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