Re: Centrifugal force

[Leigh Palmer <palmer@sfu.ca>]

> On 30/04/98 -0700, Leigh wrote:
>
> > .... In Newtonian mechanics an inertial frame is operationally defined
> > as one in which the law of inertia holds; it is as simple as that. ...
>
> The law of inertia states that "if the net force acting on a body is zero
> then the velocity remains constant". I do not see why this should not be
> applicable in the rotating frame of reference. If the bead on the rotaing
> straw does not accelerate then the net force acting on it is zero.

If the bead does not accelerate in the rotating frame of reference
then the net force which acts on it is zero. In this case, however,
there is a straw exerting a force on the bead (why a straw and not
a string?). It appears that a noncontact force must be equilibrating
the bead in this frame; if the straw is cut the bead accelerates;
ergo we are not in an inertial frame. When all constraints are
removed from the bead a force remains; the bead accelerates, and
the law of inertia does not hold in this frame

> A frame of reference is inertial if it does not accelerate with respect
> to a frame which is already known to be inertial (the frame of distant
> stars). What is wrong with this operational definition?

It requires that an experiment be done which refers to the world
outside the frame. You ought to be able to determine whether a
frame is inertial or not without referring to the outside world.
Do you think the fixed stars affect the experiment you do in
your laboratoty? To the extent that they do not, the decision
about whether a frame is inertial or not should be within the
capability of the experimenter to determine wholly within the
laboratory.

Think about Donald's hovering rocket ship (a nice improvement on
Einstein's elevator). I don't have the quote around, so I'll make
up my own. A rocket ship hovers one meter above its launch pad.
In the laboratory within an astronaut performs physical experiments
(with all manner of apparatus, lasers etc.). At one instant in time
God picks up Her fungo bat and decides to improve the situation in
the universe by removing the Earth with the speed of light to the
most remote place possible. The feat is accomplished in a direction
which is horizontal at the pad. The astronaut feels, perhaps, a
slight bump as the solar system is cleansed, but he continues with
his experiments and notices no difference whatever in the results
he obtains with those obtained in the same experiments before. The
rocket ship lab is and always was the same sort of frame of
reference, clearly not an inertial one.

Leigh