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[Phys-L] Re: A Third law question



Rick,

Did you use force probes (Vernier, Pasco, or equivalent)?

I use two Pasco dynamics carts, each of which has a Vernier force probe
mounted on it. I zero both probes and invert the output from one of
them. I then show that the forces that the carts exert on each other
are equal and opposite for all types of motion: static, constant
velocity, nonzero acceleration. One nice thing about force probes is
that they show that the forces are equal and opposite at each instant of
time, even if the forces are not constant. In particular, they show
that there is no delay between "the action force" and "the reaction
force", as many students believe there must be to explain acceleration.
I use the phrase "interaction pair" rather than "action/reaction pair"
to avoid reinforcing this misconception.

OTOH, the student's conceptual difficulty might be due, at least in
part, to an incomplete understanding of how inanimate objects exert
passive forces. Have you developed a conceptual model of solids that
explains how normal forces are associated with elastic deformations of
lattices? Increasing the force on a solid increases the deformation of
the lattices; increasing the deformation of the lattices increases the
force exerted by the solid. Most students exposed to tactile
experiences with springs recognize that more force needs to be exerted
on a spring to compress it farther, and that the farther a spring is
compressed, the more force it exerts.

Daniel Crowe
Oklahoma School of Science and Mathematics
Ardmore Regional Center
dcrowe@sotc.org

-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-L@list1.ucc.nau.edu] On
Behalf Of Rick Tarara
Sent: Tuesday, October 11, 2005 1:18 PM
To: PHYS-L@LISTS.NAU.EDU
Subject: [PHYS-L] A Third law question

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?