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Re: [Phys-l] The "why" questions



William Robertson wrote:

On that note, we should return to Bob's purpose in starting
this conversation.

I'm guessing I am the "Bob" mentioned by Bill, in which case he is
referring to a message I posted to the physlrnr list, not the phys-l
list.**

I am firmly in the camp with John Denker on this one. There is no cause
and effect between force and acceleration.

My point in the physlrnr post was that saying "force causes
acceleration" has pedagogical implications in that it seems to imply
that there is a delay between force and acceleration. That, alone,
would be problematic since, as John Denker points out, "As functions of
time, F(t) = ma(t). It is the same t."

However, there is an additional concern. Let's revisit the scenario
suggested by Bob LaMontagne:

A physics instructor places a toy car on a table and pushes it from
behind. He/she then asks the class "What just happened?"

Student: The car just accelrated because you pushed it.

At this point, I think the student really wants to say that "the car
just MOVED because you pushed it." Why do I think that? Because, in a
sense, that *would* be appropriate to say, as a force was applied and
*after* the force has been applied the object has a motion that it
didn't have before.##

[otherwise the student should say "the car just accelerated AS you
pushed it"]

So, as Bill mentioned, my concern is that saying "force causes
acceleration" allows students to use the word acceleration in place of
the word velocity and visa-versa, further muddying the distinction in
their minds.

I just don't have any research evidence to back this up -- which is why
I posted it on physlrnr.

Footnotes

**Actually, it started on an NSTA list then migrated to a private
conversation among a handful of participants then migrated to the
physlrnr list

##This idea that the motion is generated by a force is, in fact, the
same language that Newton used to describe his second law (at least via
the English translation of his Latin) so we shouldn't be too harsh on
students when they use it.

----------------------------------------------------------
Robert A. Cohen, Department of Physics, East Stroudsburg University
570.422.3428 rcohen@po-box.esu.edu http://www.esu.edu/~bbq

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf
Of William Robertson
Sent: Monday, November 29, 2010 7:43 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] The "why" questions

The underlying physics is a construct. The asymmetry is not
in my imagination, but in a practical evaluation of
real-world events. If you want to say that my evaluation of
those real world events is in my imagination, so be it. But
then everything is in my imagination. I consider myself a
radical constructivist (everything is basically a
construct) in philosophy, but I don't apply that philosophy
constantly in everyday life and in applying physics concepts.
If I get hit by a bus, I'm pretty comfortable with the
interpretation that the force exerted by the bus caused the
acceleration of my body that resulted in my injuries. People
understand abstract concepts by linking them to concrete
events, and in applying a practical interpretation we can
help students better understand the underlying physics.

On that note, we should return to Bob's purpose in starting
this conversation. If stating that forces cause accelerations
results in a confusion for students regarding the difference
between velocity and acceleration, then it's a legitimate
concern. But is it a major pedagogical concern if that isn't
the case? There are pedagogical reasons for using the
cause-effect argument, one of the most common being that
students commonly add a centripetal force in circular motion
that is caused by the centripetal acceleration. Personally, I
have found that stating that forces cause accelerations, not
the other way around, helps the students see their error.
When we set up second law problems, we begin with free-body
diagrams, regardless of the quantity we are looking for. This
process comes first, and it seems a harmless thing to explain
that we begin with the free-body diagram because the applied
forces are what set up the application of F=ma.


While we might have a discussion here, largely a
philosophical one, regarding the issue, I don't see why we
should necessarily incorporate our philosophical concerns in
all instruction. For more advanced students, yes. In more
advanced applications, yes. There have been discussions here
regarding the ability of students to reason formally, so is a
practical, reality-based presentation of the second law such
a bad thing?

I'm not saying we should avoid philosophical arguments
altogether. It is important that students understand we are
applying abstract models to the real world. But do we help
students in general if we make a point of telling them that
these forces don't cause accelerations, despite what is a
reasonable interpretation of events?

Bill