Sorry to be so late with a response, but one experiment I've done is
to have two people press bathroom scales together. The trick is to put
a thick piece of foam rubber on the back of each scale and then make
sure each participant holds the scale only by the top--don't squeeze the
top and back of the scale with the fingers. The scales I use aren't
enormously accurate, and they usually have to be adjusted so they'll
read zero when held vertically, but the readings are close enough that
students at least begin to question their notion that the force one
object exerts on another one depends somehow on the sizes of the objects.
(I usually get two people of vastly different mass to participate in
the demonstration.)
(By the way--I stole this idea from a brief unmentioned glimpse of
such a demonstration in a "New Explorers" program featuring Chris
Chiaverina and Jim Hicks that was on PBS a few years ago.)
Steve Luzader
Frostburg State University
sluzader@fre.fsu.umd.edu