Date: Thu, 18 Sep 2003 10:51:18 -0400

Author: Tom Ford

Subject: RE: Newton's Third Law

Post:

I would add one feature to Matt's demonstration. I have a large and obvious
coil spring that normally holds pencils on my desk. When I empty this with
some ceremony, and use it (in place of Matt's "bent" fingers), I get a
chance to point out the several force pairs and conclude that I am bending
the wall.

Steve DeAngelis brings out a wooden pear (the kind of painted fruit used
for table decorations) which has previously seen a bandsaw. When he
separates the two halves, each has a vector (half of the force pair)
painted on the inside. Farces Come in Pears... It is amazing how such
startling images help cement elusive concepts, but then that is why we do
demonstrations anyway.

Tom Ford

At 09:11 AM 9/18/03 -0500, you wrote:
>Dave et al,
>
>A great quickie demonstration of how normal force is not a reaction to
>weight follows:
>
>Hold a book up against the board. Make a point of showing the students
>that you are obviously pushing the book into the board (show them that
>your fingers are bent). Ask them what is keeping the book from smashing
>through the wall - 90% will say it's the board. Then you've got
>them...
>
>This can be fleshed out more if you draw a force diagram as well.
>Point out to them that the weight vector points down while the normal
>force (board vector) points outwards at a right angle to the weight.
>
>I like to use this example well before even mentioning ramps or
>anything more complex than basic force diagrams. I find this very
>effective for most students.
>
>Cheers,
>
>Matt Lowry
>Lake Forest HS
>Lake Forest, IL
>
>
> >>> David.Kardelis@ceu.edu 9/17/2003 3:53:23 PM >>>
>I am looking for a lab that will make the students really understand
>the
>third Law. Most seem comfortable with the second but not the third.
>This is for the introductory College Physics both alg and calc based.
>Could be computer based as I have Labpro's and computers in class. If
>there is a good simulation exercise that would work too. I prefer real
>objects but simulation is ok too.
>
>
>I have a couple activities that I do in lecture. Give the students
>spring scales and magnets on the ends of a paper clip. With two equal
>magnets, the two opposing scales read the same while you are trying to
>pull them apart. Add more magnets to one side making one side a
>stronger magnet, but again the two scales still always read the same.
>The other is the tug o war demo.
>
>I guess I am looking for a lab that will really make the students
>realize for example, that the normal force is not the reaction to the
>object's weight. You do problems and examples with the inclined plane
>but it doesn't seem to stick.
>
>
>dave
From rtarara@saintmarys.edu Thu Sep 18 08:15:07 2003
Date: Thu, 18 Sep 2003 09:58:29 -0500
From: Rick Tarara
Subject: Re: Newton's Third Law
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Let me say something that may be heretical. I'm not sure what
_understanding_ Newton's Third Law means. I can know the law, and can apply
the law--by invoking the mantra 'A on B, B on A', but what constitutes
understanding it? This is NOT an intuitive concept--it took a couple
millions years of human development to come up with someone who recognized
the validity and importance of it all. What I think our task is with intro
classes is to get students to accept N3. This is very difficult. One of
the most missed FCI questions is the one about the car pushing the truck and
both are accelerating. Many get the next question--car pushes truck at
constant speed correct (force of car on truck has the same magnitude of
truck on car) but few will answer this for the accelerated case.

The complex collision demo with force transducers, computer generated
graphs, and the like is very convincing--TO US---but I think less so to
students (too many black boxes). Yet it is worth doing if it convinces a
few. Hakes Socratic Dialog lab on N2 actually deals quite a bit with N3 as
well and is a good lab exercise for seeing if students really grasp Newton.

http://www.physics.indiana.edu/~sdi/

But in the end, IMO, N3 is one of those things you have to just learn,
accept, and then use.

Rick

*********************************************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
********************************************************
Free Physics Educational Software (Win & Mac)
New software now available based on project below.
www.saintmarys.edu/~rtarara/software.html
Energy 2100--class project
www.saintmarys.edu/~rtarara/ENERGY_PROJECT/ENERGY2100.htm
********************************************************
----- Original Message -----
From: "Matt Lowry"
To:
Sent: Thursday, September 18, 2003 9:11 AM
Subject: RE: Newton's Third Law


> Dave et al,
>
> A great quickie demonstration of how normal force is not a reaction to
> weight follows:
>
> Hold a book up against the board. Make a point of showing the students
> that you are obviously pushing the book into the board (show them that
> your fingers are bent). Ask them what is keeping the book from smashing
> through the wall - 90% will say it's the board. Then you've got
> them...
>
> This can be fleshed out more if you draw a force diagram as well.
> Point out to them that the weight vector points down while the normal
> force (board vector) points outwards at a right angle to the weight.
>
> I like to use this example well before even mentioning ramps or
> anything more complex than basic force diagrams. I find this very
> effective for most students.
>
> Cheers,
>
> Matt Lowry
> Lake Forest HS
> Lake Forest, IL
>
>
> >>> David.Kardelis@ceu.edu 9/17/2003 3:53:23 PM >>>
> I am looking for a lab that will make the students really understand
> the
> third Law. Most seem comfortable with the second but not the third.
> This is for the introductory College Physics both alg and calc based.
> Could be computer based as I have Labpro's and computers in class. If
> there is a good simulation exercise that would work too. I prefer real
> objects but simulation is ok too.
>
>
> I have a couple activities that I do in lecture. Give the students
> spring scales and magnets on the ends of a paper clip. With two equal
> magnets, the two opposing scales read the same while you are trying to
> pull them apart. Add more magnets to one side making one side a
> stronger magnet, but again the two scales still always read the same.
> The other is the tug o war demo.
>
> I guess I am looking for a lab that will really make the students
> realize for example, that the normal force is not the reaction to the
> object's weight. You do problems and examples with the inclined plane
> but it doesn't seem to stick.
>
>
> dave
>
From dimarco@physics.montana.edu Thu Sep 18 08:34:27 2003

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