Re: [Phys-L] inertia and the tablecloth demo

[Bernard Cleyet <bernard@cleyet.org>]

> On 2016, Aug 19, , at 15:42, Anthony Lapinski <alapinski@pds.org> wrote:
>
> As I stated before, my students (regular and honors) find forces to be the
> hardest topic. The third law is particularly challenging. A book rests on a
> table. If the book's weight (true weight, mg) is the action force, then the
> reaction force is the ___ on the ____ acting _____. Nobody gets this right!
>
> On Fri, Aug 19, 2016 at 6:24 PM, Derek McKenzie <derek@physicsfootnotes.com>
> wrote:
>
> > Richard Tarara said:
> >
> > Actually I have always been baffled by the fact that many students DO
> > NOT seem to know what you mean by push/pull or even up/down. In
> > exercises to identify the 3rd law force pairs various situations, one
> > (too) often gets a mish-mash of pushes for pulls, pulls for pushes, and
> > often in the wrong directions. :-(
> > -----------------------------------


Perhaps what may help is to use a spring "bath scale”, and a means of measuring its platform’s depression when the book is placed.  

Unfortunately, this presupposes an understanding of Hooke.   OTOH, would two students pulling on each end of a spring scale give them the idea that the bath scale is pushing up?  

bc