Date: Thu, 18 Sep 2003 09:46:38 -0500

Author: Laurent Hodges

Subject: RE: Newton's Third Law


>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 do this demonstration, too, as it is so easy and to the
point. But there is a follow-up question: What keeps the book from
falling due to gravity? Then you can discuss pushing less and less hard
until it starts to slide down, and bring in the upper limit to static friction.

From Thu Sep 18 08:11:08 2003