🚀 go-pugleaf

On Wed, 10 May 1995, Steve Luzader wrote:

>    In response to David Abineri's question about what would happen when a
> momentary force is applied perpendicular to one end of a rod floating
> freely in space, the correct answer is that a rigid rod will begin
> translating (its center of mass will move in a straight line in the
> direction of the force) _and_ it will begin rotating about its CM in a
> direction determined by the force.  If the rod is elastic, it will also
> begin vibrating in a mode with displacement antinodes at the ends.
>    I once assigned a problem like this to a junior-level mechanics class.
> Several of the students had come up with the correct answer (translation
> of the CM plus rotation about the CM), but another very persuasive student
> managed to convince everyone that the rod would only rotate.  When I gave
> everyone half credit for that answer, the persuasive student was sent to
> my office by his classmates to find out why the answer was wrong.  When
> he asked about the question, I didn't say a word.  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.
>
> Steve Luzader
> sluzader@fre.fsu.umd.edu
>
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