🚀 go-pugleaf

On Wed, 10 May 1995 Steve Koelinger wrote:
 > 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

Perhaps you have never played football, but even in pickup games I have found
from personal experience that when I jump up in the air to receive a pass and
am hit in the lower legs I do rotate.  Perhaps for further evidence watch some
highlights from NFL games.  It is quite noticeable that in mid-air rotation
occurs, just like in the pencil experiment.

Barry S. Riehle
Turpin High School
Cincinnati, OH