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

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

The approach of using the idea of interactions and that forces are caused by
something is exactly the approach that is taken by many of the reformed
physics curricula.  Students have to initially attach the idea of force to a
physical object.  This does indeed help in trying to dispel many
misconceptions.

Students in their first physics course do not generally attach much meaning
to equations.  Indeed the approach of making it an abstract relationship
shown by an equation does not work well at all.  This has been the
traditional approach for a long time, and yields very low understanding of
force and acceleration.

It is NOT a horrible mistake, to have students initially make the connection
that force causes acceleration.  This is a strong connection, and the more
abstract one that force and acceleration are related is a weak connection,
easily overcome by the force-velocity misconception.  I would say the
opposite that it is a horrible mistake to treat the equation as just a piece
of mathematics.

We have been around this discussion before, and it is now known from data
that introducing interactions first, improves Newton's third law
understanding.  There is no data for a benefit from explicitly saying that
force causes acceleration, but there is data that shows that students must
experience the fact that acceleration is dependent on the applied force.  So
immediately the idea of causality is implied in the student's mind.

It is extremely unwise to bring in a more mathematical definition of
causality, as this will be more data than can be handled by students.
Remember that working memory is small, and students have not yet chunked the
already introduced concepts.  Indeed if you look at any standard physics
book, they introduce far too many concepts at once, so that students are
easily overwhelmed.  The expert is not because the concepts have already
been chunked, so they see it as being a simple text.

There is also a good reason for categorizing forces according to the type of
interaction.  So while the notation should always be F for force, the type
of force and also the agent and object should be specified.  The reason for
the type of force is that students use this label to decide how the force
operates.  Normal forces perpendicular to a surface, tension along the
string... need to be explicitly brought out.  But, obviously labels such as
passive and active are not included.

John M. Clement
Houston, TX

> > I have had students discuss what causes a force.  I've found that
> > this helps students eliminate 'inertia' from the discussion of
> > forces.  I've taught that each force comes from an interaction and
> > there must be something causing a force.  Students eventually see
> > that inertia doesn't have an obvious cause and then can't be a force.
> >
> >
> > I am curious what others have to say about this.
>
> Well, I wouldn't recommend that approach.
>
> 1) As a minor piece of background information:  Beware that in
>  non-scientific vernacular language and thought, there is a
>  close connection between "force" and "cause", as we see from
>  the similarity of meaning in the following sentences:
>  -- I caused him to do xxxx.
>  -- I forced him to do xxxx.
>
> 2) The main point is that when it comes to physics, it is a horrible
>  mistake to confuse force with cause.  Anything that blurs the
>  distinction between force and cause is a big step in the wrong
>  direction.
>
>  In particular, it is a mistake to think that the equation F=ma
>  means F is caused by ma (unless you stretch the terminology to
>  say that ma is also caused by F).