sci.physics.relativity

On Sep 15, 2:24 am, Tom Roberts wrote:
> harry wrote:
> > "Tom Roberts" wrote in message
> >news: $ @ ...
> >> You speak too loosely. That scale measures an INWARD pull on the rock, and
> >> an outward pull on the string.
> >> The former is the CENTRIPETAL force on the rock, and the latter is the
> >> tension of the string.
>
> > That happens to be the CENTRIFUGAL FORCE (the scale does not measure
> > "TENSION"!) on the STRING.
>
> Not true.

Can you not even admit the fact that you confused (on purpose
perhaps?!) "force" with "tension"?

> A spring scale is a 2-terminal device that can only measure
> the tension applied between those two connection points. It cannot
> measure the direction of the tension, only its magnitude and the line
> along which it is applied (by the orientation of the scale itself).

Sure. That fact is perfectly in accord with what I stated.

> Moreover, these limitations are INHERENT in the design of such a
> 2-terminal device.
>
> This is NOT the typical bathroom scale, even though that
> happens to have a spring inside, too. But that type of scale
> also cannot measure the direction of the compression, only
> its magnitude and line of action; of course in the usual
> application (lying on the floor to weigh a human), one has
> no need to measure the direction as it is blatantly obvious.
>
> > Replacing "force" by "tension" and "overlooking" to indicate its direction,
> > is very telling!
>
> You clearly do not understand what a spring scale can and can not measure.

You asked:
"Centrifugal force" is directed outward -- where could you possibly
put a scale to measure that??? "

I certainly do understand such things, and even taught them. Actually
I agreed with you (eventhough you now disagree with yourself!) that
the scale measures an outward pull on the string, and I informed you
that in proper Newtonian mechanics an "outward pull" is called a
centrifugal force - as it should, by definition. The correct term
should not be confused with the PUN on that word for fictitious forces
that use Newtonian mechanics improperly.

[...]

> Yes, the Wikipedia
> article mentions it, along with the observation that your usage "is only
> occasionally used in modern discussions". I had never heard of this
> other usage, and avoid such PUNs in any case.

I avoid the use of any fictitious forces as I find them generally a
BAD idea.

[...]

> > A
> > pulling force on a scale is by definition directed away from the scale.
>
> You are not thinking of the correct type of scale. The spring scale one
> can tie into the string has two terminals and cannot measure "away from
> the scale", it can only measure the pull BETWEEN ITS TWO CONNECTIONS.

OK - you did not specify "spring scale" in your question. Examples of
scales are kitchen scales or person scales. Such a scale could be
glued on one side to the rock, and on the other side to the rope.
Thus:
<- x ->

> >>> much confusion exists between real forces and fictitious forces of the
> >>> same name.
> >> I know of no such case.
>
> That Wikipedia article introduced me to one. That's unfortunate, as that
> completely confuses the issue, and makes it essentially impossible to
> discuss "centrifugal force" in any definitive manner without being
> EXTREMELY careful and precise.

That has "ALWAYS" been the case! Why do you think that the OP is
confused? The mistaken claim that "centrifugal forces" that appear to
push you against your seat in a merry-go-round, are "real" "because
you can feel it" (that is, you can feel the outward force against the
seat) is wide-spread. Countering that with a denial of a true force
that people measure will not convince anyone.

> For the record: when I say "centrifugal force" I mean the "force" on an
> object in rotating coordinates with magnitude m*(r x w) x w, where m is
> the object's mass, r is the 3-vector from the origin of rotation to the
> object, and w is the 3-vector rotation of the coordinates. Note this
> force is completely independent of the motion of the object, and depends
> only on the rotation OF THE COORDINATES. The PUN force is not this at
> all (in the above discussion, for the PUN "centrifugal force" on the
> string, the m and r are for the rock, not the string).

Misuse of Newton's mechanics by fictitious forces is the PUN.

> > Try Newton's second law: it's about real forces, as measured in inertial
> > frames.
>
> Right. No "centrifugal force" in sight (except possibly for the PUN).

Obviously we largely agree - just not on the teaching! :-)

Harald