Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Non-inertial: part III



On Mon, 29 Apr 1996, Rauber, Joel Phys wrote:
Let me summarize the conclusions I've made so far (note: this would
not agree entirely with the conclusions Marlow makes from the discussion).

Everyone agrees that there are effects when you make measurements
in a non-inertial frame of reference. Marlow wishes to only call them
accelerations,

Sorry, but we must be clear at this crucial point: I do not propose
calling ALL effects measured relative to a noninertial frame "accelerations."
I only propose calling the kinematic quantities normally known as
accelerations "accelerations."

... I see no point in being so dogmatic about that ...

There's nothing dogmatic about it, it's simply the name that's been
given to those kinematical quantities. It's no more dogmatic than
insisting that comets be called comets and not pseudo-meteors just
because so many people confuse comets with metoers.


... and feel quite
comfortable with multiplying by mass and call the resulting entity a
"kinematical" force. I feel uncomfortable with using the term "fictitious"
to describe the resulting entity;

So do I, and I would never use the term fictitious of such entities; they
are totally real accelerations, multiplied by totally real masses. They are
only fictitious as forces.

... the reason being the effects are real as
measured in the non-inertial frame of reference and the word "fictitious"
has connotations of unreality.

Agreed.

... I don't think one can make measurements in
the non-inertial frame that can distinguish these terms

If your still talking about the distinctions between measuring forces
and measuring accelerations, I can only refer you to all that has been
previously posted on this.

... as their introduction is
precisely so you can use the 2nd law and 3rd law (and hence all
of mechanics) in non-inertial frames and analyze things consistantly and get
correct answers.

Once again, standard mechanics says that Newton's laws simply do not
apply in noninertial frames and there is no attempt to make them apply in
such frames. You do not get correct answers: for example, you calculate
nonzero work done by "centrifugal force" in the examples discussed
previously, I calculate zero work done by such a "force." Both answers
obviously cannot be correct.


...
So, one way to define a force then is operationally, by observing
the objects acceleration. If I stop there then I believe Marlow is
absolutely correct in saying I've reduced dynamics to kinematics. But there
is nothing wrong with that and "my" physics is perfectly consistent in doing
that, I get the same answers that Marlow gets to analysizing problems;

Sorry, but you do not get the same answers for important dynamical
quantities, as I have already pointed out in previous posts ( M+m vs.
M alone in Newton's correction to Kepler's 3rd law), and above (nonzero
work vs. zero work).


... we
predict the
same trajectories for the motion of objects under the influence of forces.
...

If kinematics were all there were to mechanics, then there might be
some hope for what you advocate.



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