Re: centrifugal force

[Hugh Logan <hlogan@ix.netcom.com>]

LUDWIK KOWALSKI wrote:
> http://www.observe.ivv.nasa.gov/nasa/space/centrifugal/centrifugal_entry.html
>
> The above reference (NASA educational item) is devoted to CENTRIFUGAL force.

I started to read the NASA reference, but I became a little confused by
the discussion of centrifugal force on the page entitled "Inertial
Forces, Rotational Motion and Centrifugal Force." In discussing a
passenger in a car going around a curve, the author states, "That is the
centrifugal force (i.e., a center-fleeing force). The door pushes back
and prevents you from falling out (Newton's third law)." I hope he does
not mean that the force of the door pushing back on the passenger (which
he calls the "centripetal force" in the next paragraph) is the reaction
to the centrifugal force. In any case, I think this needs more
clarification. The force of the door on the passenger is the Newton's
third law reaction to the push of the passenger on the door, not to the
(inertial) centrifugal force. I was of the impression that one
difference between inertial forces and "fundamental" forces is that
there is no reaction to inertial forces, the latter resulting only from
the acceleration of the reference frame from which the motion of the
massive body is described.
   In the next paragraph the author correctly (from the point of view of
the rotating frame) adds the centrifugal force to the "centripetal"
force, the two balancing to zero. But these can't be action and
reaction, since action and reaction forces don't act on the same body.
   I think the author's use of the word "centripetal" might cause a
little confusion, since one usually thinks of that as the force causing
the acceleration when the motion is described from an inertial frame.
(But I must admit it contributes to the zero acceleration of the
passenger as described from the rotating frame, and it is toward the
center). This may be predjudice on my part, but it reminds me of the old
books that call the Newton's third law reaction to the centripetal force
"centrifugal force" or "centrifugal reaction force." (when the motion is
described from an inertial frame) . These forces do not act on the body
in question, and they are not inertial forces.

Hugh Logan