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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.