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[Phys-L] Re: A Third law question



"We haven't done anything with springs yet and I'm afraid the deformation
argument when the object in question is a piece of cast iron in her hand
will not sway her."


This is where the moving the brick wall demo would come in handy. If
one shows pushing against a brick (construction blocks and concrete)
causes it to deform then one can believe that a cast iron "weight"
deforms when a hand's force causes it to accelerate. A force (strain)
gauge between the hand and the wall would add to the demo., especially
if the deflection is proportional to the force probe's output.

bc

p.s. I forgot to add in my post of a previous thread (magic) the
reaction engine. Such a demo (CO2 fire extinguisher or
SCUBA tank) might help. Incidentally, this one fits well w/ JD's point
(conservation of momentum). The identity is not so disguised. Force
probe here would be great also. I suppose better would be a spring
(fitted w/ a force probe) ejected mass from a single cart monitored w/ a
motion detector.


Rick Tarara wrote:
I'll refrain from commenting on all responses, but let me give a couple
here. See below:

----- Original Message -----
From: "Dan Crowe" <dcrowe@SOTC.ORG>
To: <PHYS-L@LISTS.NAU.EDU>
Sent: Tuesday, October 11, 2005 2:55 PM
Subject: 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.



This is a student in my 'gen-ed' class and it is my experience that the
above doesn't do much for them. They don't understand how the force sensors
work, there is a whole lot of electronics involved, and the computer is used
to display the results. It is all 'black box' and of course you can get the
computer to show whatever you want. ;-)


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.



We haven't done anything with springs yet and I'm afraid the deformation
argument when the object in question is a piece of cast iron in her hand
will not sway her.

These are both great suggestions for a student in my science majors class,
but neither are going to work well with students from my gen-ed class. I
talked to Joe Bellina here at length about this, and we sort of came to the
conclusion that one pretty much has to take Newton's laws as axiomatic and
then look to their applications as 'proof'. Most of my attempts were trying
to use 2nd law ideas to try and get at the 3rd law, and that can't be done.
This student is bright and WILL accept that N3 is true but is trying to
understand HOW it could be true in a situation that she feels is
contradictory. She wants a mechanism like the lattice one above, but is
most likely not knowledgeable enough to accept that particular argument.


Rick