Re: [Phys-l] F causes a

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

Students who can reason both forward and backward from NTN2 are generally
formal operational reasoners.  There is also the factor that it is possible
to improve the ability to do problems by using the rich context problems of
the Hellers, or the ALPS work sheets of VanHeuvelin.  Minds on Physics uses
some of these approaches and others to improve the ability of students to
solve problems.  Reversability of thinking may also be aided by having
students do concept maps.

Now as to F causes a, there is certainly no research evidence to show that
this way of presenting NTN2 works better or worse than using a more causal
neutral approach.  The application of force is certainly causal.  Indeed if
one thinks about what happens in a microscopic, short time frame the "push"
on a block does not immediately accelerate the whole block.  The information
about the push must propagate at a speed slower than c before the rest of
the block begins to accelerate.  The actual speed of propagation would be
the sound velocity.  And yes NTN2 as an equation does not imply any
causality.

Lower level reasoners will either make a causal connection, or memorize the
equation.  This is more of an observation based on what I have read and
observed, and not necessarily a researched conclusion.  I would submit that
this type of reasoning is a necessary stage before learning to treat these
as more abstract ideas.  I would hypothesize that to be able to not think
causally students probably have to be at the theoretical level of thinking
(See Lawson's articles in JRST).  Now perhaps your (whoever you might be)
students can think at the theoretical level, but that is very doubtful
because only 20% of HS graduates are formal operational and practically none
test at the theoretical level.

In reality since lectures are of very limited effectiveness, saying F causes
a may have little effect on student thinking.  Saying F does not cause a,
however, may be extremely confusing.

Actually I think the force causes acceleration can be treated as an analogy
or picture.  The use of analogy has been proven to be beneficial in building
understanding (see articles by John J. Clement, no relation, in JRST).  JD
seems to favor the idea that energy and momentum are quantities that can be
transferred as analogies.  But he does not favor F causes a or delta v in a
similar fashion.  These mental pictures often make the difference between
understanding and not.

Now it may be that my views are colored by the fact that I have concrete
operational students, transitional, and formal operational.  I have students
who do not understand that flattened clay weighs the same as before
flattening, or that water rises the same amount when heavy and light marbles
of identical volumes sink.  So one must ask questions like, how do you make
things accelerate?  Answer: push them. This is a cause and effect
relationship.

However, it may not be surprising to many that even college students have
similar difficulties.  A little reading in JRST, AJP, and TPT can be
revealing.

John M. Clement
Houston, TX


> >   How about these statements:
> >
> > 1) "an object has an acceleration which causes the net force acting
> > on it"
> >
> > 2) "an object has a net force acting on it which causes an
> > acceleration."
> >
> > Which one does everyone prefer?
>
> I prefer the second.  Indeed, it is at least similar to the type of
> language that I regularly use, although I think I am more apt to say
> things like "the net force on the object causes it to accelerate" or
> "the object accelerates due to the net force acting on it."
> Moreover, I like to write the second law as a = F/m when teaching
> because I think it is easier for students to make sense of.
>
> Nonetheless, I see absolutely no reason to *teach* that forces
> "cause" acceleration.  I don't perceive *any* advantage to doing so
> and I think it has several serious disadvantages not the least of
> which is that it  is devoid of physical meaning.  I want students to
> understand that physics is about determining and making use of
> relationships between measurable physical quantities, that it is not
> about distinguishing "cause" from "effect."  I want them to
> appreciate that they may just as readily determine forces from
> accelerations as vice versa.
>
> John "Slo" Mallinckrodt