the
typical discussion of this topic. Namely, the use of the term
"fictitious" force. I think the term was originally used to simply indicate
that there is no physically identifiable agent that is the cause of these
forces that are "operationally" present in non-inertial reference frames.
But it tends to have the meaning of "bad physics" because they aren't real,
as often presented.

I much prefer the term "kinematical" force to "fictitious". My
arguement is simply that there is nothing fictitious about them.
When I'm riding in a car in the front passenger and the driver
makes a sharp left turn too fast. There is nothing fictitious about
the bruise on my right arm as I get slammed into right door handle. In the
carnival rides where the people line the inside wall of the rotating "tin"
can and the floor drops out from under them, there is nothing fictitious
about the force they feel in that non-inertial frame of reference which
keeps them from sliding down to the floor (actually its the normal forces
present in my examples which are balancing the "kinematical" forces that
cause the bruise and account for the frictional force holding up the
person). In other words, I'm saying that we actually "feel" these forces,
therefore the term "fictitious" is somewhat of a misnomer.

Joel
rauberj@mg.sdstate.edu