Re: heat, centrifugal force, etc.

[John Clement <clement@HAL-PC.ORG>]

> There is a lot of physics education research (PER) which shows
> that Newtonian concept of force is very hard for students to
> understand at any level of instruction (e.g. Hake 1998). PER has
> suggested many approaches to make learning more efficient. The
> approach I have tried out is to start treating forces arising
> from *interactions*. Force is defined to be a measure of strength
> of interaction between two objects. Interaction is always
> symmetrical; this is another way to state Newton's Third Law. For
> practical purposes it is useful to discern contact interactions
> from distance interactions. This approach seems to make it easier
> for students to identify different forces acting on a given
> object. Another favorable outcome is that it is easier for
> students to master Newton's Third Law when they think forces as
> interactions. I have data on this one and I intend to write an
> article about it.

The approach of considering interactions is extremely helpful with lower
level students, but is also helpful for advanced students.  Students can
then look at a picture and easily pick out the real forces on objects, as
they are all due to something touching the object.  The only common
exception is gravitational force which they know is toward the earth.  By
such reasoning the centrifugal pseudoforce does not exist because there is
no object that can be identified to cause it.  Unfortunately concrete
operational and transitional students have extreme difficulty with the idea
of an inertial frame of reference, so it can not be introduced.  Indeed they
have difficulty seeing that a coat on top of a car that suddenly accelerates
to the right moves to the right.  You can even show them a movie of such an
event and they will all insist that the coat falls to the left.  You can be
extremely specific about the fact that you are observing it and standing
next to the car, and they will still say it falls to the left.  Even after
viewing the movie in slow motion with the original and final positions
marked on the screen they will still say it falls to the left.  Maybe after
this barrier is passed it might be possible to consider the idea of an
inertial frame.

Since most HS students tend not to be formal operational in their thinking,
and even some college physics students will be at such a low level,
non-inertial frames and pseudoforces can not always be introduced in the
beginning course.  When a student insists on centrifugal force, I will say
that it is called a pseudoforce and is that the passenger of the car thinks
there is a force throwing him outward, but what is the object that is
producing the force?  Perhaps they would be a bit more convinced if it were
called a virtual force.  I tell them we must analyze the system from outside
the car.

Even when using the idea of interactions, low level students will still put
forces in the wrong direction, and introduce fictitious forces such as an
upward applied force on a ball that is in the air with nothing (except for
air) touching it.  However the consensus of many PERers is that this
approach helps the students more than other approaches.  This particular
approach is used extensively by the UMass Amherst PER group.  I have not
seen any reported evidence that this approach is better.  Such an experiment
is very hard to do, as it would be difficult to separate this one idea from
the other PER ideas.

The centrifugal force misconception (in an inertial frame of course) leads
students to draw outward curving trajectories for balls that are whirled on
a string and then released.  This is essentially one of the important FCI
diagnostic questions.  It is not clear whether avoiding the terminology
centrifugal force or trying to deal with it is more beneficial.  There has
to my knowledge been no reported research on this particular question.  In
either case the answer may depend on the thinking level of the students.

The debate over heat and weight can get a little silly.  These terms have
not been officially standardized by the physics police or indeed by physics
textbook authors.  At this point one is free to pick a definition to use in
class until such time as there is an "official" definition.  However, there
is a fairly good reason for picking a definition, which has generally been
ignored in the debates.  The best definition is probably the one which helps
students understand the concepts.  As such I suspect that using the term
heat or heat energy to mean internal or thermal energy may help beginning
students understand the concepts.  Weight is a bit more problematic as the
simple classical definition (gravitational force) runs counter to common
usage.  On the other hand aligning the physics definition to mean the force
measured by a spring scale may be too complicated for beginning students, in
which case the term might be best avoided.  By this measure the "best"
definition may be one which produced higher gain on appropriate diagnostic
tests.  Again, to my knowledge such data has not been reported, but I have
heard that there is some data supporting the latter definition.

Without data to support the superiority of one usage over another the debate
generally descends to a very low level.   Intelligent physicists then tend
to look like mud slinging politicians.

John M. Clement
Houston, TX