🚀 go-pugleaf

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