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One of the best ways of handling the NTN3 reaction force is by
using an
anchor and bridging analogy. Preconceptions in Mechanics: Lessons
Dealing
with Students' Conceptual Difficulties by Charles Camp and John
Clement (no
relation). Some of these lessons have been used in Minds on
Physics and are
extremely convincing. I would suggest finding a used copy of it,
or getting
it from the library. Unfortunately my copy is currently on loan.
It can be reserved at Amazon, but is currently out of print.
John M. Clement
Houston, TX
Way back to basics here. During a Socratic Dialog lab on Newton's
laws,
one
student became hung up on N3 in the part of the exercise where
students
are
asked to hold a massive disk in their hand and then accelerate it
upwards.
She could understand that to do so, she had to increase the force
of her
hand on the disk to be greater than the force of the earth on the
disk.
What took the next 1/2 hour (and I think without ultimate success)
was to
try and understand how the disk was able to increase its force
back on the
hand so that N3 would hold. I talked to her about inertia--about the
resistance of the mass to changes in motion. I had her holding a
5 kg
mass
and then trying to accelerate it horizontally (to take out the
gravitational
factor). I dropped that mass into her hands to have her
experience the
increased force (of her hand and on her hand) necessary to produce
the
needed acceleration to stop the mass. She declined (wisely) to
try and
catch the falling mass with her hand in contact with the top of
the table.
We talked about moving things in space and I brought out an air
track so
we
could look at a 'frictionless' situation. In the end though, she was
still
having trouble. She could 'understand' how she increased the
force of her
hand on the disk but couldn't really accept the inertia arguments
about
how
the disk increased its force back on the hand. 'Where does the
disk get
the
extra force when I push up with more force than its weight?' was her
repeated question.
Any suggestions here? How would you try to deal with this question?
Rick
*********************************************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
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Free Physics Educational Software (Win & Mac)
Animations for Lectures
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Energy management simulations
www.saintmarys.edu/~rtarara/software.html
Energy 2100--class project
www.saintmarys.edu/~rtarara/ENERGY_PROJECT/ENERGY2100.htm
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