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Re: [Phys-l] ? passive force of constraint



Many students in introductory physics courses believe that only active
forces exist. The following is a typical statement: "The table doesn't
exert a force on the falling ball: It simply gets in the way." Physics
education research has shown that addressing this preconception helps
many of these students to better understand normal force, tension and
other passive forces.

We agree that this misconception exists. We agree that addressing it is
helpful.

Tangential remark: I see this misconception as one element of a large
bundle of misconceptions. Other elements include the pre-Galilean
notion that objects at rest remain at rest, and objects in motion tend
to come to rest.

It is also connected to innumerable problems with the terminology.

When it comes to the table "getting in the way", there are at least
two ways of explaining the situation to students:

a) Explain it in terms of a force of constraint. Explain that
in terms of Hooke's law, using a large spring constant k and
a small deflection x. This is easier to visualize if you replace
the tabletop with a not-very-taut rubbery drumhead, as discussed
at http://www.av8n.com/physics/causation.htm


Unfortunately, just explaining it does not really help students to
understand. The spring analogy can be used, but it works very well if used
as part of an anchor and bridging analogy. This is presented in
Misconceptions in Mechanics by Clement(no relation) & Camp. The students
have to actively think and discuss a series of analogies which start with a
mass on a spring, and finish with a solid surface and a model of atoms as
being "like" balls connected by springs.

The problem is the assumption that the act of explanation will work. Also
one needs a series of analogies each carefully selected to bridge to the
target conception from the anchor conception. The anchor conception is one
that students all agree on such as a weight resting on a spring. They agree
that the weight pushes on the spring, and they can see that the compressed
spring pushes on the weight.

What the students think and discuss is much more persuasive than what the
teacher tells. Indeed students have learned to distrust what they are told,
and know that they have to spout back on exams things they do not
understand, or believe. Actually students have always behaved this way.

John M. Clement
Houston, TX