I know that this topic is dying but I'd like to offer some
thoughts regarding "F causes a". I used to think that a net force
causes acceleration and I used to teach so. Many high school
textbooks seemed to do so so I thought this is the way to go. However, when we had this discussion last time I was converted
by John D's arguments. Hence, I stopped talking in terms of
causation when teaching Newton's second law.
What was good in talking about the causation? Pedagogically I
think that it perhaps guided students to look for forces when
they saw acceleration. Well, I realized that I don't need to
invoke causation to accomplish that: it is enough to teach
students that
"whenever you see acceleration (i.e., change in velocity)
look for interactions to find out the forces and their sum,
the net force. And if you see the net force, expect
acceleration".
(Note that high school physics concentrates quite exclusively
on inertial reference frames so there is no force without
interaction. I do, however, address non-inertial frames before
they finish high school physics)
The point is to have this two-fold *association* between net
force and acceleration. I haven't noticed any decline in student
learning when I dropped causation from Newton's second law.
Finally, I'm glad that John D. keeps up posting to Phys-l;
I think that his contributions are very valuable.
Regards,
Antti
Antti Savinainen, Ph.D., B.Ed.
Senior Lecturer in Physics and Mathematics
Kuopio Lyseo High School
Finland
E-mail: <antti.savinainen@kuopio.fi> or
<antti.savinainen@luukku.com>
Website: <http://kotisivu.mtv3.fi/physics/>
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