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
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Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
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