🚀 go-pugleaf

On Thu, 11 May 1995, Donald E. Simanek wrote:

> On Thu, 11 May 1995, Matt Tuley wrote:
>
> > >>...Instead, I laid a pencil
> > >> on the floor and flicked one end of it with my finger.  The pencil
 scooted
> > >> out the door, rotating around its CM.  The student's eyes got as big as
> > >> saucers and he exclaimed "It moved!"  I pointed out to him that physics
 is
> > >> supposed to be an _experimental_ science, and whenever a question is
 asked
> > >> that lends itself to an experimental check (even if its only
 qualitative),
> > >> they should do the experiment.
>
> > >There is one factor involved in the pencil experiment which does not come
> > >into play in the original situation: friction.  Friction is responsible
> > >for the rotation, not the original force.
> >
> > >Merv Koehlinger
> > >Parkway HS
> > >pw_koehlinge@noacsc.ohio.gov
> >
>
> It might be more complete to say that the friction during the impact,
> together with the force of impact, constitute a continuum of couples along
> the length of the pencil, adding up to an impulsive torque which give the
> pencil an angular momentum. After the impact, friction exerts a couple
> which slowly reduces the angular momentum till the pencil comes to rest.
> If one were to analyze this completely you'd also want to write the
> *linear* momentum equation and the conservation of energy equation, the
> motion of the pencil being the consequence of all of these, in that it
> must satisfy all of them.
>
>         -- Donald
>
Now we're getting to the heart of the matter.  It's not individual forces
that produce rotation, but couples!

Forces produce translation; couples produce rotation.

Mervin Koehlinger