Date: Thu, 18 Sep 2003 09:11:59 -0500

Author: "Matt Lowry"

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

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.


Matt Lowry
Lake Forest HS
Lake Forest, IL

>>> 9/17/2003 3:53:23 PM >>>
I am looking for a lab that will make the students really understand
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.

From Thu Sep 18 07:59:06 2003