Re: [Phys-l] Errata for FCI?

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

> I don't believe that this item belongs on the FCI.  The buoyancy/air
> pressure issue is very subtle and requires more time to address properly
> than I have available in a first year class.

A good test that discriminates between the highest understanding and just
average understanding should have some questions which are subtle.  So this
is obvious one of these.

Actually the students do not have to understand how to calculate the buoyant
force to appreciate that it is there whenever an object is immersed in a
fluid.  Of course the problem of stickyness or a suction cup effect is a
further complication which should not be brought in initially.  Students
need to first be able to identify forces, and when they are present, without
being able to exactly calculate all of them.  Then the next level is where
they know that some forces can be calculated easily, while others might have
to be deduced from other forces and the object's motion.

Students will almost universally tell you that a helium filled balloon is
being pushed up the helium.  This is a misconception that is only addressed
by the idea that the helium-balloon are one object and that the force
pushing it upward is due to some outside agent pushing on it.  Students tend
to ignore the fact that air is a "thing" which can push on objects.  So
buoyancy is another good example of an interaction which can be included as
part of the section on interactions, without getting into complications.
The idea of touch as a requirement for most common forces is something they
need to acquire.  Early on the only exception is the gravitational force.
Then they have to begin to discriminate between contact and noncontact
forces as magnetic and electrical forces are introduced.

The big issue is getting students to think about interactions rather than
making forces as something inside objects sort of like impetus.  So all of
the common forces including buoyancy are useful in achieving this objective.
Of course buoyancy could be left out, but it is easily included as just
another force without getting into the details of why it is present.

The idea of addressing things properly is unfortunately sometimes used as a
bludgeon.  It is used to force too much detail for some topics and for
omitting others.  If you were to address contact forces "properly", you
might start with electrical interactions and QM.  Buoyancy can just
initially be thought of as a force which is present when an object is
immersed in a fluid.  This model serves nicely in explaining a variety of
situations where objects float.  Then the model can be refined and changed
later on.

The "proper" way to address things is to get students to build a coherent
model for how forces work, and then to get into details about specific
forces.  Perhaps the they will not say "the balloon is pushed up by the
helium" or planes stay up because of buoyancy.

John M. Clement
Houston, TX