Re: non-inertial frames

["A. R. Marlow" <marlow@beta.loyno.edu>]

On Wed, 24 Apr 1996, Rauber, Joel Phys wrote:

> ...
> 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 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 ...

Of course there is nothing fictitious about the real inward (centripetal)
forces that produce the bruise or produce the friction that keeps you from
falling to the floor of the centrifuge.  This is what has been said from 
the beginning of this thread, and these very real forces will produce the 
same bruises no matter what reference frame you decide to view things from.
What IS fictitious in the two cases you describe is the creation of an extra
OUTWARD DIRECTED (i. e., centriFUGAL) force, acting on the person and 
somehow balancing the real inward directed force.  It is this psychological
creation of an outward directed force that causes no bruises nor any other
effects that is termed fictitious, for the very good reason that it is a
fiction invented solely to explain why we are not accelerating relative to
the side of the car or the centrifuge.  The ONLY force acting on the persons
in the examples you cite is the inward directed force of the wall on the 
person's skin  --  this is what they feel, this is what causes the bruises,
and this is what would be measured by pressure sensors. Any other forces are
fictional, and do not have any of these real effects.  Introducing other 
forces as real makes a misleading and inaccurate mockery of physics.

> ...(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).  

You have it correct here, if you mean that it is the real inward directed 
centriPETAL forces that cause the bruises, Etc., but you also have them
somehow balancing the "kinematical" forces.  What are these outward
directed centrifugal "kinematical" forces doing?   The answer is: nothing!
They don't exist, which is why they cause no effects, which is why they are
called fictitious. 
 
> ... In other words, I'm saying that we actually "feel" these forces, 
> therefore the term "fictitious" is somewhat of a misnomer.

Sorry, but the term "fictitious" is not at all a misnomer when applied to
the psychologically created outward directed centrifugal force in the 
examples you cited.  We do not feel these forces.  What we feel is the 
inward directed (i. e., centripetal) pressure on our skin, and this is 
all that exists.  

A. R. Marlow                          E-MAIL:  marlow@beta.loyno.edu
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