Re: Evolution and Creationism/Thank you/3rd Law

[Bob Sciamanda <trebor@VELOCITY.NET>]

Hi Dorrie,
Thanks for provoking some physics :-)

I use the following line of thought.  Challenge the class to consider what
wonders they might perform if they had two interacting objects which did
NOT obey the third law.  Let them think of this overnight.  If none of
their bulbs light up by the next class, then offer your own variation of
some story like this:

Suppose you had two small objects, A and B, which strongly repel each
other (Eg., like magnetic monopoles or like electric charges) in
accordance with the third law.  You might foolishly try to use this
repulsive force as a mechanism for free locomotion:

1) Fasten object A to a rigid wall; fasten object B to your "radio flyer"
(or "rosebud") wagon, and hop on.
2) You find this works only momentarily, because as A and B separate, the
force of repulsion quickly weakens.  You cannot achieve sustained
locomotion.
3) Since the increasing distance separating A and B is the problem,
perhaps the solution is to carry A along!
4) So you fasten object A to the rear of your wagon and you fasten object
B to the front of the wagon, and hop on.
5) Worse! Exactly nothing happens!  A pushes B (and the attached wagon)
forward; but B pushes A (and the attached wagon) rearward .  Because of
the third law, the effects on the wagon cancel.

6) Now, imagine that you had two magic objects A' and B' such that A'
pushed B' away with a repulsive force, but B' exerted no force on A'.  Use
them as in 4) and you not only have perpetual motion, you have perpetual
(and free) acceleration!

Hope this helps,
Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor

----- Original Message -----
From: Dorrie Bright <Dorrie_Bright@HOLDERNESS.ORG>
To: <PHYS-L@lists.nau.edu>
Sent: Friday, August 20, 1999 10:49 PM
Subject: Re: Evolution and Creationism/Thank you/3rd Law


> Dearest Fizzlers,
>
> I, at times, have come to an impasse when trying to teach Newton's Third
> Law -- I have had a student or three be quite adamant that it could not
> possibly be so . . .