🚀 go-pugleaf

   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