Re: Newton's 3rd Law

["Paul Camp" <pjcamp@coastal.edu>]

One of the nicest illustrations of this I have seen used the MBL 
materials from Vernier software. I don't have access to these myself 
(yet) but I watched David Sokoloff show them off at the last APS/AAPT 
meeting. You could follow a scheme like his:

Attach a force probe to a dynamics cart, allow it to push against a 
wall and qualitatively predict the force vs. time graph. Then do it 
and see the graph. Maybe add mass and repeat.

Allow two identical carts to push off from each other and predict 
_both_ force vs. time graphs. Then do it and see.

Add a lot of mass to one of the carts and repeat.

Now get them to explain it to you and kill that action/reaction 
thing in the egg. To me, Newton's laws are nothing more than an 
operational definition of the word "force." The first two laws tell 
you what a force does -- causes an acceleration -- and the third law 
tells you that really there are no forces. There are _interactions_ 
between particles and every interaction necessarily involves a _pair_ 
of forces -- each particle interacts with the other one. It is just a 
way of saying that in an interaction, neither particle is priviledged 
in any way.

> Can anyone share ideas on teaching Newton's Third Law?  What do you do in 
> labs, demo, lecture, simulation, worksheets, etc. when your students hit 
> this stumbling block (no pun intended) ? 
>
> Pang-Chieh Chou

Paul J. Camp                   "The Beauty of the Universe
Assistant Professor of Physics  consists not only of unity
Coastal Carolina University     in variety but also of 
Conway, SC   29526              variety in unity.
pjcamp@csd1.coastal.edu             --Umberto Eco
(803)349-2227                         The Name of the Rose
fax: (803)349-2926