Re: apparent weight

[LUDWIK KOWALSKI <KOWALSKIL@alpha.montclair.edu>]

On 18 Feb 1998 William Beaty <billb@eskimo.com> wrote:
> .... Kids are right when they say that they feel a force throwing them
> outwards.  They are wrong when they imagine that some sort of invisible
> gravity-like field has been created by the rotating object.
.......................................................................
We must recognize the net force (experienced by a bead sliding on the
horizontally mounted frictionless stick which is rotated about a vertical
axis) because we tell kids that "whenever you see an acceleration there
must be a net force, F=m*a". In this case it is the acceleration in the
one-dimensional world with respect to its own rotating frame of reference.

What is the source of this force? Without going to an explanation based
on the "curvature of spacetime" I can direct students' attention to the
energy aspect. The kinetic energy of the bead, in its own frame, is 
growing because I am turning a crank to keep the stick, and its sliding
bead, turning at a constant RPM. If the room were dark then creatures 
living on my rotating stick (inside of their habitat) would not know haw 
to explain the origin of the force but they may invent a name for it, 
for example ceight, the force due to caravity. They would have no need
for the angular momentum but the concept caravitational field may be useful
to them because the ceight, F, depends on the distance from one end of 
their habitat.

I am not saying that noninertial frames should be used to solve physics
problems. These frames should be avoided, especially in three-dimensional
situations (playing with a ball on a rotating platform). It would also be 
silly (unnecessary) to talk about caravitational field. But it is not a 
crime to invoke the centrifugal force in explaining some phenomena in 
Conceptual Science. Rapid precipitation of particles in a centrifuge tube 
(from muddy water) is one example. Nonsphericity of Earth is another. What 
is heretical about stating a rule according to which "any particle in a 
rotating frame of reference is subjected to a centrifugal force 
F=0.5*m*v^2"? The particle would accelerate (at a=F/m) if other forces, 
(due to springs, friction, viscosity, etc.) were not present.

I know, you will say "why teach them something they must unlearn in more
advanced physics courses?". And you know what my answer would be, "most of
these students ...". In my opinion, no harm can result from explaining 
certain things in terms of *centrifugal* forces, magnetic *poles*, light 
*rays*, etc. Yes, I am sticking out my neck again. Later courses, if any, 
may elaborate on limited utility of simple explanations and deal with 
hidden nuances. What is wrong with this attitude. Explain why you think 
this attitude must be rejected. 

How can the constructivist way of teaching (students rediscover the laws 
of physics through guided activities) be implemented if we say "according 
to authorities you are not allowed" to use this term or concept. The "less 
is more" principle also calls for some changes of attitude.
                                                           Ludwik Kowalski