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Re: non-inertial: part II

On Fri, 26 Apr 1996, Rauber, Joel Phys wrote:

...
...
That is the reason for my not liking the word "fictitious" because there is
a "real" effect. In this vain I'm more comfortable with "apparent force";
I also like the suggestionof "inertial force", although I understand the
objection that was raised against that. I still prefer so far the term
"kinematical force", since the effect is a kinematic effect.
...

What is the problem with using the proper kinematical name --
acceleration -- for what you agree is a kinematic effect? Could you be
very specific about the objections you feel toward this very real term
for the very real kinematic effect being described?



2) I think Marlow must have a contradiction with above definition. On the
one hand the gravitational force involves an interaction between objects and
would be called a "real" force. But on the other hand Marlow's postings
have told us that the gravitational force is a fictional force and not a
"real" force. Which is it? It can't be both?


You have misinterpreted what was said about gravitation. Gravitation is a
very real interaction -- it is simply not the acceleration it was being
confused with before the 20th century. Once this confusion was cleared
away by Einstein's thought experiments on freefall frames, gravitation was
revealed as the very real grip spacetime exerts on matter, dictating how
matter is to move in the absence of electromagnetic, weak & strong nuclear
and any other interactions yet to be discovered.


... Elsewhere (a couple of times) Marlow wrote in two different posts:

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.

just don't confuse everything by renaming
the accelerations "forces," and then expecting everyone to keep doing
the mental gymnastics necessary to keep reminding themselves that these
"forces" don't have the properties that normally define forces.

These fictitious forces to have all the effects and properties I normally
associate with forces.

We'll just have to agree to disagree here on the meaning of force. But
have you given any usable definition of force that distinguishes it from
other physical concepts, such as the kinematical concepts of position,
velocity, acceleration?

... (I'm not using the pressure sensor definition of
force, see (non-inertial: part I)). ...

Nor apparently are you using any of the other normal means of
specifying forces.


... Namely, I measure an acceleration
associated with them, by plotting the position of my object in the frame of
reference (non-inertial of course, that's where I'm doing the measurements).

This makes your notion of force a purely kinematical notion. Why waste the
perfectly good term "force" on concepts that already have perfectly good
names?


The work-energy theorem seems to work, namely when that force is the only
force present, the work done by it seems to equal the change in the kinetic
energy of the object.

So you are saying, in the context of the examples that you brought up
before (a sharply turning car, people in a centrifuge), that besides the
work and energy supplied by the real inward directed interaction exerted
on you by the door that causes bruises, there is an equal amount of work
and energy supplied by the outward directed "centrifugal interaction"
that acts on you to prevent you accelerating relative to the car? That
is a very strange doubling of energy that I have not heard of before.
You ought to be able to patent that and make some money on it. In the
present state of things, I wouldn't be at all surprised if someone in the
patent office might not accept the whole idea.


... And from all of this, all of mechanics seems to
follow. (I hope this last isn't too rash of a statement, but I'm sure it
will be corrected if need be.)
...

A very strange version of mechanics, which I wouldn't depend on to give
consistently valid results.

...
Its for this reason that the idea of treating these terms as real forces is
useful. It in fact allows me to avoid a lot of mental gymnastics to be able
to simply say that these are forces measured in this particular frame of
reference, along with all other measurements made in that frame.


How do you avoid getting the extra undetectable work and energy, and what do
you do about Newton's Third Law? Just throw it away?

Marlow has reminded us that we need a precise definition of Force, I agree
and would like to know if Marlow has one other than the pressure sensor
definition that he has given.

The best I could do was already given, in terms of springs, standard
deformations, piezo effects, and whatever else clever experimenters can
come up with that will be accepted by the physics and astronomy
community as valid measurements distinguishing force from the other
concepts of mechanics.

... (I might add that I haven't given a precise > definition, yet)


Agreed. Nor have you said what you have against the term "acceleration"
for kinematical effects measured in various reference frames.


And as long as it is recognized that they will not do any of the things
expected of forces in ANY reference frame. Might as well declare
counterfeit money to be real in certain circumstances, and expect not to
confuse anybody.

I can't resist responding to this analogy. Counterfeit money quite often
spends just as well as real money and therefore is the same as money when it
spends.


Here I have to agree fully with you -- my analogy limps badly. Counterfeit
money is much more real than the fictitious forces being invented in
this thread.

I might add, the price I pay with my viewpoint, is that I have to
distinguish between two kinds of forces when I transform between inertial
and non-inertial frames, kinematical forces and physical forces (the ones
that have a physically identifiable agent responsible for them). But Marlow
has to distinguish between two kinds of accelerations when he transforms
between two different frames of reference,

Sorry, that's just not true. Acceleration is a clearly defined kinematical
concept relative to a reference frame, and there is only one type.
(Acceleration does NOT include in its definition or detection any reference
to what causes the acceleration.)

those caused by forces and those
caused by kinematical motion of the non-inertial frame relative to the
inertial frame.


Sorry, but we'll just have to agree to disagree on that.

I think the mental gymnastics is the same for both viewpoints
(mathematically it amounts to, do I divide by the mass or not on both sides
of the equation, which is why we agree on the numerical answers to mechanics
problems, at least when we do the mathematics correctly). However,
operationally the viewpoints differ, From measuring the motion of an object
in a frame of reference (I don't know a priori if its inertial or
non-inertial)

That's very true, and until you determine this a posteriori, you had better
not apply Newton's laws, because they will give false results (spurious
mass calculations, spurious energy results, for example) when applied in a
non inertial reference frame.


I can't seperate out the acceleration into its two components,
as Marlow says I must

I have not said anything of the sort. You are the one who has introduced
two kinds of acceleration -- I only recognize one kind of acceleration.
Apply Newton's laws relative to an inertial frame, decide which of the
terms that result you would like to compute, bring everything else over
to the other side of the equation, what could be simpler.


if I'm to understand the motion in terms of forces. My
viewpoint says I don't need to perform this seperation for my two kinds of
force in order to understand the relationship between the forces and the
acceleration of my object, I don't need to because they both contribute to
the acceleration of the object in the normal manner and therefore the two
forces get treated on the same footing. This allows me to not have to
decide whether or not the measurements were performed in an inertial or in a
non-inertial frame of reference (I may not know and furthermore may not be
able to know!!!)


If you do not know whether you have an inertial or a noninertial frame,
you have no right to be applying Newton's laws! They simply give bad
results in a noninertial frame.

Students, PLEASE do not try this at home (or in homework), or you will
continually be getting wrong results, along with some right ones --
but that's the problem: you will be so confused you won't be able to
tell which are which.


A. R. Marlow E-MAIL: marlow@beta.loyno.edu
Department of Physics PHONE: (504) 865 3647 (Office)
Loyola University 865 2245 (Home)
New Orleans, LA 70118 FAX: (504) 865 2453