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RE: non-inertial frames



Your arm may be bruised, but that IS from a real force, the force of the
door handle that stopped you from moving off along the straight line path
you were taking (N1). In the rotor ride, again there IS no outward force,
only the inward force of the wall--again keeping you from moving in a
straight line. The problem with using human sensations 'to define' forces
in these situations, is that we have come to experience MANY forces
BACKWARDS! That is, our daily experience of the gravitational force (or
weight) IS NOT the actual downward force, but rather the upwards forces
that tend to balance gravity. The force of the floor upwards, or our
chair upwards is what one actually experiences. We become so used to this
that IF that upwards force is removed--we believe the downwards force is
also gone--hence the 'weightless' sensation in falling (or in orbital
motion). Conversely, given a situation where ONLY the 'upwards' force
exists, we now believe there IS a downwards force--such as in a rotating
space station or an accelerating rocket--or the rotor ride, seat back in
an accelerating car, etc. These 'experienced' forces ARE NOT there and
are therefore 'fictitious'. In fact, that knowledge coupled with the
sensation of such forces CAN be used to determine (under appropriate
circumstances) that one IS in an accelerating frame.

----------
From: Rauber, Joel

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