Re: Aberation and the Speed of Gravity

harry wrote:
On Sep 15, 2:24 am, Tom Roberts <tjroberts...@xxxxxxxxxxxxx> wrote:
harry wrote:
"Tom Roberts" <tjroberts...@xxxxxxxxxxxxx> wrote in message
news:HZwGi.7203$JD.4842@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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"?

Huh?? The tension of a string is one type of force.


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.

That PUN does not appear in Haliday and Resnick _Physics_ (first edition) that I used for mechanics ~35 years ago. The definition of "centrifugal force" I gave comes directly from that. The "outward pull" you describe is from the ROCK, not from the string, and does not meet their definition of "centrifugal force"; it does for the PUN "centrifugal force" of that Wikipedia article.


The correct term
should not be confused with the PUN on that word for fictitious forces
that use Newtonian mechanics improperly.

You are wrong. In the mechanics course I took many years ago, 'centrifugal force' was purely fictitious. As that Wikipedia article said, the PUN "centrifugal force" is rarely used in modern discussions; except for a few individuals around here I have NEVER heard it used.

I just took an informal poll of 4 professors and physicists and 1 postdoc on this specific point: NOT ONE said they would call the force the rock exerts on the string to be "centrifugal force" (it is tension), and EVERY ONE agreed that "centrifugal force" is fictitious and purely related to rotating coordinates. A perusal of the teaching version of the 7th edition of Haliday, Resnick, and Walker shows they have an elegant solution to this problem -- they make no mention of "centrifugal force" at all.


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.

In which case it can measure ONLY an INWARD pull on the rock, and that cannot possibly be the "centrifugal force" ON THE ROCK.

And it cannot be the usual "centrifugal force" on the string,
because the m and r in the equation are for the ROCK, not
the string.


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!

Not in the courses I took. Such PUNs are horrible and incredibly confusing to students. AVOID THEM. My professors did ~35 years ago. As do at least 5 of my colleagues today.


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.

Historically, "centrifugal force" was introduced JUST AS I DESCRIBED, in order to apply Newton's laws to the rotating earth in situations where that rotation is important (e.g. long-range artillery).


Tom Roberts
.

Relevant Pages

  • Re: Aberation and the Speed of Gravity
    ... it can be measured with Hooke's law using a scale. ... That scale measures an INWARD pull on the rock, and an outward pull on the string. ... You have NOT measured the "centrifugal force" ON THE ROCK. ...
    (sci.physics.relativity)
  • Re: Aberation and the Speed of Gravity
    ... an outward pull on the string. ... The former is the CENTRIPETAL force on the rock, ... That happens to be the CENTRIFUGAL FORCE (the scale does not measure ...
    (sci.physics.relativity)
  • Re: Aberation and the Speed of Gravity
    ... an outward pull on the string. ... along which it is applied (by the orientation of the scale itself). ... the scale measures an outward pull on the string, ... centrifugal force - as it should, ...
    (sci.physics.relativity)
  • Re: Gyroscopes - Usenet Physics FAQ - Reference frames
    ... >>applies and the only force is the tension of the string. ... "causes" it, of course, is the inertia of the rock (that's a naive use ... inertial frame there is no "centrifugal force", ...
    (sci.math)
  • Re: Gyroscopes - Usenet Physics FAQ - Reference frames
    ... >>applies and the only force is the tension of the string. ... "causes" it, of course, is the inertia of the rock (that's a naive use ... inertial frame there is no "centrifugal force", ...
    (sci.physics)
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