Re: Action - Reaction Demo

["Donald E. Simanek" <dsimanek@eagle.lhup.edu>]

On Tue, 15 Sep 1998, Ken Fox wrote:
> On Tue, 15 Sep 1998, Donald E. Simanek wrote:
> > On Tue, 15 Sep 1998, Ken Fox wrote:
> >
> > > Give students a small ball of clay and ask them to hold it between thumb
> > > and index finger. Then challenge them to squeeze in a manner that will
> > > make the two sides not come out symmetrical. It is a convincing effort.
> > > I wish I had remembered it in class today, maybr tommorrow.
> >
> > Interesting. But what, exactly does it demonstrate?
> >
> > 	-- Donald
>
> The concept from my mind is that you cannot push harder with one of the
> fingers. No matter which is the agent, thumb or index, that the other
> pushes back just as hard. 
>
> Am I way off?
>
> Ken Fox

My question was serious. I ask of a demo these things:

1. What do I see (observe) which convinces me of something?

2. What do I learn from it which I can apply in other situations?

3. Is it sufficiently free of other complications that the desired
proposition is clearly and unambiguously shown.

4. Can I see (comprehend) what I'm supposed to without elaborate
explanation or qualifications from the instructor?

What I see is a deformation of the putty. I certainly can't see the forces
the fingers exert, which we suppose are equal and opposite, and the net
force of each finger on the putty is directed toward the other finger. How
does the putty extrusion convince me the forces were equal and opposite?
What if the forces weren't equal and opposite (!), could that conceivably
make a different result?

I assume you are speaking of the symmetric deformation of the putty ball
along the axis of the forces. If so, a soft rubber ball would work as
well. But *is* the deformation symmetric, considering that the finger and
thumb have different shapes, and different effective radii of curvature? 

Certainly it does show that you can't press on the ball and deform only
one side, without deforming the other. This only convinces me that there
are two oppositely directed forces, those exerted by thumb and by finger,
but how do I know they are precisely equal? And even if they are equal,
they aren't an action/reaction pair, so this fact tells me nothing about
action and reaction. 

Once you introduce the putty, the action/reaction pair of forces are the
force the finger exerts on the putty and the equal and opposite force the
putty exerts on the finger. With the ball in place, the forces exerted by
the thumb and finger are no longer an action/reaction pair. I fear that
students will get from this demo a false idea of what an action/reaction
pair of forces are. Newton's third: If body A exerts a force on body B,
then B exerts and equal and opposite force on A. These forces, A and B,
are an action/reaction pair. 

The situation of this demo is far too complicated for me to get a concept
out of it. 

	-- Donald

......................................................................
Dr. Donald E. Simanek                            Office: 717-893-2079
Professor of Physics                                FAX: 717-893-2048
Lock Haven University of Pennsylvania, Lock Haven, PA. 17745
dsimanek@eagle.lhup.edu                 http://www.lhup.edu/~dsimanek
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