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-----Original Message-----Newton's
From: Forum for Physics Educators [mailto:PHYS-
L@list1.ucc.nau.edu] On Behalf Of Rick Tarara
Sent: Tuesday, October 11, 2005 2:18 PM
To: PHYS-L@LISTS.NAU.EDU
Subject: A Third law question
Way back to basics here. During a Socratic Dialog lab on
laws, oneit
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
upwards.force of
She could understand that to do so, she had to increase the
herthe disk.
hand on the disk to be greater than the force of the earth on
What took the next 1/2 hour (and I think without ultimatesuccess) was
toback on
try and understand how the disk was able to increase its force
theinertia--about the
hand so that N3 would hold. I talked to her about
resistance of the mass to changes in motion. I had her holdinga 5 kg
massgravitational
and then trying to accelerate it horizontally (to take out the
factor). I dropped that mass into her hands to have herexperience the
increased force (of her hand and on her hand) necessary toproduce
thetry and
needed acceleration to stop the mass. She declined (wisely) to
catch the falling mass with her hand in contact with the top ofthe table.
We talked about moving things in space and I brought out an airtrack
so weshe was still
could look at a 'frictionless' situation. In the end though,
having trouble. She could 'understand' how she increased theforce of
herarguments about
hand on the disk but couldn't really accept the inertia
howdisk
the disk increased its force back on the hand. 'Where does the
get thewas her
extra force when I push up with more force than its weight?'
repeated question.question?
Any suggestions here? How would you try to deal with this
Rick
*********************************************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
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