🚀 go-pugleaf

On Wed, 10 May 1995, Barry S. Riehle wrote:

> On Wed, 10 May 1995 Mervin Koehlinger 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
>
The football tackle involves another force which is not mentioned in the
original problem.  That force is gravity, which is responsible for the
rotation.  It would also be difficult to consider a human body as a rigid
object.

Mervin Koehlinger