Re: The old centrifugal force

["John S. Denker" <jsd@AV8N.COM>]

On 11/15/2003 06:21 PM, Ludwik Kowalski wrote:
> (Am I still allowed to say "what causes it?").

I still recommend against it.

> What is the nature of the force acting on the track?

That's better.

> I know
> what Millikan say about this force. In the "A First
> Course in Physics," copyright 1906, he and Gale
> wrote: Inertia manifesting itself in this tendency
> of the parts of rotating systems to move away from
> the center of rotation is called centripetal force."

That's nuts.  The outward force in a rotating
reference frame is the _centrifugal_ force.


> a) the sliding object exerts a force on a track;
> it is the force of inertia directed away from the
> center (not necessarily along the radius).

I don't like "force of inertia".  I might say that
the force was necessary to overcome inertia.  But
I would prefer to say that it is the force necessary
to keep the object on the track.

> b) That centripetal force ("due" to rotation) does
> not act on the sliding object.

In the reference frame comoving with the object,
there is a centrifugal force that does act on the
object.  Meanwhile, in the lab frame, there is
no centrifugal force.

> But the spring-like reaction to that force must be drawn.

Again, there are two ways to analyze the problem
(two different reference frames) and I fear the
two are being stirred together to the detriment of
both.

a) In the lab frame, the force acting between the
object and track is *not* balanced by any other
forces and causes the object to accelerate.

b) In the comoving frame, the force acting between
the object and track is balanced by the centrifugal
force and therefore the object remains stationary
in the comoving frame.

> c) We draw the second force and give it a name,
> such as constrain force, C. The direction of that
> force is neither radial nor tangential,

For a sliding constraint, the force of constraint
is by definition perpendicular to the constraint,
i.e. in the local "radial" direction.

Tangential components of the object/track force "C"
would be called frictional forces, and would not
be classified as forces of constraint.

> d) The net force, R, is the sum of mg and C.

Fine.

> The
> radial component of R is association (or causing
> if you prefer) centripetal acceleration while the
> tangential component is "responsible for" the
> change in the in the instantaneous speed.

Yes, assuming this is all happening in two dimensions.

> e) The mass of the object was given. Knowing
> the radius of the loop, and the instantaneous
> speed, one can calculate the centripetal force at
> the two o'clock location. Likewise, knowing the
> tangential acceleration one can calculate the
> tangential component of R.

Sure.

> Is this an acceptable approach?

Why not?

> Who was the first to declare that the concept of centrifugal force
> should not be part of our vocabulary in physics?

I don't care who said that.
I don't care how many people said that.

The centrifugal field is as real as the gravitational
field, and most students have spent enough time on
playground merry-go-rounds to know that it's real.

When somebody says physics can't be done in a rotating
reference frame, what they really mean is they don't
know how to do it.

As the saying goes: persons saying it cannot be done
ought not interfere with persons doing it.