Re: [Phys-l] force conventions

["John Clement" <clement@hal-pc.org>]

> This is pertinent to the recent discussion of centrifugal force,
> because in a typical centrifuge situation, the centrifugal force
> is exerted *by* the object upon the arm of the centrifuge.  This
> is why inexpert use of centrifugal-force ideas causes so much
> trouble for HS physics teachers.  The force is perfectly real,
> the students know it is real, and no amount of shouting from
> the teacher will make them forget it is real ... but it is just
> begging to be misinterpreted and misused.
While this is a perfectly good definition of centrifugal force, it is not
the root of common misconceptions.  The common misconception is that there
is a centrifugal force pushing the object outward when it is going around in
a circle.  Students will say this even when you are looking at the object
from the lab inertial frame.

By defining the centrifugal force as "the centrifugal foce" this
misconception does not really disappear.  The common misconception is
reinforced by common language.  Very seldom is does the person talking about
circular motion point out that the object tends to move outward because of
NTN1.  Rather they point to a mystical centrifugal force.  The way to treat
this sort of misconception is to first teach the idea that forces are always
interactions between tangible objects.  Of course at this level you are
ignoring that there might be forces such as the repulsive force causing
acceleration of the big bang expansion.

One can always bring in the idea that the ball on the string has a force
where the ball pulls on the string and the string pulls on the hand, but
that merely hides the universal misconception.

Once the students have to justify the presence of all forces by interactions
between objects, the mystical centrifugal force disappears.   If you are
going on to rotating reference frames, then it reappears as a "pseudo" force
or whatever name you wish.  But at the lowest level it can be included as
saying it "looks like" there is a force throwing you outward when you are in
a car and turn a corner.  So in common speech people say there is a
centrifugal force, but from the point of view of interactions there is no
such thing.

John M. Clement
Houston, TX