Others have beaten me to punch, particularly David Bowman; at the risk
of repeating much of what he said, and less eruditely than him, I'll
proceed, as it uses a different set of words that may be illuminating to
some; plus, as always, I use Phys-L as a way of vetting some of my
thoughts and ideas. The first part of the message was written early
this morning before my morning classes prevented completion.
______________________________
The short answer to John D's question, as others have pointed out is
that "pseudo-force" is another term used almost isomorphically, AFICT,
for "fictitious force".
I have a personal dislike for the terminology for reasons we have(and no
doubt will) discussed and argued ad infinitum.
A couple of days ago I had a few thoughts regarding my preference.
Everyone agrees about what occurs in classical mechanics for Newton's
second law when utilized in an inertial reference frame, for solving a
problem.
i) the sum over forces side only includes forces for which you can
identify a physical agent causing the force.
ii) the "m a" side involves the actual acceleration of the object as
measured in the inertial reference frame.
Now, switch over to analysing in a non-inertial reference frame. It
strikes me that we have one or two choices as to how to handle the extra
"inertial" terms that one gets when operating in such a frame of
reference. The extra terms come from a coordinate transformation and
how that alters the acceleration in N(II). Now what do you do with
these terms, now that we've gotten N(II) expressed in the non-inertial
reference frame.
A) We can put the extra terms on the sum of forces side of the equation
and call them forces. Thus making the "m a" side of the equation simply
"m a" where 'a' is the measured acceleration in the frame of reference.
This has a lot of appeal to me, particularly on operational grounds, as
measuring the acceleration is what one typically does. This is done at
the sacrifice of identifying agents for all forces in the sum of forces
side of the equation; or as Bowman said, at the sacrifice of N(III)
B) If we really want to keep to the idea of only allowing terms
(forces) on the sum of forces side of the equation for which we can
identify agent, or body that is interacting with our mass; then we keep
the extra terms on the "m a " side of the equation. Unfortunately the
stuff proportional to 'm' on that side of the equation is no longer the
measured acceleration of the object. Which means it is not directly
amenable to a kinematical measurement in that frame of reference. This
strikes me as not so good (doable, but not so good). Or as Bowman said,
one does this at the sacrifice of N(II).
As they say, one pays your nickel and takes your choice.
________________________
Joel Rauber
Department of Physics - SDSU
Joel.Rauber@sdstate.edu
605-688-4293
| -----Original Message-----
| From: phys-l-bounces@carnot.physics.buffalo.edu
| [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf
| Of John Denker
| Sent: Thursday, October 26, 2006 2:04 AM
| To: Forum for Physics Educators
| Subject: [Phys-l] pseudo-force
|
| Some questions:
|
| What do people mean by the term "pseudo-force"?
|
| Is a pseudo-force a force?
|
| Why is it called a pseudo-force?
|
| Is this concept crucial/useful/marginal/unhelpful?
|
| _______________________________________________
| Forum for Physics Educators
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|